Filter query - 1.25" vs 2"

[Yooster]

I've recently upgraded my OSC rig and there is a few filters I want to buy - Optolong L-Extreme, L-Pro etc

My setup is a Zenithstar 73 II, Flat 73A and ASI294MC Pro. Eventually I would like a filter wheel especially if I upgrade to include a mono camera, but am aware I could fit the filter in the optical chain or a filter draw, or a manual filter wheel.

Obviously 1.25" filters would save me a bit of money, but my question is, will I be at a disadvantage using 1.25" filters in terms if vignetting etc or any other reason to go for 2" over 1.25" ? I assume with the smaller 1.25" i need to have the filter as close to the sensor as possible to minimise any vignetting etc? I appreciate this can be corrected with flats etc.

 

[DaveS]

I think I would go with 2", as you're close to vignetting with the 1.25" ones. Also the IDAS filters which might be interesting are only available in 2" mounted and 52mm sizes.

[vlaiv]

I would not consider using filter drawer or wheel with OSC camera unless you have permanent setup.

I don't see much point in changing filter mid session with OSC camera?

In any case, 1.25" filters have something like 28mm of clear aperture, while 294mc has diagonal of about 23mm. No vignetting is going to occur if you mount filter relatively close to sensor, and if you don't have filter wheel/drawer, you can mount filter very close to sensor, so vignetting would not be my concern.

Here is flat with 1.25" filter on my ASI1600 - which is very similar in size. Scope is also very fast - F/6 reduced by x0.79, filter sits in filter drawer:

Image is stretched to full range 0-1.

Same flat stretched to 82% - which is caused by slight OAG shadow in bottom part. Vignetting in the corners is about 92%, but I'm not sure if it's down to filters or general vignetting from the scope and reducer.

Purchase filters based on future usage. If you plan for larger sensors, switching to mono where you'll use filter wheel, etc ... and you have budget for it - go for 2" filters. If not, there are filters in between like 36mm unmounted filters that you can consider and will cover up to APS-C sized chip.

[Yooster]

Thanks both for your advice, very helpful