[FedCom] First light testing of new homemade 18" CollapsibleClassical Cassegrain

[Richard Crisp]

sorry, wrong list

I was trying to send this to an astrophotography forum.

please disregard

----- Original Message ----- 
From: "Richard Crisp" <rdcrisp at earthlink.net>
To: "Discussion of Federal Government Communications" 
<fedcom at mailman.qth.net>
Sent: Sunday, October 31, 2004 12:05 AM
Subject: [FedCom] First light testing of new homemade 18" 
CollapsibleClassical Cassegrain


Last fall I kicked off the design of an 18" collapsible convertible 
cassegrain/newtonian scope. After waiting for nearly 9 months for all the 
parts to arrive, I was able to finally assemble the scope about 10 days ago. 
Then finally last night the skies cleared enough for me to begin testing it.

I was expecting to have to solve problems first night under the stars and I 
wasn't disappointed in that regard. The key one was coming up with a method 
of taking flats. Normally I shoot my flats in the daytime and with an open 
frame such as I have with the 18" cass I expected that would cause me some 
problems.

One thing that has been identified as an issue is the fact that my baffling 
permits "Flooding" of light past the secondary baffle to shine directly down 
the primary baffle. That causes lots of problems including shooting flats. 
That is sort of the next big task for the scope is to correct that. The 
baffle maker is planning to make me an extension baffle for the secondary 
mirror and that should cure the problem.

I also had some vignetting due to the way I assembled my beam splitter to 
permit me to use an AO7 to guide this F=5760mm beast. Once I shot the images 
last night that was apparent and I believe I corrected the problem today, 
but the clouds haven't permitted me to test it.

I also made a small baffle to insert into the primary baffle to cut off the 
flooding rays and it looks like that will at least permit me to take 
twilight flats, which I was able to do today. Unfortunately as I mentioned, 
the clouds rolled in shortly after dark so I am unable to test the items I 
corrected since the images were shot last night.

The first object was the Bubble Nebula, NGC7635. I shot this as a simple 
RGB. With the 9 micron pixels of the FLI IMG6303E camera used at F=5760mm, I 
decided to run them binned 2x2 giving me a 4 x speedup in exposure time yet 
keeping a respectable 0.64 arc-sec/pixel image scale and 1.5Mega pixels of 
18 x 18 micron equivalent size.

So this image has the resolution of an ST8E with pixels that are four times 
the area.

The Bubble was shot as a series of 3 minute exposures, again binned 2x2. 
That is like taking 12 minute exposures unbinned from a depth of exposure 
perspective.

The second image was the Trapezium region of M42. This was a series of 15 
second RGB exposures and a series of 30 second Halpha exposures. I blended 
Red and Halpha to use for the Red channel and used the Halpha for the 
Luminance channel

Because neither image was flat fielded, I have lots of ugly artifacts 
including dust motes from the optics, some ugly weak columns and other such 
blemishes that are usually corrected by flat fielding.

The things that I like are that the stars are basically round, and are 
splitting nicely. The AO7 and beam splitter did a fine job of guiding the 
AP1200GTO which has a counterweight bar completely full of weights.  That is 
6 x 18 lbs + 1 x 25 lbs + 2 x 5 lbs = 143 lbs of counter weights!

Hopefully now that things are more or less ready to test for a second night 
I will get a chance on Sunday night.

http://www.narrowbandimaging.com/ccc18_first_light_page.htm

or for scope details;

http://www.narrowbandimaging.com/cc18_page.htm
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