How important is backfocus?

[Peter Reader]

Hi all,

I'd like to step up my imaging with a 2" Optolong L-Pro Light Pollution Broadband Filter and a Sky-Watcher 0.9x Coma Corrector.

Question 1: does the filter go between coma corrector and camera or before the coma corrector?

Question 2: I'm concerned about the recommended backfocus from the coma corrector (55 mm). Does anyone know if this combination is compatible with my Canon 1000D? How do I work this out?

Best wishes, 

Peter

[rl]

How important the spacing is depends on how critical you are of coma in the corners of your image. Ideally it needs to be +/- better than 1 millimetre. If you ever use a full-frame camera it gets more critical. For me using the standard parts has always worked to my satisfaction but I'm not a pixelpeeping addict. 

The Skywatcher 0.9 coma corrector (CC) is designed for 55mm spacing from the back face of the CC to the image sensor. For this you will need a M48 thread diameter adapter whIch screws into the back of the CC and presents the Canon style lens bayonet fitting to the camera. These are different from the standard lens adapters which have a 42mm diameter thread.

https://www.firstlightoptics.com/adapters/astro-essentials-m48-camera-adapter.html

The distance is made up of about 44mm from the front face of the camera to the sensor plane inside plus the 11mm thickness of the adapter. FLO and others sell the adapter; it's mentioned in the CC blurb. 

You can screw in a 48mm threaded 2" filter into the front of the CC and you should be all set. Sometimes filter threads aren't very clever and it will only engage for 1 turn or so; it's not unusual and not normally a problem. 

Don't forget to show us your first M42...

Edited December 19, 2021 by rl

[Peter Reader]

On 19/12/2021 at 22:53, rl said:

How important the spacing is depends on how critical you are of coma in the corners of your image. Ideally it needs to be +/- better than 1 millimetre. If you ever use a full-frame camera it gets more critical. For me using the standard parts has always worked to my satisfaction but I'm not a pixelpeeping addict. 

The Skywatcher 0.9 coma corrector (CC) is designed for 55mm spacing from the back face of the CC to the image sensor. For this you will need a M48 thread diameter adapter whIch screws into the back of the CC and presents the Canon style lens bayonet fitting to the camera. These are different from the standard lens adapters which have a 42mm diameter thread.

https://www.firstlightoptics.com/adapters/astro-essentials-m48-camera-adapter.html

The distance is made up of about 44mm from the front face of the camera to the sensor plane inside plus the 11mm thickness of the adapter. FLO and others sell the adapter; it's mentioned in the CC blurb. 

You can screw in a 48mm threaded 2" filter into the front of the CC and you should be all set. Sometimes filter threads aren't very clever and it will only engage for 1 turn or so; it's not unusual and not normally a problem. 

Don't forget to show us your first M42...

Thanks for the reply. I've found a  Baader Mark III coma corrector at a good price - I don't suppose you could tell me how I use this with a filter? The SW 0.9x coma corrector specifically mentions a filter thread on the front, but I can't find this info for the Baader...

Edited December 29, 2021 by Peter Reader

[rl]

https://www.baader-planetarium.com/en/downloads/dl/file/id/483/product/3004/mpcc_mark_iii_instruction_manual.pdf

The MPCC has a thread on the front which accepts 2" filters (the thread for 2" filters is actually 48mm). That's all there is to it!

 

RL

[Notty]

I've always wondered about this and come to the conclusion, given the stacking artefacts from long sessions over multiple nights often involve me having to crop the fringes off anyway which is where any errant elongated stars would show, that I'm not going to lose any sleep over the exact spacing.