8.3.10

Cold fingers & M 96

Collimation on my main scope was continuing to be a problem despite the new spider. Although last night was nice and transparent again I resolved to solve the issue before commencing any imaging. Although the collimation looked good through an eyepiece it wasn't translating to a flat field on the camera. After spending considerable time not really improving things using well-tried methods I remembered that Metaguide gave some instructions on using the software for adjusting collimation so decided to give that a try. Within a few minutes the problem was sorted with very little trouble. Out of curiosity I put an eyepiece in to see what the collimation looked like and couldn't believe the amount of offset between the main & secondary mirrors that had been applied (interesting!). By now my fingers and ears were freezing so I hurriedly set up on M 96 so I could get in the warm and start doing some imaging at last. Details: 10" f 4.3 Newt. 10x 10 minute exposures @ iso800 with CLS in DSS, CS2, Photobrush & XAT.

6 comments:

簡單嗎 said...
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caracol_uk said...

Pete,

Seems like there's always something that needs tweaking :-). Nice to see you made the most out of the recent batch of clear nights. I've now had a proper chance to see the effect of the CLS clip in filter and although it does increase contrast, it does make everything appear blue. I even tried a quick exposure of Mars and that too came out blue! I was wondering, do you have the same problem (I'm not sure if having a modded camera would offset this) and if so do you resolve the colour balance in processing or using a custom white balance in the camera?

Simon

caracol_uk said...
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P said...

Simon,

I didn't start using the CLS until after I had my camera modified so have never seen what it does on an unmodded camera. The CLS is essential to get through the light pollution but does need longer exposures. You do need both to get the best out of Ha emissions. Images are usually heavily processed colour-wise as the stacking changes things anyway. Look at the subs I have put up as examples on some of the posts. The bluer they are the better the transparency.

Pete

P said...

PS. I leave the white balance on the daylight setting when doing astro with the modded camera. I use custom WB for doing normal photography.

Pete

caracol_uk said...

Thanks Pete, that's useful to know. I've spent much of today carrying out tests with my camera as it's much easier to do this with normal daylight exposures. I've been using Nebulosity which I now understand always downloads RAW files from the camera thus ignoring any custom white balance I had configured on the camera.

Using a colour test page I tried to adjust the colour offsets within Nebulosity to produce the same result in the image as on the page. This is quite difficult to do and I eventually realised that whatever I did the image would not appear the same as the test page. I've checked the specification of the CLS filter and it confirms that pretty much all the yellow/orange/red part of the spectrum is blocked by the filter. So, the signal is just not there so a white balance would only help a little anyway.

If the camera were responsive to Hydrogen Alpha I think that would help quite a bit, but as my camera is not modified the CCD is not sensitive to that wavelength.

It also seems that Nebulosity isn't that good at correcting the imbalance as it converts from RAW to RGB mode. Even if Nebulosity is forced to use RGB it will still download RAW and do an "on the fly" conversion to RGB. Interestingly if I capture RAW mode files using Nebulosity and demosaic them in Deep Sky Stacker the results are better. The reds seem to appear OK, though the yellows look a bit greenish. A bit of a shame really because I quite like Nebulosity. I think I'll continue to use Nebulosity to capture files using RAW mode and will simply stack them using Deep Sky Stacker.

Of course, being colour blind I have an excellent excuse for not getting colour rendition correct!

Simon