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FS-60CB backfocus woes

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#1 raadoo

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Posted 19 March 2023 - 07:12 AM

I believe I've landed in stump-land with a new imaging setup I'm putting together.

 

In a nutshell: dialling in proper backfocus for my reducer causes the focuser to lose inward travel.

In depth: I used the classic "look at the stars in the corners and add shims until they no longer radiate" and I'm very close to getting it perfect (<1mm). Trouble is, as I've increased the distance between sensor and reducer, the focuser (situated between scope and reducer) has had to move ever more inward to achieve focus. This to the point that I now have no more inward travel available and cannot achieve proper focus.

 

I'd massively appreciate any ideas, thoughts, opinions or guidance.

_

Image train is:

FS-60CB ⇢ CAA (TKA21200) ⇢ Reducer (TKA20580B) ⇢ Adapter to M48 ⇢ OAG ⇢ Filter Slider ⇢ Shims ⇢ Camera



#2 James Peirce

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Posted 19 March 2023 - 07:31 AM

What parts, specifically, are you using?

#3 raadoo

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Posted 19 March 2023 - 03:13 PM

Image train is:

FS-60CB ⇢ CAA (TKA21200) ⇢ Reducer (TKA20580B) ⇢ Adapter to M48 ⇢ OAG ⇢ Filter Slider ⇢ Shims ⇢ Camera

 

What parts, specifically, are you using?

Guessing you mean the parts that make up the image train, above?



#4 James Peirce

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Posted 19 March 2023 - 03:14 PM

Guessing you mean the parts that make up the image train, above?

Yep.

#5 raadoo

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Posted 19 March 2023 - 04:06 PM

Yep.

Takahashi FS-60CB

Takahashi CAA

Takahashi Reducer (metal back distance of 56.2mm)

Pierro Astro M52-to-M48 adapter (2mm)

ZWO OAG (16.5mm)

M48 Shims (1.9mm)

Altair Astro M48 Filter Holder (17mm)

M48 Extension (12mm)

ASI183MC Pro (6.5mm)

 

The math on the reducer checks out in practice as tests on star fields show only very slightly radiating stars in the corners, uniformly.

But as I'm now left with no inward focuser travel, my EAF is unable to run through its routine (it needs to be able to travel both in and out so as to plot a V curve).



#6 raadoo

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Posted 19 March 2023 - 04:13 PM

Perhaps some of the forum members who've likely imaged with the FS-60CB can chime in? And hopefully not be too upset about my mentioning them smile.gif  base16 Shiratar SilverLitz Rasfahan or gatsbyiv spring to mind.



#7 James Peirce

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Posted 19 March 2023 - 04:58 PM

Perhaps some of the forum members who've likely imaged with the FS-60CB can chime in? And hopefully not be too upset about my mentioning them smile.gif base16 Shiratar SilverLitz Rasfahan or gatsbyiv spring to mind.

I image with one, by the way. I just wanted to make sure the backspacing seemed to be in order as a setup like that with ZWO parts would have ended up out of range.

In my case, and generally with Takahashi telescopes and parts, I’ve found the actual mechanical backspacing they document tends to be strikingly close to what is actually needed in practice. This was true of my FS-60CB which I use with the 0.72x reducer and the ASI2600.

The numbers for your parts land you at 0.3mm inside of backfocus; and a filter, alongside what will probably be some extra mechanical depth from numerous coupled parts, probably pushes things in the other direction. So it sounds like you’re in the ballpark. A lot of precision is needed for this setup with larger sensors, but I’d have expected the 183 to be much more forgiving, other than the ~2.4μm pixels.

Which leaves me a little bit confused, because I don’t remember being so tight in focus travel that I was worried about travel room for the ZWO EAF when I had a similar setup with the stock focuser. And now I’m using a Feathertouch, which gives me plenty of travel room (albeit not with the opportunity to include the CAA), so I don’t have a point of reference fresh in memory.

But it probably shouldn’t matter. It sounds like the right connections on the focuser (focuser > CAA > reducer) so the needed adjustment probably needs to be made through changes to the imaging train itself, or the aberrations aren’t strictly a product of backfocus (though your initial explanation makes it sound that way).

How are you evaluating this? Broadband images? A shot from aberration inspector (or whatever) might be useful if someone doesn’t come along with more helpful advice.

Edit: Super basic question, but probably should be asked as I don’t know what you’ve worked with. You’ve got sufficient focus travel programmed on the EAF, right? It’s not failing because it is reaching a programmed limit as opposed to a physical limit?

Edited by James Peirce, 19 March 2023 - 05:00 PM.


#8 Vertigo

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Posted 19 March 2023 - 05:13 PM

Takahashi FS-60CB

Hello,

 

if I understood the problem correctly?! then the CSV tube could be a solution.

 

The tube from CB is 127mm. long, the tube from CSV 105mm, so you would be a good 2 cm. closer to the lens.

Ultimately, I'm only visual and don't know if it works.

 

Andreas



#9 SilverLitz

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Posted 19 March 2023 - 05:41 PM

Perhaps some of the forum members who've likely imaged with the FS-60CB can chime in? And hopefully not be too upset about my mentioning them smile.gif  base16 Shiratar SilverLitz Rasfahan or gatsbyiv spring to mind.

I have never used a FS-60B.  I have an FC-76DCU and 1.7x Extender Q for the equivalent of a FC-76Q.  I have only had this for a few months, and I have not tried imaging with it.  I really like the idea of FC-76DCU as it seems to have a lot of flexibility and supposedly in the "Q" has TOA like color correction.  It is extremely light weight and so easy to handle versus my other OTAs.  I got it mainly for a light, grab-go, visual scope, though I am almost totally AP.  I expect to use it for AP in next year's solar eclipse, as I plan to have several scope/lens/camera combos going; just need good weather in the "iffy" April.



#10 raadoo

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Posted 20 March 2023 - 02:18 AM

I image with one, by the way. I just wanted to make sure the backspacing seemed to be in order as a setup like that with ZWO parts would have ended up out of range.

In my case, and generally with Takahashi telescopes and parts, I’ve found the actual mechanical backspacing they document tends to be strikingly close to what is actually needed in practice. This was true of my FS-60CB which I use with the 0.72x reducer and the ASI2600.

The numbers for your parts land you at 0.3mm inside of backfocus; and a filter, alongside what will probably be some extra mechanical depth from numerous coupled parts, probably pushes things in the other direction. So it sounds like you’re in the ballpark. A lot of precision is needed for this setup with larger sensors, but I’d have expected the 183 to be much more forgiving, other than the ~2.4μm pixels.

Which leaves me a little bit confused, because I don’t remember being so tight in focus travel that I was worried about travel room for the ZWO EAF when I had a similar setup with the stock focuser. And now I’m using a Feathertouch, which gives me plenty of travel room (albeit not with the opportunity to include the CAA), so I don’t have a point of reference fresh in memory.

But it probably shouldn’t matter. It sounds like the right connections on the focuser (focuser > CAA > reducer) so the needed adjustment probably needs to be made through changes to the imaging train itself, or the aberrations aren’t strictly a product of backfocus (though your initial explanation makes it sound that way).

How are you evaluating this? Broadband images? A shot from aberration inspector (or whatever) might be useful if someone doesn’t come along with more helpful advice.

Edit: Super basic question, but probably should be asked as I don’t know what you’ve worked with. You’ve got sufficient focus travel programmed on the EAF, right? It’s not failing because it is reaching a programmed limit as opposed to a physical limit?

Thanks so much for taking the time to understand my setup and help with a solution!

The EAF focus is very much a physical limit as I've focused manually before attempting an AF routine and can visually see the focuser being right up to the tube. Images were evaluated in broadband with a simple UV/IR filter in place.

Comparing my own setup with yours, it looks like the one thing that's different is the focuser and the CAA. My hunch was, and someone on SGL reported the same, that it's the CAA that's actually the culprit. The CAA is introducing a fair bit of spacing which is eating some of that focuser travel.

 

Thus, it seems like possible avenues of resolving the issue are:

  1. Move the CAA after the reducer and find some way to still reach the same backfocus (narrower OAG? ditch the OAG altogether?)
  2. Ditch the original focuser and CAA and move to a FTF2025BCR (Starlight say it's Rotatable) though a bit on the expensive side
  3. Replace the CAA with a narrower rotator (my CAA is 20mm thick so doubtful I could even find one that's half that plus adapters)

I'm very open to any other ideas!

 

I have never used a FS-60B.  I have an FC-76DCU and 1.7x Extender Q for the equivalent of a FC-76Q.  I have only had this for a few months, and I have not tried imaging with it.  I really like the idea of FC-76DCU as it seems to have a lot of flexibility and supposedly in the "Q" has TOA like color correction.  It is extremely light weight and so easy to handle versus my other OTAs.  I got it mainly for a light, grab-go, visual scope, though I am almost totally AP.  I expect to use it for AP in next year's solar eclipse, as I plan to have several scope/lens/camera combos going; just need good weather in the "iffy" April.

So sorry about that, my wires got crossed between your FL55SS and the FS-60CB; I'll be forgiven as they're both Japan made flourite doublets smile.gif



#11 James Peirce

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Posted 20 March 2023 - 09:40 AM

Thanks so much for taking the time to understand my setup and help with a solution!

The EAF focus is very much a physical limit as I've focused manually before attempting an AF routine and can visually see the focuser being right up to the tube. Images were evaluated in broadband with a simple UV/IR filter in place.

Comparing my own setup with yours, it looks like the one thing that's different is the focuser and the CAA. My hunch was, and someone on SGL reported the same, that it's the CAA that's actually the culprit. The CAA is introducing a fair bit of spacing which is eating some of that focuser travel.

 

Thus, it seems like possible avenues of resolving the issue are:

  1. Move the CAA after the reducer and find some way to still reach the same backfocus (narrower OAG? ditch the OAG altogether?)
  2. Ditch the original focuser and CAA and move to a FTF2025BCR (Starlight say it's Rotatable) though a bit on the expensive side
  3. Replace the CAA with a narrower rotator (my CAA is 20mm thick so doubtful I could even find one that's half that plus adapters)

I'm very open to any other ideas!

I can’t shake that your configuration should work. I had it working myself with the ZWO EAF, and was able to run its focus routine without running out of room for travel. But I am not coming up with a reason why you don’t have enough travel. And you’re not doing anything strange, here. You’re literally using the telescope and the optical train as it was designed when you use the CAA.

 

I guess another basic question. When you started searching for focus with the EAF, did you start with close focus? It can go into a wider focus-seeking routine if it starts on the wrong side of focus, and it can seek even wider if it doesn’t start the routine in the right place. I’d double-check this if it wasn’t so much trouble to change things over from my current setup.

 

On the problem-solving side of things, you could drop the CAA and replace it with part number 81 in the system chart (TKP20006B) which, I believe, comes with the telescope. That gives you the F56>M56 connection afforded by the CAA and consumes less mechanical depth, giving you more room to focus, but you lose the CAA in favor of a basic (but well-machined) mechanical connection. That means you have your M52 connection and 56.2mm of backfocus to sort out 1) a rotator, 2) an OAG, if you want one, and 3) anything else you want. Seems like a recipe for headaches if you have any priorities in 3, which you do with a filter drawer. So that solution probably means dropping the OAG in favor of a guide scope. You do get a sort of half-arsed rotator with the ZWO OAG, I suppose. It can re-slot in some limited range of rotational positions. But that’s also not a great solution to introduce a rotator.

 

In my case I use a Baader M48 quick changer to couple my optical trains with my telescopes, and wanted to adapt that solution to my FS-60CB. It gives me what is, effectively, a rotator, and in combination with a filter wheel left me with a very fussy optical train that was hard to get dialed in exactly.

 

You are correct that the 2025BCR affords a "rotator." This, in the sense that you buy an adapter which can thread into the female connection of the Takahashi flattener or reducer, which affords a 2-inch fitting on the other side that seats in the 2-inch compression fitting on the 2025BCR. You can loosen the compression fitting and rotate the optical train that way. Which, to be fair, actually works well, as the telescope-side face of the flattener or reducer can also serve as a leveling surface to eliminate the possibly of tilt from the compression fitting and make the operation repeatable. So at least it gives you an option to have the M52 male connection with 56.2mm of backfocus with some sort of reasonable rotation solution accounted for. And yep—definitely on the expensive side, adding expense to an expensive telescope.

 

I was curious about how you were evaluating your backfocus, as when the telescope is pushed beyond its optical limits you may end up with what looks to be out-of-backfocus if, say, evaluating Hα with pixels that are so small. But it was a bit of stretch given how small that sensor is. It may well be able to handle even something that demanding since you’re so far inside the corrected image circle. But it sounds like you’re doing all of that in a sensible way.

 

This is pretty mysterious to me, on the note of that first paragraph.


Edited by James Peirce, 20 March 2023 - 09:44 AM.


#12 raadoo

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Posted 20 March 2023 - 12:00 PM

I can’t shake that your configuration should work. I had it working myself with the ZWO EAF, and was able to run its focus routine without running out of room for travel. But I am not coming up with a reason why you don’t have enough travel. And you’re not doing anything strange, here. You’re literally using the telescope and the optical train as it was designed when you use the CAA.

 

I guess another basic question. When you started searching for focus with the EAF, did you start with close focus? It can go into a wider focus-seeking routine if it starts on the wrong side of focus, and it can seek even wider if it doesn’t start the routine in the right place. I’d double-check this if it wasn’t so much trouble to change things over from my current setup.

 

On the problem-solving side of things, you could drop the CAA and replace it with part number 81 in the system chart (TKP20006B) which, I believe, comes with the telescope. That gives you the F56>M56 connection afforded by the CAA and consumes less mechanical depth, giving you more room to focus, but you lose the CAA in favor of a basic (but well-machined) mechanical connection. That means you have your M52 connection and 56.2mm of backfocus to sort out 1) a rotator, 2) an OAG, if you want one, and 3) anything else you want. Seems like a recipe for headaches if you have any priorities in 3, which you do with a filter drawer. So that solution probably means dropping the OAG in favor of a guide scope. You do get a sort of half-arsed rotator with the ZWO OAG, I suppose. It can re-slot in some limited range of rotational positions. But that’s also not a great solution to introduce a rotator.

 

In my case I use a Baader M48 quick changer to couple my optical trains with my telescopes, and wanted to adapt that solution to my FS-60CB. It gives me what is, effectively, a rotator, and in combination with a filter wheel left me with a very fussy optical train that was hard to get dialed in exactly.

 

You are correct that the 2025BCR affords a "rotator." This, in the sense that you buy an adapter which can thread into the female connection of the Takahashi flattener or reducer, which affords a 2-inch fitting on the other side that seats in the 2-inch compression fitting on the 2025BCR. You can loosen the compression fitting and rotate the optical train that way. Which, to be fair, actually works well, as the telescope-side face of the flattener or reducer can also serve as a leveling surface to eliminate the possibly of tilt from the compression fitting and make the operation repeatable. So at least it gives you an option to have the M52 male connection with 56.2mm of backfocus with some sort of reasonable rotation solution accounted for. And yep—definitely on the expensive side, adding expense to an expensive telescope.

 

I was curious about how you were evaluating your backfocus, as when the telescope is pushed beyond its optical limits you may end up with what looks to be out-of-backfocus if, say, evaluating Hα with pixels that are so small. But it was a bit of stretch given how small that sensor is. It may well be able to handle even something that demanding since you’re so far inside the corrected image circle. But it sounds like you’re doing all of that in a sensible way.

 

This is pretty mysterious to me, on the note of that first paragraph.

Your reports of a similar setup without these issues were invaluable as they forced me to take everything apart and really look at what’s what.

 

Turns out that, unbeknownst to me, some black nondescript (looks like Tak) spacer was sandwiched between the silver tube of the focuser and the CAA. Its 30mm of thickness were pushing the whole image train back leaving me no space to focus.

 

My heart and my wallet are forever thankful that you took the time and effort to help me get to the bottom of this conundrum.

 

Now to resume my fine-tuning of the reducer’s backfocus.

And we call this fun?! lol.gif




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