View Full Version here: : Field flattener and filter wheel

12-07-2017, 12:28 AM

I have a SW ED80 with a field flattener (Hotech SCA) and am planing to add a filter wheel. The imaging will be done with the home-made Cam86 CCD.

What is the usual configuration of the flattener and the filter wheel?
Scope -> Flattener -> Filter wheel -> CCD
Scope -> Filter wheel -> Flattener -> CCD

Does it make any difference as long as the correct distance is maintained between the flattener and the sensor?

Also where would the OAG sit, if I ever decide to use one?

Thanks in advance

12-07-2017, 07:56 AM
My suspicion would be the difference would be that the filter wheel aperture is likely narrower than the scope-side aperture on the field flattener, so there's a risk of cutting some light out :shrug:

OAG you would want scope side of the flattener so you can pick off nice runs stars :)

12-07-2017, 08:14 AM
You'll want to have the filter wheel as close to the CCD as possible to stop possible vignetting.

The best way is Telescope >> Flattener >> OAG >> Filter Wheel >> CCD

12-07-2017, 08:15 AM
I have mine set up as

camera -> wheel -> oag -> flattener -> scope

The difficulty is getting the main camera and the OAG to focus, so they need to be reasonably close but not too close, so you may need to experiment with the spacing. Both need to be around the same distance from the OAG prism.

12-07-2017, 04:39 PM
An alternative path I'm using now is:

camera -> filter wheel -> reducer/flattener -> oag -> scope

This is only because of some spacing issues. For the oag though, I've got it's own reducer to help with its focus path.

Eventually I hope to go back to the more "normal":

camera -> filter wheel -> oag -> reducer/flattener -> scope

I'm not sure if I'll keep the reducer for the oag at this point - it will depend on focusing issues.

12-07-2017, 09:31 PM
Thank you for your replies. Interesting variety of responses indicating that possibly it does not make much difference where the filter wheel is :shrug:

Colin, what is the reason for vignetting caused by filters being far away from the CCD? Filter size too small and cutting off light like Dunk suggested?

12-07-2017, 09:55 PM
Hi Luka

I you hold up your F/W (with adapters you expect to use in front and back of it attached) to a large bright screen (e.g. computer monitor) and experiment with looking through and moving your eye closer and further away, you'll start to see how quickly the tightest part of your lightpath (generally your filters, but sometimes M42 connectors before or after the FW) is adversely affected as your "CCD" get further away from the FW.

Getting the CCD in 10mm closer to the FW makes a huge difference to reducing vignetting on my rig.