Back focus on a C9’25

[recceranger]

Hi folks, I’ve been imaging using my C9’25 this evening and encountered an issue with focus. My imaging train has an Antares 0.63 reducer - Celestron 50mm adapter - 16.5mm spacer - 21mm filter drawer - 11mm ASI533 ring - then 6.5mm to camera sensor. I had it all fitted tonight presicely as per numerous YouTube tutorials etc. However, in attempting to achieve focus, I could get nowhere near. This was in both directions. I took the reducer off the image train and achieved focus no problem at all, but at the expense of the images not being flat across the FoV. 
 

Has anyone encountered this issue and if so did you find a work around? 

[vlaiv]

It looks like you put too much distance between reducer and sensor.

If I'm calculating things correctly, there is 8.8mm on reducer itself + 50mm + 16.5 + 21 + 11 + 6.5 = 113.8mm

That makes this reducer work at F/5.5 instead F/6.3 and requires 23.5cm inward focus travel.

Try placing sensor 84mm away from back of reducer.  Just drop 21mm extension and see what you get like that.

 

[recceranger]

1 hour ago, vlaiv said:

It looks like you put too much distance between reducer and sensor.

If I'm calculating things correctly, there is 8.8mm on reducer itself + 50mm + 16.5 + 21 + 11 + 6.5 = 113.8mm

That makes this reducer work at F/5.5 instead F/6.3 and requires 23.5cm inward focus travel.

Try placing sensor 84mm away from back of reducer.  Just drop 21mm extension and see what you get like that.

 

Sounds good. I will give it a go tomorrow evening weather permitting and post results on the discussion. Thank you sir

[newbie alert]

Celestron say to have 105mm to the sensor using their .63 reducer.. but I've found thst doesn't give a flat field or the focal length that the spec says it should be with my 8 inch,  x .63 reducer ... if I remember right it then says  the focal length is 1320.... if I extend the spacing then my focal length reduces 

Currently I'm working at 1280 focal length, but my spacing is more than the spec'd 105mm.. Stars look better at the edges, not perfect  

[recceranger]

7 hours ago, newbie alert said:

Celestron say to have 105mm to the sensor using their .63 reducer.. but I've found thst doesn't give a flat field or the focal length that the spec says it should be with my 8 inch,  x .63 reducer ... if I remember right it then says  the focal length is 1320.... if I extend the spacing then my focal length reduces 

Currently I'm working at 1280 focal length, but my spacing is more than the spec'd 105mm.. Stars look better at the edges, not perfect  

That’s very interesting. It’s a clear night tonight so I will experiment with different spacing👍

[vlaiv]

3 hours ago, recceranger said:

That’s very interesting. It’s a clear night tonight so I will experiment with different spacing👍

Antares and Celestron F/6.3 reducers have different focal length - first one has 220mm and second one has 240mm. There is also question of how much of that is taken by reducer housing (8.8mm in Antares case as far as I can tell).

You can try different spacing and calculate effects here:

http://www.wilmslowastro.com/software/formulae.htm#FR_b

In case of Antares - enter 220mm as focal length and keep in mind that 8.8mm is from lens to thread (according to this: https://agenaastro.com/antares-f-6-3-focal-reducer-schmidt-cassegrain-telescope.html )

 

[recceranger]

On 13/01/2022 at 17:28, vlaiv said:

Antares and Celestron F/6.3 reducers have different focal length - first one has 220mm and second one has 240mm. There is also question of how much of that is taken by reducer housing (8.8mm in Antares case as far as I can tell).

You can try different spacing and calculate effects here:

http://www.wilmslowastro.com/software/formulae.htm#FR_b

In case of Antares - enter 220mm as focal length and keep in mind that 8.8mm is from lens to thread (according to this: https://agenaastro.com/antares-f-6-3-focal-reducer-schmidt-cassegrain-telescope.html )

 

Thanks sir, I got it sorted. It was an issue with my EAF settings! 😃. I managed to get imaging with the reducer and all looks good. 👍👍👍👍