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RedCat51 back-focus issue with EAF/Black-CAT

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#1 glancey

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Posted 23 September 2022 - 05:15 PM

I took my new RedCat51 out for a test run last night and found that using my EAF attached with the Black-CAT accessory, https://agenaastro.com/pro-astrogear-black-cat-mount-attach-zwo-eaf-asiair-guidescope-on-william-optics-redcat-51-telescope.html, created a back-focus problem. It turns out the gearing ratio on the Black-CAT allows for approximately 13,200 steps of the EAF, not the full 65,000, and actually stops short, at even the max 13,200 clicks, of the full focuser travel on the RedCat.

 

As a result, I needed to add a 22 mm spacer to the 11 mm and 16.5 mm spacers I was already using to get my ASI294 MC Pro focused. Just something to keep in mind.

 

Glenn



#2 James Peirce

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Posted 23 September 2022 - 05:25 PM

I don't own that particular setup, but wouldn't it be feasible to get the RedCat in focus, and then install the rig in that position? That way you should have enough travel to accommodate filters, temperature shifts, etcetera?


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#3 glancey

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Posted 24 September 2022 - 02:43 PM

I don't own that particular setup, but wouldn't it be feasible to get the RedCat in focus, and then install the rig in that position? That way you should have enough travel to accommodate filters, temperature shifts, etcetera?

That's what you need to do, for sure, but you'll need 50 mm minimum for back-focus. anything less and you're just wasting time.

 

Glenn


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#4 James Peirce

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Posted 24 September 2022 - 03:24 PM

That's what you need to do, for sure, but you'll need 50 mm minimum for back-focus. anything less and you're just wasting time.

 

Glenn

Am I misunderstanding? Out of the box, with included accessories attached (e.g. rotator), the RedCat 51 is designed to accommodate ~55mm of backfocus, with some give and take of the Petzval design. Are you referring to something else?


Edited by James Peirce, 24 September 2022 - 03:24 PM.

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#5 empiredown

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Posted 24 September 2022 - 08:11 PM

If you get the "focus" just beyond Infinity before you lug everything down, you'll be just fine. 55 mm of BF, this is the way...

 

PXL 20210826 223122699
 
PXL 20210826 223134657

 

PXL 20210826 223142725
 

SS


Edited by empiredown, 25 September 2022 - 09:47 AM.


#6 KlausKlaus

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Posted 25 September 2022 - 07:07 AM

Works like a charm:

 

0199B90E-D767-49EF-8A34-9812880705E7.jpeg

 

 



#7 glancey

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Posted 25 September 2022 - 03:22 PM

Am I misunderstanding? Out of the box, with included accessories attached (e.g. rotator), the RedCat 51 is designed to accommodate ~55mm of backfocus, with some give and take of the Petzval design. Are you referring to something else?

I was under the impression that with the petzval design, there was no specific back focus requirement, not even the standard 55mm.



#8 glancey

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Posted 25 September 2022 - 03:34 PM

My concern with the Black-Cat design and it's fixed gearing, as opposed to those using a flexible pulley system, is that the Black-Cat can only turn the helical focuser 180-degrees, not a full 360. So you either have to give up some focus at the back end, or at the far end, depending on where you set the focuser when you position the gears.

 

It's just a flawed design since I imagine the tolerances would have to be too exacting to etch teeth on both halves of the ring and create a smooth movement where the two halves are joined.

 

Glenn



#9 James Peirce

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Posted 25 September 2022 - 04:47 PM

I was under the impression that with the petzval design, there was no specific back focus requirement, not even the standard 55mm.

It will find focus within a specific range, but not any range. You’ll need some amount of backfocus. I forget the exact numbers. Is there a reason why you are avoiding the standard imaging train of some 55mm?

#10 glancey

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Posted 25 September 2022 - 07:06 PM

It will find focus within a specific range, but not any range. You’ll need some amount of backfocus. I forget the exact numbers. Is there a reason why you are avoiding the standard imaging train of some 55mm?

I'm not avoiding that at all.  I was just noting an issue with the Black-Cat device, which requires a more precise adjustment than you would otherwise need due to its travel restrictions.



#11 James Peirce

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Posted 26 September 2022 - 12:19 PM

I'm not avoiding that at all. I was just noting an issue with the Black-Cat device, which requires a more precise adjustment than you would otherwise need due to its travel restrictions.


All Petzval telescopes have a limited amount of travel for proper focus. One reason why the RedCat may be a touch more limited in functional travel is because part of that travel has been afforded to the inclusion of the attached tilt plate and rotator. But it does give leeway above and below the standard 55mm of backfocus (enough to alleviate many headaches). I questioned why that might be a fuss because employing that 55mm of backfocus tends to be desirable for most setups (giving the best convenience for out of the box accessories and adapters).

#12 davidparks

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Posted 26 September 2022 - 01:23 PM

I recently sold my Redcat 51 so I can't check... but if memory serves the Helical Focuser on the Redcat 51 only move about 180 degrees throughout it's whole range.  Open or close the focus all the way, connect the blackcat gear at the appropriate "half end" and voila, you have full travel.

 

Better yet, use this : https://www.thingive...m/thing:5336214  waytogo.gif




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