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C8-SCT Back focus question


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Hello all, in preparation for galaxy season I have decided to resurrect my C8 to use with my ASI2600mc Pro, I intend to use my Antares f6.3 reducer. Can I confirm that the back focus to the sensor from the reducer is 105mm?

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The Celestron and Meade .63 reducers are 105mm. However, a discussion on cloudy nights is suggesting the antares is 85mm...

Help a brother out, what is the real backfocus spec of the Antares F6.3 SCT reducer? - Equipment - Cloudy Nights

Graeme

EDIT:  Posts further down are confirming 105mm.   Easiest thing to do would be to focus the sun through the reducer, measure that distance and halve it. 

Edited by jacko61
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Well I tried 105mm and I am struggling with round stars throughout the image. The attached image is Deneb, 180 second exposure guided. I thought it might be collimation but whilst it isnt perfect it is good enough.

FB_IMG_1641940313854.jpg

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Does anyone have any thoughts on how to fix this? Everywhere I read it says 105mm back spacing which I have achieved using the following:

Celestron T Adapter (50mm) + 16.5mm Spacer + 21mm Spacer + ASI2600mc (17.5mm)

I thought it might be collimation so last night I again used Deneb to check and whilst it isnt perfect it was the best I could get it.

 

 

collimation.jpg

Edited by stargazermanc
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1 hour ago, stargazermanc said:

Does anyone have any thoughts on how to fix this? Everywhere I read it says 105mm back spacing

Yes, 105mm is the accepted figure, but based on your star shapes in the corners, you could add a couple of mm and see if the star shape improves.

Michael

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3 hours ago, stargazermanc said:

I have no way to add as the all the adapters are fixed in length.

 

 

What is preventing you adding more adapters ?

Here's a set of 3mm to 40mm M42 adapters that you can add to or replace your 16.5mm and 21mm adapters, to get a wide range of different lengths:

https://www.rothervalleyoptics.co.uk/VSA19-10.html

Michael

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