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Tuesday, March 23rd, 2010
 Another attempt at getting better focus with my 6" f/5 imaging Newtonian.   Transparency was below average, seeing average.....at least that's my best guess, and what was predicted by the Clear Sky Chart. 
 
First I used Neil's Bahtinov mask on Capella.....my alignment star.   Then I imaged Capella without the mask, using the natural star spikes created by the Newtonian, as a guide to focus. Neil stopped by and noticed that my Bahtinov mask image showed a slight focus error.
 
Undaunted, I decided to shoot some frames of M51, to see if my focus improved the image over the M51 shots made a couple of days earlier.   I ended up shooting 2 hours - 122 subs at an average of 62 seconds per sub [65 x 65 sec, 57 x 60 sec].   Afterwards, I took a few extra minutes...it was already 2:30am....to shoot five 60 sec Darks.  I reviewed the subs and dropped the worst ones.
 
Orion 6" f/5 imaging Newtonian
Celestron CG5 GT mount  (unguided)
Canon 350D  (unmodified)
Captured in Nebulosity
Aligned and stacked in Deep Sky Stacker
Processed in Photo Shop Elements with Carboni Tools.
114  subs at 62 seconds per sub @ ISO 800
5 Darks, 25 Flats, 20 Bias frames
 
I am still not happy with the result.  The image is noisy and grainy with mediocre resolution.  I am going to ask Neil (he doesn't know it yet, LOL) to load my data into his computer and process it.   I suspect the problem is in my processing.
 


Image Gallery For This Session
Batinov mask  Capella b Capella b
M51 3 22 10 DSS PS  35 M51 3 22 10 ISO 800 113 x 62 sec BTEMP
M51 3 22 10 ISO 800 117 x 62 sec b
   

Batinov_mask__Capella_b.jpg
Click picture above for larger version
If the picture seems too bright or too dark, try adjusting the brightness below.
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Comments:
On 03/23/10 at 02:01pm Barry wrote:
Greg,
That being the case, is there a fix? In other words would "guiding" minimize the problem?

On 03/23/10 at 01:31pm Greg wrote:
Barry, the M51 image shows stars that are almost perfectly round and bloated, suggesting poor focus. But I think there is a consistent deformation of the shape that suggests a tracking/guiding error. More precisely, I'm guessing that you have small, random vibrations in the mount. I say that because tracking and guiding errors are generally more "directional" - meaning that you get elongated stars in either RA or DEC direction. Even if both axes are problematic it's hard to get sufficiently randomized errors that the stars look round. But vibrations can definitely do that.



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