Using camera lenses withs CCDs and filters

[cuivenion]

Hi all, I've recently started using this lens:

http://www.ebay.co.uk/itm/Chinon-MC-f2-8-135mm-Lens-Pentax-K-mount-/291851379498?&clk_rvr_id=1086354185802&rmvSB=true&clk_rvr_id=1104019952035&afsrc=1&rmvSB=true

with my Altair GPCAM to get some wider field images, and I've been using it with the supplied IR cut filter. Should I stop using the filter for objects such as M42? I took a test shot of it last night and the colours didn't seem right:

It 7 x 2 minute subs quickly processed in DSS (with mismatched darks). There seems to be a lot of blue and not much red. Is it a case of more subs? or is the filter causing this. I have read that you should use an IR cut filter with lenses to reduce star bloat.

[ronin]

Do you know which IR cut filter it is?

Since Ha is not really IR it shouldn't make any significant difference as the Ha should be passed by your filter. But I guess that some filters are more well defined then others. Additionally what is the "natural" colour of M42? Just about everything is processed and stretched that the colours we are used to seeing on an image will be very much artifical. Enter Messier 42 into Google then select Images, M42 is available in every colour combination you may or may not like. I have seen purple, red and green and M42 cannot be all of them.

[ollypenrice]

45 minutes ago, ronin said:

Do you know which IR cut filter it is?

Since Ha is not really IR it shouldn't make any significant difference as the Ha should be passed by your filter. But I guess that some filters are more well defined then others. Additionally what is the "natural" colour of M42? Just about everything is processed and stretched that the colours we are used to seeing on an image will be very much artifical. Enter Messier 42 into Google then select Images, M42 is available in every colour combination you may or may not like. I have seen purple, red and green and M42 cannot be all of them.

If an OSC or RGB M42 comes out green the imager has made a mistake. Simple as that. Ignoring false colour images it has a fairly narrow range of credible colours just based on the astrophysics and the result from carefully calibrated images. It lies somewhere on a red-magenta continuum. The colours are not, in my view, artificial if done properly. The star colour can be checked against spectral class, for instance. 

In the image posted above the stars are mostly showing a pale magenta so that can't be right, but the colour isn't a mile out. The cluster in the lower left are mostly blue stars (many are B class). 

I'd say just try with and without the filter.

Olly

[cuivenion]

Thanks guys, is it possible that the star colour is due to CA? As far as I understand it stopping down the lens a couple more steps would help with that wouldn't it?

[cuivenion]

1 hour ago, ronin said:

Do you know which IR cut filter it is?

Since Ha is not really IR it shouldn't make any significant difference as the Ha should be passed by your filter. But I guess that some filters are more well defined then others. Additionally what is the "natural" colour of M42? Just about everything is processed and stretched that the colours we are used to seeing on an image will be very much artifical. Enter Messier 42 into Google then select Images, M42 is available in every colour combination you may or may not like. I have seen purple, red and green and M42 cannot be all of them.

It's the IR cut filter that comes with the camera.