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Old 12-01-2011, 01:56 PM
Eris
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Imaging problem using the Gemini System

Hi everyone,
I have recently upgraded my Losmandy C11 with a Gemini Goto system. While everything is going well in that department I'm finding that when imaging in 'Photomode' using CCDops with a SBIG ST2000 stars look slightly elongated after about 1minute of exposure. It's not a polar alignment issue as that's already been done with a polar alignment scope and the main scope working at f6.3 is balanced.
On re-reading the manual I have gone back to the box and discovered a component that was not attached (see images) that I'm thinking may be a "tracking interface cable"???.
If it is should this part be attached to the CCD connecter which in turn will send corrections to the drive to offset drift of 10 pixels + ?

I would appreciate any constructive advice you can give.

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  #2  
Old 12-01-2011, 02:32 PM
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Is this an SBig optoisolator?
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Old 12-01-2011, 08:36 PM
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wasyoungonce (Brendan)
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Looks like an Isolator. The ST-2000 has a self guiding sensor. Just plug this in line between the ST-2000 and the auto-guide port.

But I ask "what type of motor plugs do you have on your Gemini"?.."the old flat telephone type, RJ12 and cables/connectors" or "the newer round DIN type connectors"? This is important!

This is important as the older Gemini units must have the "opto isolator" in line between the guide head and the Gemini auto-guider port.

Page 42 of the Losmandy Gemini manual states:
IMPORTANT NOTICE:
(1) Never connect TTL output stages directly to the Hand Controller or auto-guider ports of an older version Gemini! Severe damage may occur to the connected devices. Use a Relay Box (as provided by SBIG) (aka your opto isolator) or the optional Losmandy Optocoupler unit instead. By so doing, you get the additional advantage of providing electrical protection to these sensitive units.
..........
Detailed instructions for connecting an SBIG camera with TTL outputs to the older version of Gemini can be found in Appendix 8.6 (Autoguider Port).

Appendix 8.6 states:
8.6.1 Connecting Autoguiders with TTL Outputs to Older Gemini
Systems SBIG ST-237/ST-7/ST-8/ST-9/ST-10/ST-2000 series cameras with TTL outputs must be connected using the SBIG Relay Box, the optional Losmandy Optocoupler or equivalent when using an older version of Gemini.


I don't want to scare are alarm you but.....you may have damaged your Gemini unit (or camera). I do not know enough to tell at the moment ....so you may be best to head over to the Gemini Yahoo group for more info. Others more wise than me can chime in here....


Next...you stated "...It's not a polar alignment issue as that's already been done with a polar alignment scope..".

In reality any polar scope alignment will not be accurate enough for imaging, you need to perform a "drift alignment".

Last edited by wasyoungonce; 12-01-2011 at 08:57 PM.
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Old 12-01-2011, 10:26 PM
Eris
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Thanks for your suggestions and advice. The Gemini console has the newer round DIN type connectors. Does this mean it is ok to connect it?
When I added the Gemini to the CG11 scope I also replaced the old stepper motors with servo motors.
I see what you mean about drift alignment as it looks like there is some field rotation in a small section of an image I'm attaching.
I thought the "Track & Accumulate" option n CCDOPS would have counteracted this though.
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Old 13-01-2011, 08:21 AM
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For some reason I was thinking you had the older Gemini model (you are entitled to slap me silly)....since you have the round DIN motor connectors...you are sweet..all is ok.

You can still use the opto isolator in line with your auto-guider. It doesn't need it but some people like the extra "isolation" protection it gives.

Sorry I have never used CCDOPS so I cannot comment on it's effectiveness. I still suspect a good drift align is the best method.

Ok your image is not that bad. It's a bit hard to tell but it looks like some field curvature and maybe the CCD is not quite square on. Is your spacing to you CCD correct from the .63 flattener? This is the biggest influence of field curvature. I have made this mistake myself , recently .

As I said It doesn't look like miss-tracking as I would have suspected that the self guiding head would be the bee's knees' in tracking. I suspect the guide head also does it's own calibration routine but you can go into the Gemini menu and set the Gemini guiding rate between 0.2x sidereal and 0.8x sidereal. This may help your guide head corrections (although I don't think this is the issue).

Something that may also help is the Gemini ASCOM driver. You can use this like the Gemini GCC software..but this one is free (you need Microsoft net 3.5 and ASCOM 5.0a (or more) installed for this to work, just read the requirements).

With this you can check all your Gemini settings with this ASCOM driver and save them to and from the Gemini/Computer.

Well that's about all I can add apart from saying it looks like field curvature and I'm probably completely wrong! but don't rule out polar alignment.
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Old 13-01-2011, 01:21 PM
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Hi again. Thanks for your reply. I'll check the spacing and see what happens.
Hopefully there will be some clear sky here on the weekend to test these things out.
As for the drift alignment issue the website below is quite interesting. I believe the simulator training helps before going out to do the real thing.
I'll do this as well and see what improvement it makes.

http://www.petesastrophotography.com/
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Old 13-01-2011, 03:44 PM
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yep thumbs up do post back.

Also consider your image scale. You are imaging at around .86 arc seconds per pixel, now it's possible (I have done .54 arc sec's per pixel) but difficult due to "everything and anything".

Maybe try 2x2 binning?
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Old 13-01-2011, 05:55 PM
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Thanks for the suggestion. I'll put it on the list for the weekend if it clears up. Taking great images seems to elude me. There is no substitute for knowhow and experience. I'll keep plodding on and let you know the outcome.

Cheers
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Old 13-01-2011, 06:10 PM
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There are way more knowledgeable folk on this and the Losmandy and Gemini Yahoo forums. Maybe also check out yahoo SBIG forum.

What I know...you could fit in writing on a postage stamp!
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Old 16-01-2011, 08:46 PM
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I had another go last night to try and correct some of the problems mentioned earlier. I'm starting to appreciate that the little things all need to be accounted for while trying to beat the weather. I think I'm going to need more practise especially with fine tuning in polar alignment and collimation. I've included a pic of Omega Centauri one of last night efforts.

Cheers
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Old 16-01-2011, 09:21 PM
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Hi Eris. Looks good.

You can post jpg images up to lets say around 800 ~1000 pixels wide. This makes them much bigger and easier to see for forum member to evaluate.
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Old 16-01-2011, 09:59 PM
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Thanks for the info. Here 'tis.
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Old 17-01-2011, 08:26 AM
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Ok I had a look at the stars at the corners.

Looks like some field curvature (that is it looks as if the stars are flowing into/out of the centre of the image), not a lot though and this error is small. This is normal, all image planes are curved and the CCDs are flat. now if someone could make a curved sensor

Also, the stars are all slightly elongated to the left, showing a little tail as if: the image plane is tilted; or it's not quite tracking correctly on one axis. The error is small. This can probably be corrected in Photoshop.

The images here are from your and are crops. They look worse than the original because I had to cut them from a small jpg and enlarge them as a jpg, hence they look more fuzzy etc: Top left crop. bottom left crop. Bottom right crop. Top right crop. Middle left crop.
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Old 19-01-2011, 06:39 PM
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Thanks for your analysis. I'll keep trying to refine my technique and improve on my image taking. I'll also need to explore ways on how I can fix up these distortions in Photoshop.

Cheers.
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Old 19-01-2011, 08:23 PM
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Star repair...yep can help here..only a little:

atalas.net videos
Waid observatory.
Bf-astro.


Or the professional videos like adam block. There are quite a few like this
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Old 20-01-2011, 10:49 PM
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Hi again,
Yes I have Don Waid's CD and went back to it after your suggestions. The smudge tool in Photoshop does a great job and is very useful on a small selection of stars but I think I would pull my remaining hair out doing this job on all the stars on the outskirts of Omega Centauri.
I also had a look through Don Waid's tutorial on "Star elongation repair using the offset filter" and went through the process in Photoshop. While it looks straight forward I was not able to reproduce the blue selection line he shows (unless it was one of his highlights) which was necessary to use as a reference point when going to the next quadrant to repeat the process.
Could you let me know how to bring up this coloured selection line (not marching ants) when this selection is made?

Thanks again for all of your suggestions so far.
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Old 21-01-2011, 08:40 AM
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Sorry I do not have any of those CD's.

I have applied star repair using offset filter a few times, but, with my experience level I really could give instructions as I'm still following others instructions.

So I could only Parrot what others say...add nothing useful.

Sorry.
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Old 21-01-2011, 07:29 PM
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Thanks anyway. I'll plod along with it over the weekend and see what happens. No doubt something needs to be turned on or another box ticked somewhere in the menu.
Cheers
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