ASI 294

[Simon Pepper]

Hi all,

I am making the transition from DSLR to dedicated camera (turn up next week) and wanted some insight on back focus. I opted for the 294 and as per FLO it shows a back focus of 6.5mm. I assume this just means from the back of my scope which is a redcat to the front of the camera should be 6.5mm is it that simple or is it 6.5mm from the glass? I have a filter in the redcat will this affect back focus? Also will the 294 have all the necessary spacers as looking in the FLO page I can only see a spacer and adapters all of which are over 6.5mm so confused how they will help it used for?

 

if my back focus is wrong what are the signs? Elongated stars in the corners?

thanks

 

Edited February 20, 2021 by Simon Pepper

[Clarkey]

Look here - this is the ZWO guide on back focus. The 6.5mm if the distance from the front of the camera to the sensor.

The best solution of 55mm back focus length | ZWO ASI (astronomy-imaging-camera.com)

Most of the spacers and adaptors come with the camera (I think).

[Simon Pepper]

Thanks that looks perfect will reference that when it arrives. 

[teoria_del_big_bang]

The 6.5 mm will be the length from the camera sensor to the front of the camera mounting face.

Your scope and flattener / reducer will have a certain back focus.
From what I can see the Redcat is 59.7.

So you would then need spacers to make up 59.7 - 6.5 = 53.2 mm less the width the filter wheel or drawer takes up (if yo had one).

Also in theory you need to add 1/3 the width of the glass in the filter usually around 0.7 mm to 1mm.

But watching this video on the Redcat you do not have to achieve the exact back focus as in this video he only sets it to 56mm and then uses the focussing mechanism on the Redcat to adjust the actual back focus.
On most setups the back focus has to be exact to stop elongated stars in corners but I think the design of the Redcat means this is not required.

Backfocus

I think this is all correct, that is my understanding but admit I always get confused about back focus so I am sure somebody will correct me if I am wrong.

Steve

 

[david_taurus83]

The Redcat is a petzval design which means that the optimal spacing is reached by just bringing it into focus.

The camera should come with a bunch of spacers to make up 55mm. Put them on the camera and then onto the Redcat. You should be in focus when you rack out around 5mm.

[Simon Pepper]

Thanks all this is great info and hopefully should mean one additional night not wasted trying to troubleshoot. These clear nights are precious 

[Clarkey]

It's always worth getting your focus sorted in the day using a distant object. You can then just tweak when it comes to imaging time. Even when I change my rig slightly I tend to check everything in daylight to save on rare imaging time.