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#1 bob2ride


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Posted 26 October 2020 - 06:26 PM

Hi Everyone

I have a nexstar 8SE and am about 2 years into astronomy with maybe 6 months in attempting astrophotography. I have been using my Canon M50 and that's worked out ok.  I recently bought a second hand ZWO camera.- ASI 1385 MC camera.  I have used it thru the eyepeice but that's not really great.  So I am planning to attach the camera at Prime focus.  To to that I think I need to know the back focus for the camera.  That's something I don't know and can't seem to find.  Secondly I need a few adapters - I ahve the T adapter for my scope and a focal reducer is on the way. 

Any suggestions as to

1. how far to offset the camera sensor

2. where do I buy the needed adapters?


Yeah, I know i have an alt/az mount.  and it's not going to be perfect but this is the first scope not the last one so one step at a time.



#2 AZ Maverick

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Posted 26 October 2020 - 07:11 PM

The back focus to the sensor in that camera as listed in the specs from ZWO is 12.5mm.


Edited by AZ Maverick, 26 October 2020 - 07:11 PM.

#3 Noah4x4



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Posted 27 October 2020 - 02:15 AM

Hi Bob,


A bit more information is required as there is more to this equation given you mention Focal Reducer.


The 12.5mm mentioned in post #2 is merely the internal back focus distance to the sensor within the camera. The overriding back focus measurement will be the far longer requirement of the Focal Reducer (FR). 


To explain, I am going to assume a Celestron SCT scope and Celestron FR (Meade might differ). Bob,  it would be helpful if you add a full list of your equipment to your Cloudy Nights signature, then we can guide you more specifically for your kit. However, for example;


The back focus for a Celestron x0.63 Focal reducer is 105mm. This means that it is desirable to have 105mm distance between FR and camera sensor. The back focus of the ZWO provides 12.5mm of this. Hence you need to create 105mm minus 12.5mm = 92.5mm spacing in between. This might vary for other makes/models, but I hope this helps you grasp the principle. The focal reducer back focus is the key parameter.


The T-adapter will provide either 50mm or 55mm of this (depends on make/ model) leaving perhaps another 42.5mm or 37.5mm to be bridged. For that, I use a Baader Varilock as that can be adjusted for my different cameras (my ZWO has 12.5mm internal back focus and my Atik 17.5mm). You can alternatively use loose spacers. Both can be purchased from any quality astro-retailer. But my optical train is therefore OTA > FR > T-Adapter > Varilock > camera with 105mm between FR and camera sensor. BUT......There is a potential complication....


You mention owning an Alt-Az. The solid all screwed affixed route to achieve 105mm that I describe might not offer enough back end clearance if using a longer cooled camera. This would mean a need to include a diagonal (which I believe is what you describe, but instead refer to eyepiece). However, the principle is the same, you need 105mm through the diagonal if using a Celestron Focal reducer. The ZWO will have a 1.25" nose-piece for use with a diagonal, and that gives you a good deal of adjustment. The T-adapter might then be redundant. 


In essence, you need identify the specific backfocus requirement of your actual kit which you can glean from its specifications, but the above describes the principles.. If you need more help, just ask. However if your ZWO is not cooled, then the route I suggest should hopefully offer enough rear end clearance if you set your OTA fully forward. As I said, if we know your make/model/size  of scope we can give you more specific guidance.

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