From: "Milton Aupperle" <milton@outcastsoft.com>

Date: January 22, 2009 3:07:01 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: M1 "The Crab Nebula"

Hi Folks;

I managed to image M1 the Crab Nebula this week:

http://groups.yahoo.com/group/Astro_IIDC/files/DSO/M1CRAB_20090119_MJA.jpg

Even with the IDAS LPR Filter and 10 minute exposures, M1 is a tough

target to separate from the light pollution here, as sky background is

23,500 out of 65,535 and M1 is a maximum of 28,500 out of 65,535 in

the Luma frames for 10 minute exposures.

Turbulence was not too excessive (+/-1.9 arc seconds) with sporadic

spikes to 6 arc seconds, which Astro IIDC handled very well while guiding.

I also discovered that the guide prism for the Celestron OAG is 

causing darkened shadowed areas on the Grasshopper 2/3" EXHAD CCD,

which is why the left side of the image is so much darker. For my flat

frames, it causes a darkening of around 3,500 units out of 65,535

which does not seem to be too much, but it actually is huge when

processing. As I rotated the guide camera around the Axis, this

darkened different areas on the CCD opposite the prisms side, so my

flat frames were useless. I added a step out ring that shortens the

amount the prism extends into the light path by 4 mm and that has

solved it. The total variance across the 1384x1036 CCD from corner to

corner is now < 500 units for flat frames. This didn't happen with my

1/3" Flea camera but the 2x sized Grasshopper CCD it sure is. I never

really noticed it until I started taking longer exposures with high

light pollution backgrounds and couldn't figure out why the Flat

frames didn't compensate for it.

Hope that helps someone..

Milton Aupperle