C8-SCT Back focus question

[stargazermanc]

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?

[jacko61]

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 January 11, 2022 by jacko61

[stargazermanc]

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.

[michael8554]

The star shapes in the corners suggest you could increase the 105mm spacing.

But does the Focal Reduce act as a Coma Corrector too ?

Michael

[stargazermanc]

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.

 

 

Edited January 14, 2022 by stargazermanc

[michael8554]

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

[stargazermanc]

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

 

 

[michael8554]

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

[Same old newbie alert]

On 14/01/2022 at 13:32, stargazermanc said:

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

 

 

You could add baader aluminium shims.. come in various thickness, gold a 1mm, black .5 and red .3 if I remember correctly..you have a APS-C sized sensor so you need more critical spacing, the best collimation you can get.. I've said this elsewhere but I don't think 105 is the correct spacing as that gave me a focal length of 1320, which shouldn't be correct..

I've adjusted the spacing to give me the 1280 it should be, stars look much better..sure I can improve it very slightly but as I'm using a smaller sensor how precise do I want it