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Yet Another BackFocus Question

Astrophotography Beginner EAA
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#1 Cbchhaya

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Posted 20 July 2021 - 12:49 AM

I apologize if my searching skills have failed me and this question has been answered before.

I have an AT72EDII that I have been using with the 0.8x focal reducer and my 60D camera. I recently got the ASI533MC Pro and took the rig out for a spin tonight and couldn’t get the thing to focus.

My setup is an IDAS LPS D3 filter screwed into the reducer, the 16mm M42-48 adapter that comes with the camera, the 21mm M42-42 adapter, the 11mm ring on the 533 and the camera itself.

What am I doing wrong here? I tried focusing (using Focus routine in ASI Air Pro) but could not hit focus anywhere in the full range. Did I just miss a focus point? Or is my reducer+filter+extender configuration incorrect?

Thanks for your help!



#2 blazek

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Posted 20 July 2021 - 01:50 AM

If your previous setup was working, and your 60D sensor to reducer distance was OK, you just have to compensate the difference in sensor-thread distance between cameras.For your Canon the sensor-flange distance is 44mm plus adapter comes to standard 55mm. For your 533 sensor to thread distance is ,I think, 17.5 mm, so if you want to put your 533 instead your 60D, you need to put 37.5mm "extension" on your 533. And btw, think about putting the filter as close as you can to your camera, not directly on reducer. 



#3 Cbchhaya

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Posted 20 July 2021 - 05:14 AM

For your 533 sensor to thread distance is ,I think, 17.5 mm, so if you want to put your 533 instead your 60D, you need to put 37.5mm "extension" on your 533. And btw, think about putting the filter as close as you can to your camera, not directly on reducer.



Ok so then I need the two extension tubes since they add up to 37.5 and in that case they couldn’t be the problem. The filter adds a few mm which puts spacing beyond 55mm. Is that OK?

Edited by Cbchhaya, 20 July 2021 - 05:17 AM.


#4 blazek

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Posted 20 July 2021 - 06:06 AM

I assumed you had filter also with Canon and your connected your Canon with Canon to T2 adaptor. If you unscrew your Canon and screw in 37.5 mm extension from that point (either made of one or several pieces) and than screw in your 533 , you should be fine , because that will be the same as you using Canon. 



#5 Cbchhaya

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Posted 20 July 2021 - 06:47 AM

I assumed you had filter also with Canon and your connected your Canon with Canon to T2 adaptor. If you unscrew your Canon and screw in 37.5 mm extension from that point (either made of one or several pieces) and than screw in your 533 , you should be fine , because that will be the same as you using Canon.


The filter with Canon was clip in. This one is screw in one.

#6 alphatripleplus

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Posted 20 July 2021 - 07:17 AM

Moving from EAA to Beginning Deep Sky Imaging for a potentially better fit.



#7 blazek

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Posted 20 July 2021 - 07:20 AM

Good then you have to put it as close as possible to camera and account for its ring thickness within this 37.5 (deduct filter ring thickness from 37.5. ) . Or consider adding 

https://astronomy-im...lter-drawer-m42

to jour camera is not bad idea. It would let you add/remove filters easy. This drawer goes directly onto the camera , and will "consume" 21mm out of your 37.5 that you have to add. 


Edited by blazek, 20 July 2021 - 07:22 AM.


#8 fewayne

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Posted 20 July 2021 - 07:35 AM

You should be able to achieve focus even if the distance between flattener-reducer and sensor is pretty far off. Correct backfocus is important, of course, but you have something else going on. Try the old hold-the-camera-up-behind-the-telescope trick, see if you can achieve focus on a (distant!!) daylit scene.


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#9 sbharrat

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Posted 20 July 2021 - 07:38 AM

If your previous setup was working, and your 60D sensor to reducer distance was OK, you just have to compensate the difference in sensor-thread distance between cameras.For your Canon the sensor-flange distance is 44mm plus adapter comes to standard 55mm. For your 533 sensor to thread distance is ,I think, 17.5 mm, so if you want to put your 533 instead your 60D, you need to put 37.5mm "extension" on your 533. And btw, think about putting the filter as close as you can to your camera, not directly on reducer. 

The 533 came with all the extensions needed in order to make it exactly 55mm. So you *should* have everything you need to get it equivalent to your Canon. (I went from a Canon 1000D to a 533 myself).

 

Regarding the filter, I had a 2" L-Enhance. I did exactly what you did screwing it into the reducer. Eventually I switched to using a filter drawer but what you did worked for me for a while. 



#10 Cbchhaya

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Posted 20 July 2021 - 07:44 AM

The 533 came with all the extensions needed in order to make it exactly 55mm. So you *should* have everything you need to get it equivalent to your Canon. (I went from a Canon 1000D to a 533 myself).

Regarding the filter, I had a 2" L-Enhance. I did exactly what you did screwing it into the reducer. Eventually I switched to using a filter drawer but what you did worked for me for a while.


Then, as suggested above, maybe it was something else if this arrangement has worked for you in the past. As for the filter drawer will the 48mm filter slot right in or will I need to change filter dimensions?

#11 Cbchhaya

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Posted 20 July 2021 - 07:46 AM

You should be able to achieve focus even if the distance between flattener-reducer and sensor is pretty far off. Correct backfocus is important, of course, but you have something else going on. Try the old hold-the-camera-up-behind-the-telescope trick, see if you can achieve focus on a (distant!!) daylit scene.


Makes sense. As mentioned above by another helpful member using extensions that came with 533 has worked in the past so I now doubt that is the issue.

I’ll try focusing during the day and see if it gets close then probably just mark that spot on the scope.

#12 sbharrat

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Posted 20 July 2021 - 07:50 AM

Makes sense. As mentioned above by another helpful member using extensions that came with 533 has worked in the past so I now doubt that is the issue.

I’ll try focusing during the day and see if it gets close then probably just mark that spot on the scope.

Just to be clear... I was answering the thread about getting your backfocus right. The asi533 came with everything you need to make your setup equivalent to your Canon setup.

 

What fewwayne is pointing out is that wrong backfocus (unless it is really far off) shouldn't have prevented you from being able to focus as all (your initial problem). So even after you get the backfocus right, it may not completely solve your problem. But for me, I would get the backfocus close, then try to do the focus as suggested using a far off object in the daytime.



#13 blazek

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Posted 20 July 2021 - 08:13 AM

Small apology, I was not aware that ZWO includes 16.5. + 21mm T2 extensions. (Not all 533 manufacturers and sellers do) . 

If the difference would be that big (37.5 ) and short exposure/low gain on camera it could happen that you see nothing.

However , agree, if you are just few mm off , you should see "donuts" .  So above advices from sbharrat are of course good and correct 

Btw, IDAS filters that are marked 2" or M48 do fit into the ZWO filter drawer that has M48x0.75 thread for filters. 



#14 Cbchhaya

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Posted 20 July 2021 - 08:17 AM

Small apology, I was not aware that ZWO includes 16.5. + 21mm T2 extensions. (Not all 533 manufacturers and sellers do) .
If the difference would be that big (37.5 ) and short exposure/low gain on camera it could happen that you see nothing.
However , agree, if you are just few mm off , you should see "donuts" . So above advices from sbharrat are of course good and correct
Btw, IDAS filters that are marked 2" or M48 do fit into the ZWO filter drawer that has M48x0.75 thread for filters.


So a very quick experiment this morning. I tried focusing with 21+16.5+11+filter on a distant building about 150m away. And guess what? It achieved focus very close to the spot I’d marked for my 60D so it certainly seems the issue wasn’t with the extensions added in.

Unfortunately I will have clear skies only about 2 weeks from now :-( so will rest uneasy over this problem.

#15 acrh2

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Posted 20 July 2021 - 08:23 AM

Unfortunately I will have clear skies only about 2 weeks from now :-( so will rest uneasy over this problem.

 

Weather people are full of ****. 



#16 nic35

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Posted 20 July 2021 - 08:25 AM

I suspect your exposure/gain/histogram settings are off.



#17 Cbchhaya

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Posted 20 July 2021 - 08:42 AM

Weather people are full of ****.


You mean it could be worse than that??!! ;-)

#18 Cbchhaya

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Posted 20 July 2021 - 08:44 AM

I suspect your exposure/gain/histogram settings are off.

Unfortunately I couldn’t capture the screen (thinking there was nothing interesting to note) but I couldn’t see a single star in FOV so there wasn’t much besides random noise in the histogram to look at. Gain 100 (also tried 150).

#19 nic35

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Posted 22 July 2021 - 09:14 AM

Unfortunately I couldn’t capture the screen (thinking there was nothing interesting to note) but I couldn’t see a single star in FOV so there wasn’t much besides random noise in the histogram to look at. Gain 100 (also tried 150).

I made my comment because the camera worked in a high light environment.  So, try cranking the gain up to about 350 or higher.  Exposures of 8 seconds or so. You might even remove the filter, just to test things out.  

 

I don't use an AAP, so i can't comment on the histogram settings.  Just make sure your black level isn't set too high,

 

Also, you might find that celestial objects focus at a significantly different position than do objects 150m away.   You might try a daytime test with an object 1/2 a mile away, or more to see where the focus position is.

 

j


Edited by nic35, 22 July 2021 - 09:20 AM.


#20 weezer

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Posted 05 August 2021 - 04:28 AM

Check out Carl Smith's You Tube presentation on EAA Focusing:

 

https://www.youtube....h?v=c1NarA4_qxY




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