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flattener/reducer backfocus issue

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#1 kd0ar

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Posted 09 July 2020 - 09:37 AM

I image using the AT-72ED and AT102ED scopes.  I also use the AT ATR8 flattener/reducer on both scopes.  I recently bought a filter wheel.  Now I dont have enough back focus.  Almost, but cant quite attain focus.

 

I would be able to get focus if I could swap out the 2" focuser adapter that fits on the back of the flattener, but I have no idea what the thread size is on the body of the optics.  I'm talking about the end that normally fits into the focuser.  If I could find a shorter adapter to replace the 2" focuser piece, I would be able to image with it with the filter wheel.  any ideas?  TY.



#2 Stelios

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Posted 09 July 2020 - 01:20 PM

If indeed you lack back-focus, any of the many focuser tube extensions should be of help.

 

In any case, with a flattener you need a set distance from sensor to reducer (usually 55mm), so  if you were able to focus without the filter wheel, you should be able to focus with it.

 

Perhaps if you tell us what camera and FW you have, (or better give a picture of your imaging train) we could help more.

 

Your solution makes little sense at first blush--the piece that fits into the focuser fits *into* the focuser, so what difference does it make if it's longer or shorter?



#3 kd0ar

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Posted 09 July 2020 - 06:25 PM

I may have my terminology wrong.  Basically, I've run out of inward travel.  I need to shorten the light path from the scope to the camera.  In the image below, shows the ff.  The section I marked is the section I would like to remove from the FF, but the threads on the glass are unknown, and I do not have any way of adapting the FF glass to the filter wheel.  The threads in the 2" focuser adapter on the FF screw into the filter wheel.  I guess I'm looking for info as to how I can shorten the image train.  If I were to get the flattener that does not reduce, that may allow me to achieve focus, but I'm not sure, and I'm not sure I want to get one if it still dont work.

 

Camera - Canon T3  (or a Celestron Nightscape)..  Neither will focus.

Field Flattener is the ATR8 reducer/flattener.

Filter wheel is the Astromania 5 pos 2" wheel

Scope - AT102ED  or AT72ED.  Neither will achieve focus.

 

 

 

107693705_2989939681055002_6327590767709ht path


Edited by kd0ar, 09 July 2020 - 06:28 PM.


#4 Stelios

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Posted 09 July 2020 - 06:41 PM

It's all clear now. You need in-focus. Not sure why you are using a DSLR and a filter wheel, to try unattended narrowband? 

 

I assumed (from the mention of filter wheel) that you had a CMOS or CCD camera which takes up far less of the reducer's back focus than a DSLR. The usual setup is [CCD/CMOS]=>FW=>[spacer]=>reducer=>scope. That won't work with a DSLR which eats up the full backfocus.

 

My guess would be that the piece you show (the piece that inserts in the focuser) would be an M48 thread, in which case you should be able to easily find an adapter. Why not call Astronomics and ask? They are the Astrotech retailer, as well as our friendly CN sponsor. 

 

If worse comes to worst, perhaps you could replace (sell/exchange/return) the FF for a less bulky one. The ES ones I believe have the optics inside the focus slide tube, and are shorter. They exist in all shapes and sizes. 


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#5 kd0ar

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Posted 09 July 2020 - 06:49 PM

I have actually had really good luck imaging Ha and O3 with the dslr.  I do plan on getting a mono CCD, but I'm having too much fun imaging Ha with the dslr.  I just got the filter and filter wheel a couple weeks ago, and im surprised with the results I'm getting.  This is without the FF using the AT102ED.

 

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Edited by kd0ar, 09 July 2020 - 06:51 PM.



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