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Old 09-09-2013, 07:27 PM
E_ri_k (Erik)
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G2V Calibration

Hey guys, I have been reading about white balancing, and calibrating exposure ratios using a G2V star, for RGB imaging. I found a few lists naming suitable stars. I have read ALOT of different pages. Our Sun is a G2V star, yes?

Is there any reason why you couldn't use the sun to calibrate? Obviously not using the telescope, so not to start a fire and destroy the camera! But perhaps with a camera lens stopped down to the maximum?

Or am I missing something, and it's this a stupid idea......

Erik
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Old 10-09-2013, 08:18 AM
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I think that even with your camera stopped down to max you would still over saturate your camera, blind yourself and possibly ruin your camera lol.

I've been reading a bit on this as well as I'm have a hard time getting this right, and from what I've read so far you are better of with a mag 4-6 G2V star so as not to saturate the pixel. otherwise your calibration will be off.

I've downloaded a trial version of CCD Autopilot as it has an automated calibration routine for G2V as well as a automated filter offset routine. The trial is 60 days and is fully functional. I'll probably end up getting the professional version, a bit exy but what isn't in this game.

Cheers
Sandy
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Old 10-09-2013, 08:38 AM
E_ri_k (Erik)
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Thanks Sandy. With my kind of luck I'd wreck something and manage to start a fire somehow haha.

I was reading on Stargazers Lounge, where someone had imaged a white piece of paper in direct sunlight, and a pinhole in the lens cap of the scope. Apparently it worked?

It's cloudy of the daytime here as well as the nights haha, so I might do some more reading on the subject!

Erik
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Old 11-09-2013, 07:28 AM
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Hi Erik. The 2nd method you mentioned seems reasonable. Another alternative we used to use for calibrating spectroradiometers is to use a D65 lamp from an Over Head Projector (OHP). These are designed to mimic the sun's colour at 6500K. Imaging the light produced by these on a spectrally flat surface (PTFE/Teflon) should give you a decent colour balance.

Cheers,
Cam
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Old 11-09-2013, 08:25 AM
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Quote:
Originally Posted by E_ri_k View Post
..., and it's this a stupid idea......
Well, I completely agree with you on that one...
Any type of G2V calibration or anything involving Sun and telescope without filter is not so "bright" idea.
Why not just use G2V stars?
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Old 11-09-2013, 08:30 AM
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multiweb (Marc)
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Originally Posted by E_ri_k View Post
Is there any reason why you couldn't use the sun to calibrate?
Only if you're quick.
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Old 11-09-2013, 09:48 AM
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Erik,

I used G2V calibration quite seriously for a while (doing all the extinction calculations with a spreadsheet, etc.) and found it sometimes produced decent results. These days I often use eXcalibrator which is much quicker and easier so long as you can plate solve. It's worth checking out: http://bf-astro.com/excalibrator/excalibrator.htm

I generally use a variety of methods to determine ratios for my colour combine and then pick the version I like the most. Most people appear to want pleasing rather than accurate colour

Cheers,
Rick.
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Old 11-09-2013, 01:57 PM
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I generally use a variety of methods to determine ratios for my colour combine and then pick the version I like the most. Most people appear to want pleasing rather than accurate colour .
Rick makes a really good point - if you really needed to be accurate then you'd need to form an actual series of spectroscopic data points. Namely you'd want to have a calibrated blue and red point as well, at the very least.

Cheers,
Cam
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Old 12-09-2013, 08:15 AM
E_ri_k (Erik)
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Originally Posted by orion69 View Post
Well, I completely agree with you on that one...
Any type of G2V calibration or anything involving Sun and telescope without filter is not so "bright" idea.
Why not just use G2V stars?
Haha, thanks I'm glad you agree, however I wasn't considering pointing the scope at the sun, completely open and unfiltered, I think that would be worthy of an idiot award! Just thinking of different ways of doing things, using a nearby source. Maybe there is a reason why there wasn't much info on this method
No big deal. I'm sure I can find a G2V star.
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Originally Posted by multiweb View Post
Only if you're quick.
Lol
Quote:
Originally Posted by RickS View Post
Erik,

I used G2V calibration quite seriously for a while (doing all the extinction calculations with a spreadsheet, etc.) and found it sometimes produced decent results. These days I often use eXcalibrator which is much quicker and easier so long as you can plate solve. It's worth checking out: http://bf-astro.com/excalibrator/excalibrator.htm

I generally use a variety of methods to determine ratios for my colour combine and then pick the version I like the most. Most people appear to want pleasing rather than accurate colour

Cheers,
Rick.
Thanks Rick. For me, at this stage I am generally looking for a pleasing colour, however I would like it to be an accurate representation . My concern was if I used equal exposure lengths, I would end up with a completely messed up image! However I don't know how severe it might be. If it's only subtle, I'm sure that can be processed out? It's been horrible weather here for a long time now and I haven't had a chance to test anything. When you say it "sometimes" produced decent results, what do you think caused them to be "not decent"? I have never plate solved before, but I'll read into it and check out your link.
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Originally Posted by LightningNZ View Post
Rick makes a really good point - if you really needed to be accurate then you'd need to form an actual series of spectroscopic data points. Namely you'd want to have a calibrated blue and red point as well, at the very least.

Cheers,
Cam
Thanks Cam. It's not that critical to me (at this stage) to be extremely accurate. I'm just after a generally accurate colour.

Thank you everyone for your input. I think what I'll do is just find a G2V star and give it a shot?

Erik
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Old 12-09-2013, 02:50 PM
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Originally Posted by E_ri_k View Post
When you say it "sometimes" produced decent results, what do you think caused them to be "not decent"?
G2V calibration using standard extinction factors is only a rough approximation. Atmospheric conditions vary over time, etc. I found that the results didn't always look as expected.

Quote:
Originally Posted by E_ri_k View Post
Thank you everyone for your input. I think what I'll do is just find a G2V star and give it a shot?
Certainly a worthwhile experiment. You could even do it at a couple of different altitudes and see if extinction factors produce results that match your experimental values.

Cheers,
Rick.
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