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Old 02-10-2014, 09:25 PM
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BruceG (Bruce)
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First Light for GSO RC8

Hi all, I was helping a good friend with his new RC8 on Monday evening and we found the first images not as amazing as we were expecting.

Below are the first images left to right tarantula with a CLS clip filter (taken from a very LP affected site), 10x90 sec subs and only a few darks with a spectrum modified 450D at 1600 ISO, the same target with Ha filter with 180 sec subs firstly in colour and then the same data in B&W, Finally an enlarged section of the first image to show the bloated stars with red and blue fringing.
He is using the Orion 50mm mini guide scope and starshoot autoguider camera. The focal length of the guide scope is 145mm and the RC8 is I believe 1600mm. Do you think the issue is poor guiding with the guide errors causing the fat stars or do you think it is an optical issue? The PHD2 graph was not too good and PA could also be improved.

Any insights or steps to investigate the issue will be greatly appreciated.
Bruce.
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Old 02-10-2014, 09:59 PM
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Quote:
Originally Posted by BruceG View Post
Hi all, I was helping a good friend with his new RC8 on Monday evening and we found the first images not as amazing as we were expecting.

Below are the first images left to right tarantula with a CLS clip filter (taken from a very LP affected site), 10x90 sec subs and only a few darks with a spectrum modified 450D at 1600 ISO, the same target with Ha filter with 180 sec subs firstly in colour and then the same data in B&W, Finally an enlarged section of the first image to show the bloated stars with red and blue fringing.
He is using the Orion 50mm mini guide scope and starshoot autoguider camera. The focal length of the guide scope is 145mm and the RC8 is I believe 1600mm. Do you think the issue is poor guiding with the guide errors causing the fat stars or do you think it is an optical issue? The PHD2 graph was not too good and PA could also be improved.

Any insights or steps to investigate the issue will be greatly appreciated.
Bruce.
GSO+Canon - 0.66 arcsec per pixel (ASP).
Miniguide+SSAG - 6.7 ASP
6.7/0.66=10.15

That's about the edge of the envelope. You should look for a ratio under 10. If it was a mono chip instead of a DSLR you could bin it 2x2 which would help a lot.

If the PHD graph (measure in ASP) is bumpy you need to improve the mount alignment. You didn't say what the mount was. That could be important too.
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Old 02-10-2014, 10:08 PM
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BruceG (Bruce)
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Quote:
Originally Posted by mithrandir View Post
GSO+Canon - 0.66 arcsec per pixel (ASP).
Miniguide+SSAG - 6.7 ASP
6.7/0.66=10.15

That's about the edge of the envelope. You should look for a ratio under 10. If it was a mono chip instead of a DSLR you could bin it 2x2 which would help a lot.

If the PHD graph (measure in ASP) is bumpy you need to improve the mount alignment. You didn't say what the mount was. That could be important too.
Thanks for the guidescope to imager info Andrew - the mount is an EQ6 on a new pier - Cheers.
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Old 02-10-2014, 10:21 PM
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cometcatcher (Kevin)
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With focal lengths over 1500mm most people move on to an OAG.
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Old 03-10-2014, 05:17 AM
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What about the achromatic abbération? Any thoughts?
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Old 03-10-2014, 09:10 AM
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traveller (Bo)
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Very interesting discussion so far.
Following from Andrew's comments, I did a quick spreadsheet of my scope and camera set up and then did a traffic light analysis of each combo.
I was conservative in my analysis as I am a noob and live in heavily light polluted inner Melbourne.
(Sorry to hijack the thread )
Bo
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Old 03-10-2014, 09:53 AM
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What about the achromatic abbération? Any thoughts?
Can't be. RC's don't have any lenses. One would have to suspect either the clip filter or in camera settings.
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Old 03-10-2014, 11:31 AM
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Very good point. Will try sans filter and to ensure PA is better aligned before next session.
Thanks Kevin.

Last edited by BruceG; 03-10-2014 at 01:28 PM.
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Old 03-10-2014, 11:32 AM
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[QUOTE=BruceG;1122436]Very good point. Will try sans filter and ensure PA is better aligned before next session.
Thanks Kevin.
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Old 05-10-2014, 02:14 PM
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Nice shots Bruce, just a thought with the colour fringes. Did you refocus after each filter change as some brand filters are not parfocal.
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Old 05-10-2014, 03:10 PM
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Yes we were pretty fanatical about focus each session began with focusing in BYEOS and confirmed by a bahtinov mask. Perhaps it is more critical to guide in that area being closer to the pole?? I just setup last night with a very bright moon to improve my PA and tried the drift align routine in PHD2. Seems to have sorted it mostly although I dont have any lowish views to either the East or West due to tall trees, so declination is set using a supposedly accurate digital incidence gauge (not an iPhone app).
Anyhow to get back on topic, after the alignment I took 50x1 minute subs if the Tarantula area and I was not at all pleased with the results as seen below: Heavily cropped to get rid of the worst of the field aberrations (without my flattener / reducer). This was my Megrez 90 from Blue Mountains. What are your thoughts for astrophotography in this region?
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Old 05-10-2014, 03:48 PM
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Hi Bruce

Did you guys check collimation on the RC?

It can get knocked about in shipping but I have found that once done its pretty much stable especially if pier mounting on a permanent basis.

Easiest way to check is to find a busy star field, say around NGC104, get focused, then defocus till you see the rings and take an exposure. Check all the corners to see if the stars are pointing into the centre and also get a bright star in the center of the FOV and make sure the shape of the circles is exactly round. I used to use BYE focus screens to help with that but do a search for ALs Collimation tool on this forum which is a godsend

You should be able to get it right just with the secondary adjustments. Note though it's worth investing in a set of Bobs Knobs.

Last edited by DJT; 05-10-2014 at 04:27 PM. Reason: More info
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Old 05-10-2014, 10:45 PM
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Great , thanks David. We did wonder about collocation. Me not being familiar in any way with RC's didn't know how to make a diagnosis. Now at least we have some stuff to try. Thanks for your valuable input!
Bruce.
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Old 06-10-2014, 12:55 PM
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No worries

Instruction leaflet here. Basically same scope, different brand but will get you the same result. Seem to recall the GSO didn't have one in the box when I got mine.

https://www.astronomics.com/document...c%20manual.pdf

Really small adjustments needed. Don't go mad.You shouldn't need to touch the main mirror and if new to this, and are not sure, just focus on the secondary and see how you go.
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Old 06-10-2014, 01:21 PM
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That's great David - thanks heaps - you're right, there were no instructions. My friend is away for the weekend so this will have to wait for next weekend to get checked. Cheers,
Bruce.
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Old 09-10-2014, 07:03 AM
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First light with RC8

Hi All,
I am the friend that BruceG has very kindly sought advice for. It's about time I popped up in the exchange. Great thanks to all who have so far contributed their thoughts and particular thanks to David for the AT8RC manual link. Very informative.
I have just purchased a Thin OAG so our next steps will be to fit this for the SSAG. I have also purchased a Concentric Ring attachment for my Howie Glatter laser collimater as recommended by Howie himself (really hope this is just a backup - all that I have read about collimating R-Cs terrifies me!).
We'll report back on our results.
Clear skies.
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Old 30-10-2014, 11:52 AM
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christian.ckr (Christian)
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Hey Bruce,

I had the same imaging scope and guidescope as you have used but I used an NEQ6 and a 7D, my results are similar to yours but I never had issues using the starshoot autoguider package at 1600mm, maybe PA or PE is your main issue...

I know it says LCM instead of LMC but every time i try to change it, flickr gives me an error... https://www.flickr.com/photos/kriis__xx/9719713829/
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