Focal Reducer - Beginner Question II

[Leedsgreen]

Dear All,

I recently bought a 1.25" 0.5x focal reducer to use on my 127 SkyMax. As many warned, there is some strong vignetting when using it afocally with my Canon Powershot, but visually it appears to work. I have a few more questions:

1. Try as I might, I can not get the 0.5x reducer to focus with my Philips SPC900NC webcam. I have tried the webcam with the reducer on both my APO 66 refractor as well as the SkyMax 127. Could this problem be due to the 1.25" barrel adaptor I have attached to the webcam? What I mean is, does the barrel adaptor hold the reducer to far away from the CCD chip?

2. If I manage to get the set up to work, using the reducer on my APO 66 could turn it from f6 into f3 (and even f2 is spaced correctly). At such short focal lengths would I be able to capture a bright galaxy (such as M31) at 1/25s (i.e. the slowest shutter speed of an unmodded SPC900NC)?

Thanks very much for any help,

Nick

[rikyuu]

hi,

I will try to answer;

1) focal reducers are designed to work optimally at a specific distance from the sensor ie. their focal length. This can be found by placing the FR in front of a strong distant light, eg a torch, spotlight etc. and then measuring the distance between the FR and the focused image of the light source. This should then be the optimal distance between the sensor and FR. As you say, the webcam barrel adapter may be the wrong length.

2) looking through some of the people images of M31 on SGL, even with a 5 second exposure with a DSLR at f/7.5, M31 is a faint blob. Your webcam will typically take 1/25s exposures (125 times shorter than 5s), but sometimes this can be lengthened to 1/10s, but I don't think it will see much I'm afraid.

hope this helps

[Leedsgreen]

Rikyuu,

Thanks very much for your reply and advice. Much appreciated.

Nick