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Achieving focus with ASI462MC

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#1 desert_skies

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Posted 25 October 2021 - 08:56 AM

Hi 

 

I have been plodding along with my Canon DSLR for some time now, and that’s gone reasonably well. So as a next step I got myself a ZWO ASI462MC. However I am having problems achieving focus, especially with a Barlow attached. I suspect it is something to do with back focus, and I have searched high and low for info on this, but to no avail, with the result that I am pretty much stuck and each session ends with me ditching the Astro camera and running for my DSLR in the hope that I can at least capture something for my night’s work.

 

so, basically, this is where I am:

 

I attach the nosepiece to the camera, put it into the diagonal - cannot see anything.

 

Remove the diagonal and insert the nosepiece directly into the visual back, can achieve focus.

 

Insert the nosepiece into a Barlow 2x lens and insert the Barlow into the visual back- can get some way towards focus but never reach it before it starts going out of focus again. (Technically I know it should be a 1.5x with this camera, but I am told that with good seeing it should push to a 2x with good results).

 

As I said I can’t find anything on this. How do I set up this camera for use with a 2x Barlow and Celestron 9.25 SCT?? I am using Sharpcap for capturing.

 

 Thanks for any tips!

 

 



#2 kathyastro

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Posted 25 October 2021 - 09:07 AM

The DSLR consumes 55mm of back focus.  The ASI only consumes 12.5mm.  So if you just removed the DSLR and added the ASI, you would need 42.5mm of extensions to get it to focus at the same place.

 

The barlow complicates things.  Usually, it will move the focal point outwards, so you would need even more extensions.

 

One way to estimate the amount of extension needed is to hand-hold the camera (or the camera + barlow unit) and move it back until you get close to focus.  It doesn't have to be precise, since SCTs have a huge range of focus.


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#3 matt_astro_tx

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Posted 25 October 2021 - 09:32 AM

Yep, ditch the diagonal.  

 

Have you tried removing the barlow lens element from the barlow EP itself and screwing it onto the 1.25" nosepiece?  That's what I do.


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#4 JMP

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Posted 25 October 2021 - 09:52 AM

Are you using a UV/IR cut filter? Mandatory with the 462!

Also, shoot at f/10 (no barlow) as a first step.

Edited by JMP, 25 October 2021 - 09:53 AM.

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#5 Tapio

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Posted 25 October 2021 - 10:01 AM

Use some easy target like horizon object in daytime or moon.

First without barlow (your field of view will be really tiny with such combo) and then with.


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#6 RedLionNJ

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Posted 25 October 2021 - 01:28 PM

You should have zero problems reaching focus with your setup.

 

SCT-barlow-camera.    No need for a diagonal, etc.   The SCT has an enormous range of focus. Ignore the term "backfocus" - it's totally not relevant here.

 

You state you "almost reach focus" before it goes further out of focus again.

 

This suggests you are indeed reaching focus at some point in the range. But the seeing/collimation/whatever wasn't up to generating a truly sharp image.

 

Check your collimation and try again on a different night, maybe on a high overhead star.


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#7 desert_skies

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Posted 25 October 2021 - 01:56 PM

Thanks for the tips! At least I was right about the diagonal! Filter is arriving any day now. I thought about just attaching the lens cell from the Barlow, but I believe that will reduce the magnification to around 1.5x. Still, I should try it. But ‘walk before run’ is probably sound advice- get comfortable with the camera first, f10, no Barlow, easy target… that will be the first step. Will check collimating before anything else as a matter of course! Thanks to all.


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