Vigenatting with 36mm filters.

[Tandum (Robin)]

Does any know of any software than can tell me the smallest filters I can use before I get vigenatting?

I found a 36mm filter option which would give me 7 filters in the same wheel but the qhy9 sensor is a long way back inside that camera.

[RobF (Rob)]

I'm pretty sure there is light/ray tracing software you can use to map your lightpath Robin. Amateur telescope makers use these sorts of things. CAn't recall any names, but have seen mention here. I'm sure someone will pipe up.

[Tandum (Robin)]

You're thinking Newt25, trying to get a pc going that can run it, it's old

[bmitchell82 (Brendan)]

hey Robin,
I use newt 2.5 on win7 without issue and in XP should work no problem. Though it wont tell you the information that you require as it only takes into account distance of back focus + focus draw tube diameter.

If it was me i would use trig to work out the un obstructed max light cone that could pass at least you will know that its not the filter doing the vignetting. That should give you a "rough" idea

[RickS (Rick)]

Robin,

The things you'll need to know to figure it out are scope aperture and focal length, sensor size (easy to look up for QHY9) and distance from filter wheel to the sensor. I don't think you'll even need trig. Similar triangles should get you there. I'm happy to have a go at the calculation if you like.

Cheers,
Rick.

[Tandum (Robin)]

I got newt25 to go on XP but I think it will be a lot easier to cut a 36mm mask out of cardboard, bang it inside an existing filter and shoot some flats to see what happens. I'm sure the tak @ F6, F8 will be fine but the newt is @ F3.8 and if I ever do splurg and get a 2" Wynne corrector, it will drop to F2.7.

[Tandum (Robin)]

I got around to doing this today if anyone else is wondering about the difference. Although there is some minor vigenatting in the faster scope with any filter it's only minor. When you look at the difference between 2inch and 36mm filters, the 48mm filters are mounted so have a clear aperture of 43mm while 36mm filters are unmounted and should have a clear aperture of 34mm which is only a couple of mm difference all round.

I saw no difference at all @ F8, but here's what ccdinspector says about the flats from the newt @ F3.8.

1st is no filter, 2nd is 2inch and 3rd is 36mm which was aproximated with a carboard mask inside a 2inch filter.

I also found I have a bit of tilt in the camera and it looks like this newt needs a tune up

[RickS (Rick)]

Thanks, Robin. Interesting results...

[Tandum (Robin)]

One thing with these results is I used a QHY M54 to 2" nosepiece whereas the starlight wheel would have an M42 to 2" nose.