A Visual Aid to Understanding Backfocus; Revisiting My Adapters for a New Camera

Here's a little set of slides I made that helped me understand backfocus. Maybe you'll find them educational, too!

Basic prime focus imaging relies on placing the imaging device's sensor at the focal plane of the optics. The position of the focal plane depends on the properties of the optics. In a refractor or reflector the focal plane distance can be measured from the objective or, since the objective is often hard-mounted in a mechanical system, the distance from a reference point somewhere else is given. Figure 1 might help visualize all this by using my FSQ-106EDX4 as an example. 

Figure 1. Definition of backfocus

The FSQ reference point is at the rear end of the focus tube when it's fully retracted. I will call the distance between the reference point and the focal plane the True Backfocus;  it's 178 mm for the FSQ (Figure 1, top). I call it this because it is usually the only number you're given by the telescope maker. The FSQ focuser travel is a modest 30 mm (Figure 1, bottom). Notice that when the end of the drawtube is at the middle of its travel range the distance from it to the focal plane is 163 mm (Figure 1, middle).

Now examine Figure 2. The purple adapters are supplied by Takahashi; they are four adapters that connect the drawtube to the manual rotator and extend a nice wide optical path out to a female M54 thread. Their total backfocus (optical thickness) is 110.2 mm. These adapters are between the end of the drawtube and the camera sensor, so their backfocus must be included in the total backfocus. You may have something analogous to them for your telescope, so read your scope's documentation! 

The green adapters are the items I add: In my case they're mainly working the M54 down to a male M42 thread to connect with the EFW or camera's tilt plate.  In your case you may also have an OAG, a electronic field rotator, or a filter tray in this green area. Again, since these all sit between the drawtube end and the camera sensor, their individual backfocuses must count towards the total backfocus. 

The total backfocus will be the distance between the end of the drawtube and the sensor regardless of the drawtube's extension, as shown in Figure 2.

Figure 2. How Practical Backfocus positions the sensor

Figure 2 (middle) shows the drawtube half extended, and if we choose the total backfocus correctly the will be at the focal plane! So what's the correct total backfocus? It's going to be the True Backfocus minus half the drawtube's travel.  I'll call this value the Practical Backfocus. The following two statements now apply:

Practical Backfocus is typically found by subtracting half the focuser travel from the True Backfocus. 

The total backfocus of all the items sitting between the end of the drawtube and the sensor should equal the Practical Backfocus. 

It's that easy! Just add up all the backfocus values of your adapters and spacers (and camera, too) and make sure it equals the Practical Backfocus.

This is exactly how Takahashi calculates "best" backfocus (see steps 1 and 2):

Imagers should supply adapters with the following total backfocus (Numbers for my FSQ are in brackets):

1. Maximum focuser back focus [the distance from the end of the fully retracted metal drawtube to the focal plane, 178 mm]
2. Minus ½ focuser travel [half of 30 mm, or 15 mm]
3. Minus necessary telescope accessories [the four adapters (645 RD, CAA Rotator, Aux Ring (S), and Coupling) provided by Takahashi that have a total backfocus of 110.2 mm]
4. Minus Camera backfocus [17.5 mm for the 2600's with their tilt plate attached, 12.5 if it isn't]

Why is the Practical Backfocus the "best?" Mainly for safety. If you're using motorized focusing, the autofocuser software will move the drawtube in and out. Being in the middle of travel makes it unlikely those motions will ever carry the drawtube to one of its stops, making the autofocus fail and possibly damaging your motorized focuser.

Let me be perfectly clear about the subjective nature of the Practical Backfocus. Any backfocus that allows your sensor to get to the focal plane is perfectly fine, although it may not work well with autofocusing. 

This raises an interesting point: if your autofocuser movements away from focus are smaller than half the focuser travel, you can make the Practical Backfocus larger by adding an extension tube. As a result the drawtube be less extended when focus is reached, possibly reducing the dreaded "drawtube droop." This might be an adjustment that's also worthwhile if your focuser has a great deal of range that you don't need, and it might save a few dollars by eliminating backfocus extenders.

Again using the FSQ as an example, if I know my autofocuser will never move the focuser more than 3 mm in and out, I could "live dangerously" by adding an extension tube that has a backfocus of 12 mm (half the focuser travel  minus 3 mm).  This effectively makes the Practical Backfocus 175 mm, and the drawtube needs only extend 3 mm for the sensor to be at focus. (3 mm + 175 mm = 178 mm, the True Backfocus.)

I could also reduce the Practical Backfocus 12 mm givin me a backfocus of 151mm. In this case the focuser would need to extend 175 mm for focus, but I can't think of why that might be desirable.

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If you're using a focal reducer, tele-extender, or flattener, things are different (Figure 3). The reference position is now a mark on the reducer or perhaps is the plane of its back plate; be sure to check its documentation. 

Figure 3. Reducer backfocus

When focus is correct the focal plane of the reducer will be the manufacturer-specified distance from the reference position. The total backfocus of your adapters and camera will need to equal or be very close to that.

For reducers et al., Practical Backfocus = True Backfocus. 

The CR 0.73X reducer I use has a required backfocus of 72.2 mm.

There are other situations where precise backfocus may be required:

• Camera Lenses (you absolutely must match the lens backfocus)
• Telescopes with built-in flatteners or reducers
• Coma Correctors
• RASA astrographs

Any time you put something that bends light into the optical path, read its documentation!  

Note that in these cases we can't play around with the Practical Backfocus as we did for prime focus imaging. When using a reducer et al. with a with its own True Backfocus, you will need to get the total backfocus as close as possible to that -- within practical limits. Most people aim for getting within +/- 1 mm. But given the software that now exists (I'm looking at you, BlurXTerminator) it's possible to correct star distortion caused by backfocus error. Up to a point, anyway.

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Several years ago I went into gory detail about what adapters were needed to connect my ASI𑽈2600MM and Canon DSLR to my FSQ-106EDX4 and Canon EF lenses. I've been imaging only LRGB since then and the adapters have worked fine. I'm going to switch to OSC this year despite this.

No, I'm not getting lazy in my advancing years. When I started CCD imaging it was almost all narrowband because that made it possible to image from my Bortle 7.5 backyard.  Back then I had sufficient open sky above me, but in the last fifteen years the trees have eaten up the sky. All I have now is the area around the north celestial pole, and I can tell you --- in narrowband, it's not exactly an area that's rich in narrowband targets!

I now travel to open sky sites where the sky is usually Bortle 5 or darker and I can LRGB image. Technically I could be using OSC! OSC data is easier  to process than combining three or four channels. So, ok, maybe I'm a little lazy.

So shortly I'll be diving into OSC by buying a ZWO ASI𑽈2600MC, the almost-identical twin of my 2600MM. I am going to hold onto the MM, though. I really do like its higher sensitivity and greater effective resolution, particularly when imaging with my FSQ at f/5. I'll definitely do some L-OSC imaging, and may want to augment some OSC images with H alpha. 

As in past years my primary imaging optics will be the Takahashi FSQ-106EDX4 in its native f/5 mode, the FSQ with a 0.73X focal reducer, and my Canon EF compatible lenses (at focal lengths of 200, 135, 70, and 50 mm). 

And now on to the adapters! First, some notes:

• The FSQ modes need new adapters because (a) I won't use the 2600MC with an EFW and (b) When I did this last time I missed adding the Takahashi Rotator Adapter's 10 mm backfocus. This error seemed not to affect image quality in the least, and was possibly even beneficial (see earlier discussion)
• I have an urge to reduce the vignetting when using the FSQ so I'll use M48 when possible instead of M42 adapters. Even better would be to use M55 extensions with the CR 0.73X reducer -- if I could find any
• I've removed the M42 adapter from my Samyang and restored its original Canon plate, making it compatible with my other lenses. This will let me use the 2600MC with all of them
• Modes that use the EFW have an added 0.6 mm (about 1/3 of the filter thickness) backfocus
• The FSQ prime focus modes are within 1 mm of their Practical Backfocus value. They differ from each other by 0.6 mm so that they'll share the same focuser setting
• There's a huge number of ways to combine adapters to get these results; what I show here is only one of them
• Most (if not all) the items in "My Adapter" green come from AgenaAstro; The exceptions are noted below. A few are so old they may have been found at an archeological dig

Here are my adapters for each mode: 

Figure 4. How I'll achieve the desired backfocus

That rightmost mode (2600MM + EFW + Canon lens) is a very tight fit.  I found the thin Canon-M48 adapter on AliExpress; the thin (2 mm backfocus?!) M48 to M42 adapter is coming straight from Astromania Optics. Will it work as advertised? And if it doesn't, will BlurXTerminator be able to correct the distortions? Oh, the suspense!

Of course my optics may not exactly match Takahashi or Canon specs, which will only be important when imaging with the reducer or lenses. There may be some tuning necessary, so field testing will be high priority when the weather finally warms up.

And finally, yes, that's a lot of different adapters/extenders/spacers!

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An little fiscal side note: When I needed to connect my old SBIG ST-8300 to a Canon lens in 2012, the SBIG adapter cost $295 (this is also the current price!). this year I'll pay only $59 for the ZWO Canon lens adapter, and the AliExpress adapter was even less at $35. Am I justified to feel as if the SBIG adapter was a bit overpriced? 

Using PixInsight PixelMath to Correct a Flat Frame Issue

In my last post I talked about a problem I had with flat frames collected at f/2. To correct the issue I used the processes AutomaticBackgroundExtraction and GradientCorrection. This worked very well for two of my images, but I recognized it might not with the huge nebula Sh2-264, (AKA the Lambda Orionis / Angelfish Nebula).

I tried the ABE+GC method on this and it failed spectacularly by obliterating the eastern half of the nebula. So my approach will instead be to use a method that employs PixelMath and an ad hoc model of the donut hole.

Here is the hole as seen in the green master frame from one the other images collected the same night:

Green Master Frame with the donut at center in all its glory

This donut appears in the master green frame of Sh2-264, if not as prominently. So I'll use the above photo as the basis for the correction's structure. It's worth noting that the master frames have already been flatted, so either my system is dust-free, or the flatting took care of any dust donuts. What I'm about to do is apply a simple correction that's limited to the central area of the image.

At the very center the brightness doesn't seem diminished and no correction is needed. As distance from the center increases the distortion progressively darkens to form a ring, then quickly brightens again. It's this ring that must be brightened. Because I'm correcting the master linear frames I'll use simple multiplication of the existing pixel values.

Eyeballing this led me to a polynomial model of the needed correction. Here it is in an Excel graph.

Sixth-order polynomial fit to estimated donut darkness

The horizontal axis is scaled distance from the donut's center, with the value 1 corresponding to radius of the ring's darkest values at about 700 pixels from center. The vertical axis is a dimensionless value for the darkness of the donut relative the image's true background.

This is a complex shape that is fit well by a sixth-order polynomial 

So, how to make use of that polynomial? Let's look at the PixelMath script!

Script for hole correction. Click for larger version

This is a simple script. Basically it looks at the distance a pixel is from the center of the hole, calculates the factor by which to increase the pixel's brightness, and multiplies the pixel's existing brightness by that factor. Pixels outside the donut (R > 1.38)  are left unchanged. 

The constants are essentially input parameters that can be played with to refine the correction. The main adjustment is Amplitude; too large a value and the donut becomes a light ring, too small and there's not enough improvement.

The multiplier is 

        1.0 + (polynomial value times Amplitude). 

I found the best result used Amplitude = 0.04, so at most the correction is a 4% increase in linear value.

So what did this do? Here is the Sh2-264 Green Master before and after the script is applied:

Uncorrected

Corrected

Note that these are given the "Boosted" PixInsight stretch to emphasize the hole; it won't be this evident in the final image.  The script did a fair job, I think. 

 

My post-WBPP workflow for this image uses ABE to subtract first order light pollution from each master light frame; the script is then applied to each channel. After that I return to my conventional workflow: CombineChannels, BlurXTerminator to reduce chromatic aberration prior to color calibration, and so on. 

The result:

Finished Image, corrected (1/4 scale)

North is up; The Angelfish swims nicely westward! I think the best way to validate any image is to compare it with clearly superior one by an accomplished imager, so I used Adam Block's superb image for this one. I think my processing does a very decent job of reproducing his in the central area of the nebula, so I'm happy!

This completes the rehabbing of the Iowa Star Party images.  Next I'll be revisiting some of my images from 2022. Also, Masters of PixInsight are doing a workshop on mosaics in December, so maybe I'll finally process that Veil Nebula mosaic data I've been sitting on.

Iowa Star Party Images: Repaired!

To recap: I took one night's data at the October Iowa Star Party.  As was my usual (rather lazy) practice I deferred shooting flat frames until my return home. When I processed the data I got something of a shock. The stacked calibrated frames had very obvious dark "donut holes" in the middle. Here's what I mean:

Master Green after ABE

This was the result of not matching the exact focus used while gathering light frames. The remedy would be to obtain correct focus and retake the flats, so the next clear night when the temperature was close to what it had been in Iowa I set up in my back yard, got focused, shot flats, and hoped for the best. Something was off, though; the new result after recalibrating the light frames was an even larger donut hole than what you see above.

I've read that flatting at f/2 is a fiddly thing, and based on this I have to agree.

The only real solution is to collect future data and flats at a slower speed so that focus is less of an issue. And to take flats in the field at the time of data acquisition. But that's too late for the Iowa data, which I really don't want to throw away. So what can I do to salvage it?

How about creating a sort of secondary multiplicative layer of my own? One way to do this might be by using using the GAME script for PixInsight, which basically would create a round mask that I could use to stretch the hole area a bit, or even to create a synthetic secondary flat to use in recalibrating the already calibrated light frames. Nice thought, but I tried both methods and didn't get satisfactory results. 

More labor-intensive would be to create applications in a high level programming language that could directly adjust pixel values in the master frames. It's been maybe 12 years since I did any serious coding and had none of the needed compilers installed on my computer, so no -- that would be too involved. Much simpler would be to use PixInsight's PixelMath to do the same thing. I used Microsoft Excel to model the hole as a 6th-order polynomial and used a simple PixelMath script to transform the master images. This actually worked to some extent, but not well enough. I could have fiddled with that and eventually found the right polynomial, scale size, and amplitude to make a good correction, but I really didn't have the patience for that. There had to be a better way.

And in this case, there was. I wouldn't recommend this as a cure for any other flatting problem, but it seemed to work well enough for the Iowa data. The nice thing is that it used two standard PixInsight processes.

Because the data for two of the targets was low in the eastern sky over a rather conspicuous light dome, it suffered from a considerable light pollution gradient. I used PI's ABE to make a first pass at getting rid of that. This was a fairly typical application of ABE, using subtraction and a function degree of 4:

ABE settings

This essentially revealed the hole and whatever else the bad flat didn't correct. GradientCorrection was then applied with some non-default settings designed to work better on the hole's small-scale structure.  

GradientCorrection settings

Here is the result. The hole is almost completely gone, as are the edge issues left behind by ABE.

I applied this to all three channel masters and processed normally. Without the hole I could process a little more aggressively to bring out Barnard's Loop.

My image based on Iowa data

Wikipedia image (an RGBHa image by Hunter Wilson),
cropped and scaled to match my image above.

Below my image is the picture of Barnard's loop on its Wikipedia page for comparison. I've rescaled and cropped the image to match mine. Wilson's acquisition data is RGBHa, which probably accounts for the very red nebulosity.

Am I totally happy with my image? Not entirely. Theres still some weak signal suppression at the the very center of the image, and I wish the Running Man was bluer. Given that this is based on only half an hour of total exposure (ten minutes per color channel) I am pleased with is how it reveals a lot of the blue reflection nebulae in western Orion. It's a shame it didn't quite extend far enough to catch the Witch Head. It would be wonderful to devote hours to this area, but that's not going to happen.

Here's the other de-holed image from Iowa:

L to R, Soul Nebula, Heart Nebula,
and the Double Cluster (1/4 scale)

As with the Orion image, the hole is essentially eradicated. Too many stars, though. 

Reprocessing the third image, the huge Lambda Nebula, will be problematic as the hole is very entangled with the nebula. I may need  to use my PixelMath script method for that.

Next spring I'll try to get dithering working more reliably so that I can drizzle process and get better stars. With some luck I may get my long-sought wide field image of the Polaris-area IFN after all.

But now it's time for winter hibernation and reprocessing of old data. With what I'm learning from the Masters of PixInsight folks, I may even tackle that big old Veil Nebula mosaic data I've been sitting on for over a year!

Next time: the Lambda Orionis Nebula (Sh2-264) after de-holing!

Iowa Star Party Results

Back from the Iowa Star Party, and my first field use of my Samyang lens. Mostly the results were good; shooting with the Samyang at f/2 appears to have great potential, but I need practice to make it really great.

The first night of the three-night party was on-and-off cloudy with enough breaks to keep visual observers happy, but it was not sufficient for the deep sky imaging I wanted to do. I collected some frames mainly to practice with the system.

The second night was almost perfect. Thunderstorms had moved through during the day and were gradually drifting off to the east as the night progressed. Frequent lightning flashes illuminated the eastern sky at for a while but diminished so that by the time I stopped (around 3 AM) they were not a factor. I used the first three hours of darkness to image the Heart Nebula / Soul Nebula / Double cluster area:

Heart and Soul Nebulae, Double Cluster

Next up was an hour on the central Orion area (20 minutes per channel). I didn't do a good job with processing this.

Central Orion (M42, Horsehead, Flame)

To illustrate what the lens can do, here's the Orion Nebula from the above image, post-processed for higher dynamic range:

M42 from above image

It's pretty, but it's only 1293 x 1326 pixels, so at 300 DPI it's only good for a 4 x 6 inch print. 

It was getting into the wee hours of the morning, but I couldn't stop--I get so few opportunities to image in Orion. I just had to image the Lambda nebula around Orion's head, something I had imaged eleven year earlier in H-alpha on an unusually warm February evening. This is based on a ridiculously short 30 minutes of total exposure (10 minutes per channel) and really illustrates the power of shooting at f/2: 

Nebulosity at the head of Orion (Betelgeuse is at lower left)

(All three of the above full images are at half-scale. For full scale images, see my AstroBin gallery.)

Alas, the third night was not to be. The cold front came through and the day was windy and brisk. late in the afternoon it started raining; online sources suggested that clearing might happen around midnight -- or later. I decided to call it a star party and get some good sleep before the return drive the next morning.

After an imaging session it's always good to reflect on what was learned, and this trip had some lessons beyond learning that my sleeping bag advertised as being good to 20 F was definitely not.

Rotating the system proved very doable. It was awkward at first, but so is everything. I found it almost impossible take the focusing belt on and off without messing up the focus, but the penalty for that is minor. My NINA advanced sequence design with added pauses seemed to work out fine.

The first night I found that R-G-B-Dither cyclic acquisition was problematic. I was doing this to make drizzle processing possible. Mostly this worked, but occasionally PHD2 went wildly unstable in RA and suffered death-by-overcorrection. I'm not sure if this is a settling time issue or what. The second night I turned dither completely off and guiding was fine. The ASI 2600MM seems not to need dithering, but if I'm to drizzle, I'll need it.

Look again at the above images, and notice the slightly darker circular area at the center of each image: a donut hole. This arose because of bad flats; here is the stretched value of the ratio between older, correct flats and those from Iowa. 

Lovely radial symmetry about the image center, but this should be a nearly uniform field aside from any changes in dust. 

I tried using the older flats in place of the new ones, but that failed. Because my system is currently almost dust free, I tried skipping the flats and using PixInsight to correct the vignetting. That also failed. My next attempt will be to use Photoshop to manually correct this issue. 

My guess for what caused this is improper focus. I deferred shooting flats until I returned from Iowa, and in that time the zero reference point on the EAF was lost. This made it almost impossible to reproduce the focus used for the light frames, resulting in the poor flats. One clear night should make it possible to shoot some new flats at proper focus.

The weather forecast is mildly optimistic for the first week of November with highs in the 60s; will that warmth will bring the clear night I need? 

In preparation for the Iowa Star Party I'm doing a some imaging housekeeping. The guider needs a fresh calibration and the filter offsets need a good redoing. More complicated are the changes needed for a basic imaging sequence using the Samyang lens. Because rotating the camera/lens requires the EAF belt to be disengaged and then re-engaged, one needs to put some breaks into the sequence to make opportunities for working with the belt. Here's what I'm setting up for the Iowa party.

The following instructions are for the NINA advanced sequencer. If you're not using that, you really should be! Also note that this is specific to a short focal length system for which you intend shoot RGB suitable for drizzle processing.

• NINA's Advanced Sequencer (If you are not, you should be)
• Your EAF position is zero when the lens focus is at the infinity stop

All good? Great. Here's what you need to do if you plan to orient the imagining camera while it sits in an Astrodymium cradle. It's a little complicated by the need to disengage the EAF belt while turning the lens.

Rotating the lens requires that you add two Message Box (MB) instructions to the sequence. The MB instruction is found in the Utility section. Obviously, if you don't care about the camera orientation, you can just skip the following.

If you're doing a Slew, Center, and Rotate (SCR) and have connected the Manual Rotator to NINA, place one MB just before the SCR command and another just after it. When the sequence reaches the first one, it will stop. You should then disengage the belt from the lens, being careful to not disturb the focus. Then click away the Message Box. NINA will slew, center, and tell you the amount and direction to rotate the lens. Do this until NINA is happy. It may also do some additional centering, but eventually it will reach the second MB and stop. Re-engage the focus belt. Verify that focus is good enough for autofocusing, and click away the second MB. The sequence will now resume normal operation.

If you're using Slew and Center (SC) and simply eyeballing the orientation, just use one MB immediately after the SC instruction and a second SC right after that*. When sequencer get to the MB, Disengage the EAF belt and manually orient the camera using snapshots. When the orientation is correct, re-engage the belt, check focus, and click away the MB.

*The second SC is needed to insure the target is at frame center; it acts as the post-rotation recentering performed by SCR but not by SC.

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I still wasn't happy with the tilt I was seeing, and I wondered if maybe it was coming from the way I had the camera attached to the Samyang -- that partially attached 5 mm spacer seemed iffy. I sent a note to Nic at ThinkableCreations asking if he had some advice about how to remove about 2 mm from the M42 threads on the adapter he sells. He didn't, but he expressed some concerns about the shorter thread length not providing a solid connection. I thought that would not be a problem and went ahead with my plan to sand off the end of the threads.

I used 600 Ultrafine "Wetordry" sandpaper from 3M for a while and made little progress, so I switched to 400 and it went nicely. Eventually I was able to attach my 7.5 mm spacer to the Thinkable adapter, giving me 45.5 mm of backfocus, close enough to the 45.0 desired.

Sounds good, but the reality was that this change once again made the lens unable to focus infinity.

Swapping off the 7.5 mm spacer for a 5 mm, back focus was now 43 mm and focus was restored at a EAF position of about 3200. Still bad tilt, possibly because I didn't sand the Thinkable adapter down far enough. I added two spacer shims to bring the back focus up to 44 mm. The focus position was now around 2300. One last tilt tilt-correcting shim took it up to 44.2 mm and an EAF position of 1400. Possibly I'll add another thin spacer in Iowa.

Evidently the ability of a lens to reach focus depends strongly on backfocus, something I didn't know. 

EAF position (vertical axis) vs. Backfocus (horizontal axis, mm)

(The zero position in the diagram corresponds to the maximum lens movement in the direction of infinity focus. The negative position at 45.5 mm is a guesstimate.)

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Anyway, here I am a few days away from the Iowa Star Party and I'm going to shove my perfectionist nature to the curb and settle for "good enough." I'm going to rely on BlurXTerminator to handle aberration and tilt issues.

Here's an idea of what the lens can do. This is about a 16th of the full image area, unscaled. M31 straddles the right edge of the image. This is a single 20 second frame of luminance; click to see it at 1:1 scale.

Getting The Samyang Setup Ready For Imaging

[This is Part 2 of 2 about my new Samyang 135mm f/2 lens imaging system]

What is the Samyang Setup? 

It's the same setup I use for imaging with the FSQ-106 -- with a few changes. Obviously the imaging scope is now the Samyang lens and the Pegasus FocusCube 3 is swapped out for a ZWO EAFN. The connecting hardware between the lens and my imaging camera (ASI 2600MM) is different as well because of backfocus needs.

Questions to Answer

Is the lens optically sound?  Can it provide focus at infinity? Does it have significant aberration? Will it work well at f/2, or does it need to be stopped down to f/2.4, f/2.8, or f/4? Do the lens adapters introduce significant tilt?

Does autofocus work well?

How much can I reduce the time it takes to make a single dither?

Given the cloudy nights typical at this time of year it will take a while to get things sorted out. Because it only requires stars to do this I can stay in my back yard; dark sky is not necessary.

Night One (10 September)

The lens would not focus at infinity.  This meant autofocusing and image quality assessment were off the agenda. 

What did work was tracking. Plate solving was 100% despite the stars being somewhat out of focus.  I was able to slew and center without any issues. 

Clouds came in before I could look into dithering -- or anything else, for that matter.

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A letter to the M42 adapter person  at Thinkable Creations got a fast reply that pointed me to this video that shows how to remove the focus travel stop. This was an easy fix and with the stop removed the lens should be able to focus stars. 

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Night Two (22 September; summer is over!)

Really, it was almost two weeks between clear nights that I could use! Worst Summer Ever: clouds, smoke aloft, smoke at ground level with air quality alerts, rain, and the abundance of mosquitoes that the rains produced. Onward to Autumn!

First business: star focus. I set the EAF zero position at the full out focus, and infinity focus is near  position 750. Park position will be a little larger than the backlash.

I used a standard methodology* for getting autofocus configured.  

1. Manually** find a very good focus. 
2. Change position** gradually until you see greater than 50% growth in star size. Set step size to the amount of position change.
3. Run autofocus and see if the ratio of defocused:focused HFR is about 3:1 to 4:1; estimate how much backlash is in the system and enter that in the OUT field of NINA's autofocuser settings. Backlash will appear as unchanging HFR in the first few measurements. The change in position from the first measurement to the last one at the same HFR is the amount of backlash.
4. Run autofocus again and adjust step size and backlash accordingly until a decent hyperbola emerges
5. Repeat Step 4 until HFR ratio is about 3:1 to 4:1 and hyperbolic quality is close to 1.00
6. (optional) Reduce number of autofocus points and run autofocus to confirm it still works well 

*This is described by another fine Patriot Astro video starting at the 16:29 point, where the process is used with a ZWO EAF.

**My suggestion is to start at focuser position zero (the new "infinity" stop, or close to it) and move to best focus. Stop at a good focus and don't try for perfection; don't decrease the focus position at any time while hunting for focus. Then continue increasing the position while determining the step size. If you happen to pass through a better focus, note its position and measure step size from it. This insures that backlash does not factor into step size.

I did get AF to work reasonably well, with a focus step of 100 and backlash also at 100. However, this was with NINA's built-in AF, not Hocus Focus, so for Night 3 I'm going back with Hocus Focus

Sample frames were also collected at f/2.0, f2.4, f/2.8, f/3.3, and f/4.0. 

This gives me hope that I can image at f/2.0. Night 3 will be tuning Hocus Focus for better focusing and seeing if I need to adjust backfocus. If this works out Night 4 might be trying to create an actual RGB image!

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Loading the sample frames into ASTAP suggests dreadfully large tilt: 42% at f/2.0, and 16% (barely tolerable) at f/4.0. Here is are the diagrams of interest at f/2.0:

f/2.0 Tilt Original

This indicates a strong bottom to top tilt.

f/2.0 Aberration Inspector Original

The bottom row has badly elongated stars, but the top row isn't too bad at all. I think the tilt adds elongation in the bottom row while essentially nulling it out the top row. If I could selectively remove the tilt I'd probably have a better idea of the aberration due to backfocus error and  could possibly fix it mechanically.  

Toward that end I've ordered some very thin 3D-printed tilt shims. (Hardware doesn't permit me to use the tilt plate that came with the camera). Correcting some of the tilt might help with focus and other star-diameter calculations. An alternative it to use software to correct both tilt and any other aberrations simultaneously. The software of choice for doing this is BlurXTerminator (BXT).

Applying BXT (using its default settings) gives me this:

f/2.0 Tilt after BXT

f/2.0 Aberration Inspector after BXT

Quite an amazing improvement, isn't it? Tilt has essentially vanished and corner stars are much rounder.

Night Three (23 September)

Hocus focus worked well with the existing values of backlash and step size. I did bump backlash upward a little to 150 after looking at a few runs. With HF running the hyperbolic fits were much better and the luminance focal position seemed more consistent.

I ran the filter compensation calculator with mixed results. Red and green were basically parfocal with luminance, but blue was quite offset. This might be because I need to adjust exposure times? I'll repeat this.

Night Four (25 September)

This time the best focus (smallest NINA HFR) determined manually was at focuser position 735. Blue best focus came at 835, so the offset was +100. This is essentially the same as the software-determined +93. 

I took a baseline R-G-B-Dither 10 times; the target was M52. My main goal is to get a baseline for how long it takes to gather this data. It appears that a simple 60 s frame consumes about 70.4 s; a frame followed by a dither uses 101.4 s. Ignoring autofocusing, this means a single RGBD(ither) sequences uses about 242.2 s to collect 180 s of data. Roughly speaking, multiply the total exposure time by 4/3 to get the actual acquisition time. It's pretty much the same as if I was shooting LRGB.

An "adequate" data set of 40 frames per channel, suitable for drizzling, means 2 hours of data. This means acquisition time will be about 2.7 hours, plus some for refocusing. This isn't half bad, and might be bettered by adjusting settling times and optimizing the filter order.

Anyway, here is the first light image for the lens:

M52 at center

This is surprisingly good, at least to me. I'm under a Bortle 7 sky and not using filters of any kind. The ability of PixInsight to remove light pollution boggles me, and how well BlurXTerminator reduces aberrations is equally amazing. At f/2.0 the lens has considerable chromatic aberration:

CA in corner star

This star elongation is almost all chromatic aberration, and amounts to about 4 pixels between blue and red. Because BXT is able to correct this I'm going to go with f/2.0 for my first "real" image. If that doesn't turn out well I may move to f/2.8.

Questions Answered?

Is the lens optically sound?  Can it provide focus at infinity? Yes, after a little surgery. Does it have significant aberration? Yes, but it appears to be correctable using BlurXTerminator. Will it work well at f/2, or does it need to be stopped down to f/2.4, f/2.8, or f/4? It's adequate at f/2.0, but might be better at f/2.8. Do the lens adapters introduce significant tilt? I suspect this is the source of the tilt I'm seeing, but I need to look closer at this issue. Maybe the shims I ordered will be the remedy, or I may revert to using a Canon to M42 adapter to see how that works.

Does autofocus work well? It seems to work well enough.

How much can I reduce the time it takes to make a single dither? I still need to play with the dither settings and find out.

------------------------

That's the last of the prep nights!  Next I'm going to try to resolve an issue I've had while using two ZWO cameras (one for imaging, one for guiding) at the same time; this was a problem that first popped up while at a remote dark-sky site in which the two cameras switched roles. Using the ASI2600 as the guide camera really does not work.

Another issue I need to explore is why it takes NINA so long to connect to my Losmandy Gemini 2, why it throws an error at first and then makes a good connection. Strange!

The Samyang 135mm f/2 Lens; Setting It Up and How to Use It

[This is Part 1 of 2 about my new Samyang 135mm f/2 lens imaging system]

Assembling the System

My Samyang 135 mm f/2 imaging system comprises the Canon version of the lens,  a ZWO EAFN, the M42 adapter from Thinkable Creations, and finally the Astrodymium rings. 

The rings (made in Canada) were held up by the changing import rules and regulations about tariffs and fees because I had ordered them from the manufacturer in Canada. The seller was scrambling to sort out what it all meant for any delivery issues and added fees, and was good enough to contact me about what was going on and suggest I cancel the order and buy it from Agena Astro.  Which is what I ended up doing.

The Astrodymium rings/cradle went together like clockwork. I managed to take a couple of missteps by not following the directions and paying attention to the animations in the instructions. I don't use ASIAIR and probably never will, so instead I got a second accessory rail instead. I know when one is accustomed to machined aluminum for tube rings and dovetails the 3D-printed plastic parts may seem a little suspect, but when assembled the entire thing is quite rigid. I don't expect to see any flexure at all.

Through no fault of Thinkable Creations the install of their adapter was more difficult than anticipated. Installing it involves first removing a plate on the Samyang that interfaces with a Canon DSLR and then detaching a spacer ring that's held in place by four tiny screws. The plate came off easily, but two of the spacer ring screws were crazy tight and it took some time (plus a little penetrating oil and elbow grease) to get them out. Other than that the install went well. 

One thing about this adapter that prospective buyers should know: the M42 threads are very long. When I screwed it onto my ZWO EFW it came within a mm of the filter carousel. This seemed dangerous; I imagined it snagging on the carousel and possibly damaging the EFW and filters. 

But fortunately it all worked out. The required backfocus when used with filters is 45 mm. With the adapter in place I have 12.5 (camera) + 20 (EFW) + 5.5 (adapter) for a backfocus of 38 mm. My plan was to add a 7.5 M42 spacer ring to bring it up to 45.5, which I should have been close enough to the magic number of 45.0. Unfortunately those long threads wouldn't allow the ring to fully screw onto the adapter, and instead of 7.5 mm it added 9.5 mm. That put backfocus at 47.5, much too long. Luckily, I had a 5 mm spacer ring on hand. When it screwed on as far as possible there was a gap between it and the adapter face of about 2 mm. This effectively made the adapter's back focus 7.5 mm. Plus the spacer ring's threads don't intrude into the EFW anywhere near as far as the adapter. So if you intend to use the adapter, buy a 5 mm spacer ring, too. The diagram below illustrates how the backfocus works.

Backfocus for ASI 2600 minus tilt ring (red),
ZWO EFW (blue), 5mm spacer (green), and M42 adapter (black);
Diagram is not to scale!

The only thing I didn't get (but should have) in the initial round of orders was a short (150 mm) Vixen-style dovetail, but that's on order and will arrive about the time this is posted. The short length allows the camera/EFW to have full rotation and lets me do flats by resting the light atop the lens shroud.

Imaging at a focal length of 135 mm

Undersampling

Imaging with the Samyang 135 mm f/2 lens is going to be different from my usual imaging mainly because of its short focal length. This will cause what's called undersampling, in which pixels scale is smaller than what the seeing scale. When this is the case, a star's light will illuminate a pixel, but probably not the pixels around it. The star is imaged as a square of pixel size. (When pixel scale is much smaller than seeing scale, a star will illuminate many pixels which makes for nice looking stars at the cost of resolving detail.) 

How do we know undersampling will occur before even taking an image? All we need to do is compare our imaging setup's pixel scale to a value of seeing expected for an imaging session. Suppose we take average seeing as 2.0 arcseconds per pixel  ("/px).

The formula for a setup's image scale is base on the camera's pixel size and the focal length of the imaging telescope or lens: 

Pixel scale (in "/px) = 206 x camera pixel size (in microns) / focal length of lens (in mm)

If you look closely at my recent IFN image, you can see it's on the edge of being undersampled: many of the smaller stars look blocky. According to the above formula, my setup for that had an image scale of 

Pixel scale = 206 x 3.76 microns / 387 mm = 2.0"/px

This confirms the idea that it's mildly undersampled.

Now let's repeat the calculation for the Samyang. We have

Pixel scale = 206 x 3.76 microns / 135 mm =  5.74"/px

This is much larger than 2"/px, so it's safe to assume stars will be undersampled, probably badly.

Drizzling

The way to compensate for undersampling is to drizzle during processing. Drizzling can make those blocky stars rounder and fuzzier, at cost of extra processing time and worse, an amplification of noise. The noise can be reduced by acquiring a large number of light frames and by using a utility like NoiseXTerminator. Drizzling raises the bar on how many light frames to collect and may require frequent dithering. 

If you read forums there seem to be two common answers for how many frames you need -- at least 100 or at least 40. The former comes from those who want the very best images, while the latter is for people like me who are happy with satisfactory results. I'll probably go with 40 for the color frames, but closer to 100 for luminance. There's also disagreement about how often to dither -- once every few frames or with every frame. I'll probably choose to dither after each luminance frame and after each third frame for color channels, if I can reduce the time it takes to dither to something like 20 seconds or so. This might be unrealistic, only testing will tell.

 Dithering

The distance to dither on the imaging camera is generally accepted to be 10 px or so. NINA lets you set this by specifying how many pixels to move on the guide camera. To determine the value to use requires that pixel scale formula again, applied to the imaging system and again to the guiding system:

Imaging Scale = 5.74 "/px (from previously)

Guiding Scale = 206 x 3.75 microns / 130 mm = 5.94 "/px

This means moving one pixel on the guider corresponds to moving 5.94", and moves the imaging camera (5.94 / 5.74) px, or 1.04 px. In other words, the motions of the imaging camera essentially are the same as those of the guider. If I want 10 px dithering on the images, I should use 10 px for the NINA "PHD2 Dither Pixels" setting. The number to use is open to guesswork. Maybe 5 is fine? I'll have to try different values.

Other NINA dither settings are related to the mount. "Settle Pixel Tolerance" is basically how close PHD2 has to be to the guide star before it allows the mount to start settling. You can also set the minimum and maximum times for settling. The defaults for these are 10 and 40 s, respectively. My plan is to experiment with the minimum time and pixel tolerance values to see what works fastest with my mount. 

Some people dither only in RA, but the general advice is to use random dithering.

Exposure time

This is really a guessing game with many trade-offs. For fun I'll use the Sharpcap Sky background calculator for imaging with the Samyang at the Iowa Star party. Sky brightness there is 21.60 magnitude per square arcsecond, and the resulting sky electron rate is 6.49e-/px/s. Read noise is a negligible 1.4e- at gain 100, so to get the sky up to about 1/6 of full well would take 333 s (5.5 minutes). Pretty sure most of the stars in the field of view would be blown out by that. How about simply making sure that the sky signal swamps the read noise? Let's say by a factor of 100? That would only require an exposure of about 21 s. So now I have the exposure time bracketed: 20 to 2000 s!

It's worth noting that some people will shot light frames with only 20 s exposure.

I've been using 90 s exposures and I really like the star color I got in the IFN image so I think I'll stay with that. A test image around the next new moon would be really useful.  

Next Post: Testing