Posted this on Cloudy Nights, but no one seems to know the answer, so here goes:

I want to use my dslr for prime focus photography with my Nexstar 8SE using a 1.25" adapter and t-ring. I found this site:

www.telescopeadapters.com

I talked to a guy there and he said that their 1.25" and 2" adapters are both threaded for filters. Since I have 1.25" filters, I want to get the 1.25" adapter so I can use my filters.

Now, my question is will I see any vignetting if I go with the 1.25" set up? The camera I will be using is an olympus e520, which has a smaller sensor than APS format cameras, so Im hoping I wont see any vignetting.

Here are the specs on the ccd in mm:

22.500 (diagonal)
18.000 (width)
13.500 (height)

Hi Alex:
With a camera DSLR you have a matrix of bayer, your chip is one color shot, http://en.wikipedia.org/wiki/Bayer_filter , you are not going to need colors filters, seems to me that it is not recommendable.
For this reason, it would prefer to use the adapter of 2" , and this it can even cause something of Vignetting, but always you will be able to eliminate it, calibrating, with a good series of Flats.
Many greetings
Cesar

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http://www.astrophoto.com.mx

Thanks for your help. I live in a heavily light polluted area and the filter I was going to use is a Skyglow Imaging Filter, which is just a broadband filter used to filter out skyglow and street light pollution. Im wondering how to calculate the amount of vignetting involved with a sensor of the dimensions I posted. I saw that the clear aperture of the filter is 26mm and the diagonal of my sensor is only 22.5mm, so it should fit without vignetting-- however I think the distance between the sensor and the filter is also of concern, and probably narrows the clear aperture of the light pathway. I just wish I knew the physics involved and could make the calculation myself to determine the amount of vignetting. Im hoping one of the kind folk on here can help

Flat fielding will easily remove any vignetting. Even if present it wouldn't be a problem at all. Even non vignetted images should be flat fielded as no image is evenly illuminated nor is each pixel of the same sensitivity. Flat fielding corrects all of this.

Under a heavily light polluted sky a mono camera with narrow band filters will give the best results. One shot color will require very many frames to get the signal to noise ratio down. If it is really bad you might need several nights worth of data to get very deep. Otherwise star clusters, bright nebula such as many planetary nebula, double stars etc. would be your best targets. You'll need good gradient removal tools and expect color casts (especially green) to add to your processing woes. I have a friend that does very well from "downtown" Berlin. Last I knew he gave up on any filters with one shot cameras. They just meant even more exposure time. Though his mono camera can go far deeper than his OSC using narrow band filters. He's been using the OSC for color data and the mono for the luminosity layer to good results. So it can be done with enough exposure time.

Rick