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

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

Basic prime focus imaging, where the objective directly forms an image on the camera sensor, relies on placing the sensor at the focal plane of the objective. To do this we need to know where the focal plane is relative to the optical tube assembly. In a refractor or reflector the focal plane location can be measured from the camera end of the focuser's drawtube when it's fully retracted, also called the reference point (Figure 1, top). For my FSQ-106EDX4 the focal plane is 178 mm beyond reference point. 

Figure 1. Definition of backfocus

I will call the distance between the reference point and the focal plane the True Backfocus. I call it "True" because it's commonly a number you get from the manufacturer, so you know it's realiable. The FSQ focuser travel is a very 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, where I've added all the hardware that sits between the end of the drawtube and the sensor. 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 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 check your scope's documentation! 

The green adapters are the items I add: In my case they're mainly working the M54 down to an M42 male thread that will  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 toward 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 because I've chosen the total backfocus "correctly" the sensor is at the focal plane. So what's the numerical value of 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 equals True Backfocus. minus half the drawtube travel 

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 and autofocusing software, the drawtube will move in and out during autofocus. 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 increase backfocus beyond Practical Backfocus by adding an extension tube. As a result the drawtube be less extended when focus is reached and possibly reducing dreaded "drawtube droop." This might be an adjustment that's worthwhile if your focuser has a great deal of range that you don't need.

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 can "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 total backfocus 175 mm, and the drawtube now needs only extend 3 mm for the sensor to be at focus. (3 mm + 175 mm = 178 mm, the True Backfocus. Effectively I've brought the sensor 12 mm closer to the OTA.)

I could also reduce the Practical Backfocus 12 mm giving me a total backfocus of 151mm. In this case the drawtube 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 may be different (Figure 3). The reference position is now a mark on the reducer or is perhaps 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. Most imagers try to get within 1 mm of the correct value. Given the software that now exists (I'm looking at you, BlurXTerminator) it's possible to correct star distortion caused by backfocus error if it's not too great.

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

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

If you image using any of the following you may need to match a specified backfocus:

• Camera Lenses
• Telescopes with built-in flatteners or reducers
• Coma Correctors
• RASA astrographs

Anytime you put something that bends light into the optical path, read its documentation to see if it requires a specific backfocus.  

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 its own True Backfocus, you will need to get the total backfocus as close as possible to that -- within practical limits. 

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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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A 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-M42 lens adapter, and the AliExpress Canon-M48 adapter was even less at $35. Am I justified to feel that the SBIG adapter may have been a bit overpriced? 

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.

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

Backfocus & Hocus Focus (a NINA plugin) and a New Image

Backfocus

When I first heard about the focusing plugin Hocus Focus (HF) I was unsure if it was something I wanted. NINA's built-in autofocus seemed to work perfectly. Then I watched one of the Patriot Astronomy videos that demonstrated what else HF can do and I realized I not only wanted it, but I needed it!

HF (created by George Hilios) features what he calls Aberration Inspector. Much of what it does is beyond my ability to intepret meaningfully, much less act on. But it has one key ability: to measure error in backfocus. Having proper backfocus is a problem when using focal reducers.

If the backfocus isn't right, you won't be in focus across the field; if you focus on stars at the field's center, stars in the corners will be out of focus. This is why an old fix for this problem is to focus on stars some distance out from the center. Like most compromises it's hardly a perfect solution: stars in the center and corners will be slightly out of focus.

The better remedy is to get the backfocus as close to correct as possible. Usually this means taking images, examining them closely, guessing the distance of the correction to make, making the correction, then shooting more images, etc. It's time consuming and inexact. Fortunately there's a better way--use Hocus Focus.

HF can quickly estimate the magnitude and direction of your backfocus error during a special autofocus session. Then you can make the correction if you have the proper spacers on hand. And you're done.

I have a Takahashi CR 0.73X reducer That I want to use with my FSQ-106. The CR wants a backfocus of 72.2mm. My camera and filter wheel add to 32.5mm, so I need my adapters to provide 39.7mm.

For my first run of the Aberration Inspector I had this:

• M56 to M48 adapter, 12.1mm
  
• M48 to M42 adapter, 16.5mm
  
• M42 spacer ring, 10mm

These add to 38.6mm. Close, but 1.1mm too small. The Inspector told me I needed to add 3 focuser steps to the backfocus, which is the right sense of change, but the magnitude seems off. 

One of my focuser's steps is about 0.004mm (30mm/8000steps), so 3 steps is a mere 0.012mm. My assumption is that somewhere in how I set up NINA or HF a factor of 100 error sneaked in. If that's correct, then the correction it's suggesting is to add 1.2mm.

So I added two thin spacers totaling 1.2mm and ran the inspector again. Here is the result 

As you can see (if you click the image to enlarge it), the Inspector now says the error is zero steps and the difference in star quality between center and corners is almost imperceptible. I think it's safe to say that I'm now within 0.1mm of having correct backfocus!

This means the next clear night I'm going for a larger target, maybe the Elephant's Trunk, M31, or the entire Veil!

Some incidentals for those of you who like miscellaneous information...

• All of the goodness of fit (R squared) values were 1.00
• This was performed without polar alignment or guiding, the exposure time was 2s through my luminance filter
  
• I had to increase the autofocus backlash from 450 to 600 steps. I should probably redo all my filter offsets, too, if only to see if they have changed. The autofocus step size was unchanged.
  

New Image

I've imaged the two nebulae (IC 59 and IC 63) near gamma Cas before, and it was time to revisit that to see how I have progressed. 

Here is my 2009 attempt

This poor image was tortured with wild stretching and clipping, then oversaturated to show some color. 

In 2022 things are looking better: better mount, better camera, better telescope, better processing. Here is the full frame

And here is the nebular part of the image at full scale

Acquisition details are at AstroBin. 

What really surprised me about this image was that it looks so good for having so little data. It's based on about 82 minutes total exposure spread across the LRGB channels. And short exposures, too: only 90s each! 

The optical performance of the FSQ is--at least to me--breathtaking.