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Stacking two focal reducers - backfocus?

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7 replies to this topic

#1 Noah4x4

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Posted 25 September 2020 - 02:25 AM

I normally use Atik Horizon on Hyperstar or ASI294mc at f/6.3. But I have two Celestron x6.3 focal reducers that I have yet to try in tandem due to incessant cloud ever since purchase of the second (sorry if my purchase has affected others). 

 

Has anybody worked out the best optical train for dual stacked FRs? What back focus from the second focal reducer to camera sensor is ideal (e.g. without diagonal)? I have a T-adapter, Baader Varilock, and assorted spacers so I can experiment, but if somebody already has the accurate number, I would be grateful.

 

My enquiry embraces 2 x 0.63 Celestron reducers. If others can append data for other combinations of stacked FR then this thread might become a useful resource.

 

BTW, I am aware that increased vignetting is likely with larger camera sensors, so let's keep this thread focussed on backfocus required. As an EAA observer rather than an EAA photographer that doesn't trouble me.

 

 



#2 sg6

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Posted 25 September 2020 - 03:03 AM

Undefined if it is possible.

The first reducer will put the image at some position that is basically unexpected by the second. So in the optics the object position/distance is unknown.

 

A chance that the created image is virtual and so you don't get one that can be projected on to a sensor/eyepiece.

 

If the idea was that 2 0.63 reducers make a 0.4 reducer, they don't.

 

Best is: Put them together, measure the new image position (if there is one) and tell us.



#3 ColoKid

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Posted 25 September 2020 - 09:43 AM

Check out this posting.

 

https://www.cloudyni...focal-reducers/



#4 DSO_Viewer

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Posted 26 September 2020 - 11:28 AM

I really do not understand why you would want to stack two 6.3 reducers esp. since they are not made for this purpose when you already have a nice Hyperstar to use? 

 

Steve 



#5 Noah4x4

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Posted 28 September 2020 - 03:31 PM

I really do not understand why you would want to stack two 6.3 reducers esp. since they are not made for this purpose when you already have a nice Hyperstar to use? 

 

Steve 

I love Hyperstar, but some targets are too small at f/2. I also embrace f/6.3, but a problem there is that the backfocus required is 105mm and when on an Alt-Az hence only a short uncooled camera will go to the zenith. I want to try my 12.5cm long Atik Horizon camera at a higher f/ratio that f/2, but it is too heavy for rear mounting using a diagonal and too long behind a single reducer.

 

I suppose that the bottom line is I don't want to buy a GEM simply to gain rear end clearance. I realise there are other benefits, but my Evolution is great except for rear clearance. From what I have gleaned, Starizona NightOwl at f/4 might offer a compromise, but it is not currently available. I have also read that stacked 6.3 reducers might potentially offer a solution. There are a few threads where people appear to have succeeded, but finding greater information such as what backfocus was desirable behind the second reducer isn't easy. It won't be the first time I have wandered up a blind alley and found it is a cul de sac!


Edited by Noah4x4, 28 September 2020 - 03:34 PM.

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#6 DSO_Viewer

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Posted 29 September 2020 - 10:33 PM

I love Hyperstar, but some targets are too small at f/2. I also embrace f/6.3, but a problem there is that the backfocus required is 105mm and when on an Alt-Az hence only a short uncooled camera will go to the zenith. I want to try my 12.5cm long Atik Horizon camera at a higher f/ratio that f/2, but it is too heavy for rear mounting using a diagonal and too long behind a single reducer.

 

I suppose that the bottom line is I don't want to buy a GEM simply to gain rear end clearance. I realise there are other benefits, but my Evolution is great except for rear clearance. From what I have gleaned, Starizona NightOwl at f/4 might offer a compromise, but it is not currently available. I have also read that stacked 6.3 reducers might potentially offer a solution. There are a few threads where people appear to have succeeded, but finding greater information such as what backfocus was desirable behind the second reducer isn't easy. It won't be the first time I have wandered up a blind alley and found it is a cul de sac!

That now makes more sense to me so thank you for helpful information Noah.

 

Steve



#7 roelb

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Posted 02 October 2020 - 04:45 PM

https://www.cloudyni...-atik-infinity/


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#8 Noah4x4

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Posted 03 October 2020 - 07:44 AM

I knew I had seen this somewhere. Thanks Roelb.


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