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? 

My 135 mm Tamron Delusion Ends, It's Plan B now

I really thought I could get away with using my old Tamron lens for imaging Integrated Flux Nebula (IFN), but it's not to be. The number of minuses kept growing as I spent more nights practicing with the setup. I realized that my desire to use the Tamron and was blinding me to the issues that doomed it.

The greatest difficulty was being unable to rotate the field of view in a reproducible manner. This happened every night I used the setup with one night being 45 degrees out of kilter from the others. Keeping the camera orientation consistent between multiple imaging sessions is essential; without this, some of the field will need to be discarded during stacking. It became obvious that the amount of lost field would probably be substantial, resulting in a much retained field of view. The advantage of the wide 135 mm field of view would be lost. I think the cause was the ring clamp I was using to join the lens and camera to the dovetail. To permit manual rotation of the camera this ring had to be loosened and retightened many times, resulting in misalignment.

Image quality was also not what I wanted. There was no reasonable solution for the back focus error and I would be stuck with strong aberration needing to be corrected by BlurXTerminator. I knew some residual aberration remained, and I was concerned that this might show itself during the aggressive processing I would use to draw out the IFN. 

I came to realize that the stepdown ring I was using to produce spikeless stars was causing severe vignetting. While flat frames could somewhat compensate for this, too much signal was being lost -- again diminishing the effective size of the field of view. Using the lens's internal blades to stop it down was an alternative that created large, flaring spikes around the stars. I found this unacceptable.

Plainly, the lens was not up to the purpose and it was time to move to Plan B.

Plan B

The fallback is to use the FSQ-106 + focal reducer operating at a focal length of 387 mm and focal ratio of f/3.65. This California Nebula image used that configuration and gives you an idea of what it can do. The advantages are many over the Tamron: 

• 17% increase in speed (f/3.65 vs f/4)
• A very flat field with modest vignetting  
• An actual rotation ring
• Amenities like autofocus, autoguiding and dithering, easier creation of flat frames, and automatic meridian flipping

This is a much heavier scope to tote around, but the only real disadvantage it has is the smaller field of view. Here's a comparison:

135 mm Tamron field (outer box) vs 387 mm FSQ field (inner box)

The star cluster is no longer in the FOV, which is fine as the IFN is the real target. Does the smaller FOV (about 3.5 x 2.3 degrees) include enough IFN to be worth imaging? This image of Polaris IFN by another imager has essentially the same FOV as my setup will produce. I think there's enough IFN there to make it worthwhile particularly if Polaris can be reduced in size. I'll also compose the image to have Polaris much closer to the north edge of the frame, making more room for the IFN. 

Other aspects of the comparison image are worth looking at.

The scope used was an f/5.5 refractor with a flattener that didn't affect the focal ratio (so far as I can discover). The camera, an ASI 6200 color camera, has a quantum efficiency about the same as my ASI 2600 mono camera. The total integration time was 5 hours and 10 minutes. My thought is that if I want a reasonably deep image with low noise I should try to get at least twice the total time that went into the image. I'll probably use a plan that requires two nights of imaging: 5 hours of luminance one night, 3 hours of Chrominance another night. That's roughly the equivalent of 11.7 hours of one-shot color gathered at f/5.5. 

Yes, I know, that's a LRGB ratio of 5:1:1:1 and instead of the usual 3:1:1:1. I just like working with luminance; one night a few years ago something glitched and I ended up shooting only luminance. The final ratio was 6:1:1:1 and the result was quite nice (in my opinion, of course).

The comparison image was created using 5-minute light frames, which is probably why star colors are muted and Polaris is bloated. I've had much better luck with shorter exposures, and may simply go with 90 s lights. Both of my linked images used 90 s lights exclusively.

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Tariff watch: the Rokinon 135 mm lens (Plan C) is holding steady at $449 and in stock at B&H.

A Brief Pause

The seasons have changed again, at least according to the calendar if not the thermometer. Despite the pleasant autumn I managed to be lazy and not haul my imaging gear out. I think an early cold snap convinced me it was winter. The reality is it's almost the new year, so time for a look ahead to 2024.

C9.25 and TV-102 back in service

As a last gasp of 2023 activity I did do a little bit of scope maintenance. First it was the turn of my old Celestron C9.25 to get collimated; I had cleverly manage to mess up its alignment to the point it was useless. This time I was very careful. Only small adjustments were made until I had it close to correct. I'll still have to do a finer tuning in the spring/summer when I can get the scope out under some decent stars.

The other bit of work was for my TV-102. Some years ago (nine? Amazing how time flies!) when imaging  IC 348 in Perseus, I noticed an irregularity in the flare around 4th magnitude omicron Persei. You can see it as a notch at about 5 o'clock.  

Omicron Persei with a mysterious dark notch

Here's a link to the full image on AstroBin. For comparison, here is Gamma Cas with my FSQ106, showing what a flood of starlight should look like.

Gamma Cas (FSQ-106EDX4)

The dark notch isn't awful, but it is a serious distraction. The primary suspect was something in the optical path. It was a simple thing to point the scope at a light surface and look through the objective from the focuser end. Clearly apparent was a sort of bump protruding into the circle of the objective. But what was it? Had something fallen into the OTA?

In the dangerous tradition of "monkey see, monkey disassemble," I took off the objective cell and discovered the problem. A bit of the TeleVue flocking paper inside the telescope tube had blistered up and gotten into the way of light. The fix was obvious to this monkey so I carefully filed down the petrified bump, put the objective cell back in place, and called it a done deal. 

Then something amazing happened: I had a thought. I wondered if I had gotten the cell back in place correctly, and was the scope still optically aligned? Dashing to the Internet I started reading posts about the travails people had collimating TV-102s. Also how they often had to fabricate clever jigs to do the alignment. Said jigs were far beyond my technical skill to create, so I was depressed. Not wanting to admit to myself that I might have made more trouble than I had remedied I set the scope aside and moved on to  imaging with my C9.25 (not yet misaligned), AT65EDQ, and eventually FSQ-106. I'd revisit the potential damage later.

Fast forward nine years to the other night when I put the TV-102 on the mount and actually looked through it at a star. It wasn't a complete star test, but at 90X Polaris A and B looked like pinpoints. At  450X racking in and out a touch gave decently symmetric airy disks. So maybe I got really lucky and it's fine. The old scope can rejoin the fleet now. I would like it functional for that eventual time when I return to visual observing. It works wonderfully with my 31mm Nagler Type 5 hand grenade, and to me it looks like the classic telescope. A picture of it from 2008 still graces the WhiteRock Conservancy (home of the Iowa Star party) web site.

AT65EDQ

After adding the FSQ-106 for imaging my poor AT65 has languished. It's an imaging scope that's corrected to provide astrograph flatness, meaning it's not a so great for visual use. Its FL of 420mm (f/6.5) is close to that of the FSQ with the 0.73X reducer (f/3.65 and 387mm) so it's somewhat redundant. But it is small and light and could easily sit on something less substantial than my G11. 

All it really needs to be a little sibling to the FSQ is electronic focusing, so that's what I'll be adding to it. And a nice Seahorse hard case for travel. Then I only need to rig a way to mount the Pegasus powerbox to it. More about that next time.

Veil Mosaic

I have all the data, I've been delaying processing until snowy winter hibernation mode starts. In the Twin Cities in a given year a "white Christmas" with 1" of snow or more on the ground has a chance of about 71%. But that won't happen this year!  The ground is bare and the forecast is for a record-warm 53 on Christmas Eve with a possible thunderstorm. The non-winter of 2023-24 is on track to continue into January.

When the snow finally arrives I'll start with a mosaic of the luminance data first to see what I've got, then repeat for each of the color channels.

 AstroBin Voting

I like to post my better pictures on AstroBin. It's mostly a vanity exercise but I think that posting technical details about images can be helpful to others. I know I've learned some things by seeing how other people acquire their data. 

Because AstroBin is a sort of social medium members are allowed to vote for images and there are best image designations. Some people complain that it's highly subjective, motivated by people trying to build their posse of followers, get votes in return, etc. All of that is possible but I don't care. It's fun, and I like it when I get up-votes. Although I admit I don't often understand why some of my images get more votes than others I've posted.

I've noticed something odd in the last year or so, though. Some of my dusty old posts that have been sitting quietly will suddenly get a flurry of votes from unfamiliar people. An example is this image of Jones 1 from 2015. Earlier this month it got four new votes within a few minutes of each other. Other people have imaged this object and created far better images than mine, so I have to ask why I got those votes.

Possibly it's people fishing for votes, but why would they reach back to an image from eight years ago? Why not take the time-honored approach of following someone so that they'll reciprocate and build your posse?  The near-simultaneity of the votes makes me think it might be something more devious.

Oh well, maybe the applicable adage is "don't look a gift horse in the mouth." So I say: Thank you  for your votes, mysterious time-lagged strangers! Maybe next time don't wait 8 years!

Bumps on the imaging highway; more (unfinished) images

This is astrophotography, so things are never as easy as one hopes. 

At the beginning of September I had an all-night imaging session at the club's Eagle Lake Observatory site.  I was trying to do a few targets: M31, The California Nebula, and because a little time was left over as the dawn was approaching M42. Let's start with the latter because it illustrates the main issue.

I had only about a half an hour before the morning sky started to really brighten, so I set up NINA to do five 90-second frames for each LRGB channel. Here is the initial stretched result:

 

 

I did say LRGB, but this is mono! It turned out I took 20 L frames; the mystery is how that happened. I wish I could tell you, but I don't know the actual cause. My best guess is that NINA lost contact with the filter wheel and couldn't execute the filter changes. So this is not quite the image I wanted, but eventually I'll be able to shoot the chroma and make a nice image.

 

The same thing happened with my California Nebula frames and I've got 96 L frames which should make for a really nice eventual image. Here's a quick processing of the luminance frames:

 

This seems to have started as soon as I began imaging M31, as those are all (so far as I can tell) luminance frames. All 125 of them! I can say this from the early look at the M31 stack that if you want a very smooth background, shoot 125 frames! This is a first look at the luminance image, where I've somewhat de-emphasized field stars using PixInsight's Morphological Transformation:

 

 

Incidentally, these are all imaged using the Takahashi CR 0.73X focal reducer on my FSQ-106, so I was operating at  f/3.65 and an effective focal length of about 387mm.

 

Another mystery was easier to solve. I noticed that autofocusing was having a very difficult time. This was traced to a loosening of the connection between the motor and focuser shaft. I've tightened that up now, and just in case that was happening because of overly tight parking of the focuser I'm going to change the focuser's home position.

 

This weekend I hope to actually collect the M31 chrominance, and maybe deeper into September get the California Nebula color data.

 

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I'm returning to something I started back in 2020: spectrographic imaging. More about that next time (I hope!)

 

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.

 

FSQ106 First Light Image; Astronomical League Open Cluster Observing Program: A To-Do list for Astro Photography Tools (APT)

At last! My FSQ system was complete and I could take it out for a first light image. Nothing fancy, just a lot of stars. I also made it a point to process it only with PixInsight, replacing ImagesPlus for calibration and reducing my dependence on Photoshop. The image is here at AstroBin. Below is a preview, along with a 1:1 scale of M13 itself. Click each for larger views.

 

 

The image is based on a bit under three hours of LRGB. There's a lot of room for improvement in this image, I know, but it's wildly better than my previous efforts.

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I think one of my first long-term projects for the FSQ is going to be the AL Open Cluster Program list. The AL provides a PDF document listing the 125 objects in the program along with their coordinates which is helpful because quite a few of the clusters are from obscure catalogs like Berkeley, Trumpler, and Dolidze Dzimselejsvili.

I'm using Stellarium and APT to control the G-11 mount. Wouldn't it be nice if there was an observing list of the AL open clusters for one of those so that I could do simple a simple go-to for each object? Unfortunately Stellarium doesn't seem to support user-defined object lists but APT does. 

Some poking around failed to turn up a list. In APT it's fairly easy to create lists by loading objects into the ToDo list. Start by opening the Object Browser. If a cluster is in the Deep Sky list, find and select it, then click the "Add in ToDo" button. If an object isn't in the list, use the ToDo list's Add New button. When you're done creating a list, just export it. The data is put into an easy-to-read XML file.

This is just what I did! If you use APT and want a copy of the list, click this link:

https://app.box.com/s/l3ji97b1ysm3xxvpve2mgud34nv4x34c

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And now a footnote to my last post in which I was saying goodbye to the Nebraska Star Party for a while. I should mention the same applies to the Iowa Star Party. The last two times I've attended that the heat and humidity were incredible. Those were on the Labor Day weekends, and this year it's a week earlier--meaning the probability of excessive heat is even greater. Much as I hate to give up on ISP, I probably won't go back until they move it into late September or October. 

Yes, I know, I won't be missed by either event, and yes, I'm a weather wimp 😄

FSQ-106 arrives; a new hobby for winter

As the pandemic slogs on with a second flavor of Omicron and war rages (ha ha, I meant of course a "special military operation" per war criminal Putin) there's a little bit of good news.

The FSQ arrived in early January. I'll spare you the unboxing video I didn't make, but I have to say the way Takahashi packages their scopes is nothing short of amazing. Three boxes in one, fitted together with almost surgical precision. Because it's been winter outside I've yet to take the scope out for any sort of star testing. What I am doing is getting it all accessorized. (Pictures to follow eventually.)

First, a Pegasus FocusCube. This went on easily and works just fine. Next, a Pegasus Powerbox Advance riding a dovetail bar fit into a new finder shoe. Finally, the guide scope on a bar that slips into the handlebar spanning the Primalucelab tube rings. 

The next step was putting it on a Losmandy D plate, adding the ASI 2600 + filter wheel combo and getting it to balance on the G-11. This took a little finagling (largely the creation of 1/8th inch risers to provide a touch more clearance between the scope and dovetail).

Once it was balanced it was cable time, so I connected everything through the PowerBox and did a little testing. ASI imaging camera + filter wheel, QHY guide camera, PoleMaster, FocusCube, G-11, PHD2, APT: Everything worked in harmony. The only thing I didn't test were the dew straps, but I'll do another dry run in March and verify their operation.

Lastly, I did some box-crazy myself. I got a Seagull hard shipping cases for the FSQ and the ASI. The old AT65 went into an old tool box, and with the SBIG ST-8300 sold I used its case as home for my modded Canon DSLR.

So basically, everything is boxed and ready for the highway. All I need is warmer weather.

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One problem with winter is that I tend to shut down my astronomy hobby in the cold. When I was younger and doing strictly visual astronomy I didn't mind going out when temperatures were in the teens or single digits F. 

When I started imaging that changed. Cables get very stiff in the cold, and the required fiddling with fingers leaves them stiff, too. I had thought that I might do mirror making during winter, but my work area (at a perpetual 65F during winter) produces hard pitch and figuring is difficult. So this year I decided to make a bold leap and find a non-astronomy winter hobby!

When my mother died in February of 2021, I inherited a couple of family heirlooms. The one I value most is a pocket watch owned by my great great grandfather. Amazingly it still runs, but I knew that it had probably sat in safe deposit boxes for over 50 years and was badly in need of service. I found a watch repair shop ( a real one, not just a place that changes batteries in quartz watches) and set them to work in April 2021. Ten months later the watch has been cleaned and oiled, but the watchmaker has been working on repairing a broken piece that is used for setting the time. Because this watch is very old and of unknown manufacture replacement pieces are impossible to find; instead, pieces must be fabricated.

During the long wait I wondered what was involved in servicing mechanical watches and found some videos on YouTube showing the process. They were fascinating. Many videos later I decided I'd like to try this and started gathering the needed baseline set of tools. I've already dismantled several old pocket watches (none of which were running) for practice, and actually coaxed two of them back to almost running. I know, "almost running" is the same as "not running," but it was fun and I learned some things. Next, I'm going to work on disassembling/reassembling running inexpensive Chinese and Indian movements.

You may think amateur watch servicing is nothing like astrophotography, but you would be wrong. Both:

• Require expensive, specialized, and breakable hardware -- you would not believe how expensive watchmaking tools are
  
• Considerable experience
• Are largely fading hobbies
• Are mainly practiced by older men

There are a lot of differences, of course,  but fun is fun. And so is staying inside during winter.

ASI 2600MM; FSQ-106EDX4 imaging trains

ASI 2600 Glitch:

First, bad news then the good: My new ASI2600MM Pro was delivered last month. As it sometimes happens with new things, not all is well. The camera's temperature sensor was not reporting. From reading forum posts, it appears that ZWO, like every other maker of advanced astronomy gear, has occasional blips in their quality control. A non-functional temperature sensor is not common, but it does happen.

I got in touch with OPT (the vendor) and they had ZWO contact me with the fix. The fix is so simple I wondered if it could possibly work: Shoot some compressed air around the sensor's flat cable, pop open the little clip holding it in place, shoot some more air in, jostle but do not disconnect the cable (that part was left very vague in the instructions), close the clip, and button the camera back up.

It worked!

FSQ-106EDX4 Imaging Optical Trains

CAVEAT: I have not taken delivery of the FSQ so none of the following has been verified in the field. A vendor post on CloudyNights suggests delivery may have to wait until February!

Now on to the main topic: how to get ready for imaging with the FSQ-106EDX4.

The imaging cameras will be an ASI2600MM Pro and a modded Canon T2i. I'll be using the FSQ in two modes: native f/5 and f/3.65 using the CR 0.73X focal reducer, and I would like to use the ASI with my existing EOS lenses. (I resisted the temptation to buy the 0.6X reducer, opting instead to save a thousand dollars and make the optical train a little more user friendly.)

I'll be imagining with two devices: a Ha-modded OSC Canon T2i and the ZWO 7x36mm EFW (M42 connections, optical thickness 20mm) with ASI2600MM (backfocus of 12.5mm after removing the tilt plate).  The following configurations are based on what Takahashi has published about their adapters (see link below) and forum comments on CloudyNights.com.

FSQ106 Native Mode + ASI2600:

The FSQ focuser has a travel range of only about 30mm, so back focus should put the sensor near the center of this if possible. Sources suggest this is at a distance of 163mm (178mm minus half the travel range) beyond the end of the focuser. Already in the optical train are the 645 RD adapter (TKA36581, 10mm backfocus), CAA-250 (TKP86200, 38.5mm), Aux Ring S (TKA38205, 27.5mm), and Coupling TW (TKP36003, 34.2mm); their total is 110.2mm. This has stepped us down from M98 to M54 thread and we still need about 53mm. To get us down to M42, we use Blue Fireball's adapter with thickness 6.9mm. Now we add the EFW (20mm) and 2600's backfocus (12.5mm without the tilt plate) and we're at 110.2 + 6.9 + 20 + 12.5 = 149.6. We add a simple 12mm M42 spacer and get 161.2. You could use a 5, 10, or 15mm spacer, or even a thicker M54/M42 adapter. "Close" is good enough.

REVISED 8/11/2022 The diagram above is not longer correct. FSQ106 + CR 0.73X Focal Reducer + ASI 2600

Takahashi says the optimal distance from FR back to sensor is 72.2mm. The camera backfocus and filter add to 32.5mm, leaving 39.7mm for adapters that will take us down from M56 to M42. The M56 adapter in the diagram proved to be problematic so instead I'm using these adapters: Blue Fireball M56(f) -M48(m), M48(f)-M42(m), and M42(f)-M42(m), with respective lengths of 12.1, 16.5, and 10mm. This gives a total of 38.5mm, about 1.2mm short. I used the Hocus Focus plugin for NINA to see if this was close enough. The plugin suggested I need to add 3 focuser steps, which translates to a mere 0.011mm. In other words, the plugin agrees with the new setup. I will add another 1.2mm to the adapters and run the plugin again to see what it says.

It's not clear to what extent vignetting will be an issue, but my hope is that it will be largely correctable using flats.

Canon EOS lens + ASI2600

(Note: This has been tested and shown to work. See later post.) The backfocus target here (per the ZWO documentation) is to put 43.8mm between the lens and sensor (26.3 for their adapter and 17.5 for the camera with tilt plate attached). I want the EFW in this train, though, so I remove the tilt plate and use a thin Canon lens to M42 adapter (10mm). This puts it at 10 + 20 + 12.5 = 42.5. Thin spacer rings are used to bring this up to the required value.

Incidentally, this configuration is the reason the tilt plate is missing in all the other configurations. 

Correction: Cam sides of scope and FR are M56 female

 

FSQ-106 Native mode + Canon T2i

The FSQ-106 System Chart tells us we need only spacers provided with the telescope and the Wide T-mount for EOS.

FSQ-106 + CR 0.73X Focal Reducer + Canon T2i

The FSQ-106 System Chart tells us we need only spacers provided with the telescope, TKA35201 and the Wide T-mount for EOS.

LIST of PARTS NOT INCLUDED with FSQ-106EDX4

This lists the adapters and spacers needed to supplement the parts provided with the FSQ-106. They are listed along with their: backfocus, manufacturer, vendor/part #, and price. Very possibly you can find alternative products and vendors, but I'm very happy with Agena and OPT.

    M54 (f) to M42 (m), 6.9mm, Blue Fireball, AgenaAstro/PAAR-BF-T-16, $25     

    M56(f) to M48(m), 12.1mm, Blue Fireball, AgenaAstro/ PAAR-BF-M-24, $43    

    12mm M42 spacer, 12mm, Celestron, OPT/CE-93618, $40 

 

    10mm M42 spacer, 10mm, source unknown        

    EOS to M42 (m) adapter, 10mm, QHY, AgenaAstro/PAAR-QH-20072, $35

    TKA35201, NA, Takahashi, OPT/TK-TKA35201, $82

    Wide T-mount for EOS, NA, Takahashi, OPT/TK-TMW0004, $167

    Thin M42 spacer rings, 0.1 to 1.0mm, Blue Fireball, AgenaAstro/PAAR-BF-S-SET7, $40

HELPFUL LINKS

https://agenaastro.com/

https://optcorp.com/ 

https://www.takahashiamerica.com/

https://astronomyplus.com/wp-content/uploads/2019/08/Takahashi-Adapter-Specifications.pdf