freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Autoguided PE -- how good is good?
[Re: idahoman]
#2973741 - 03/09/09 01:34 PM
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Thanks for the replies. From memory (I'm still figuring all this stuff out -- ha, ha) I think my FWHM's are about 4 to 5 arcseconds. What does that mean? Is that an indication of seing?
Hi-
For guiding quality the fwhm of raw frames of decent length - say 5-20 minutes - is a key thing to go by. It is affected by seeing, focus, guiding, and pixel scale to some extent.
If seeing is decent and guiding is good, I think fwhm's should be down around 2.5". If you have AO and a brightish guidestar, it should go to 2.0" and below.
I don't know of a fundamental difference between GEM and fork - I would think it mainly depends on the quality of the bearings and gears.
I am in the northeast U.S. not known for great seeing, and I guide a CGE and C11 to get regularly 2.5" and below fwhm - sometimes even to 1.5" - and that is without AO.
Examples are at:
MetaGuide C11 CGE images
Another thing is - guiding errors can be misleading and much smaller than the true error seen at the imaging camera. So fwhm's always tell the full story.
I think you should be able to get smaller fwhm's with AO, especially on a night of good seeing. But if seeing is really bad - maybe it is a fundamental limit for you. Even then - watch for nights of steady air.
Frank
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Jaxdialation
Post Laureate
Reged: 04/01/07
Loc: Northeast, FL
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Re: Autoguided PE -- how good is good?
[Re: freestar8n]
#2975516 - 03/10/09 10:52 AM
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I think there are too many variables, which are unknown to give 2" as an expected outcome.
Quote:
If seeing is decent and guiding is good, I think fwhm's should be down around 2.5". If you have AO and a brightish guidestar, it should go to 2.0" and below.
I don't know of a fundamental difference between GEM and fork - I would think it mainly depends on the quality of the bearings and gears.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Autoguided PE -- how good is good?
[Re: Jaxdialation]
#2975690 - 03/10/09 12:48 PM
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I think there are too many variables, which are unknown to give 2" as an expected outcome.
One thing to look at is if someone nearby with an AP1200 is getting below 2" fwhm, then seeing isn't a fundamental limit. And a medium grade mount plus AO with a bright guidestar should make the mount less of an issue if things are tuned right.
I guess another question is, what are the fwhm's without AO? If they are also around 4", then the AO may not be operating correctly.
A basic thing is just to make sure it is well focused in the first place. What are the fwhm's in a 0.5 second exposure of brightish stars without AO?
Frank
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TxStars
Pooh-Bah
Reged: 10/01/05
Loc: Lost In Space
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Re: Autoguided PE -- how good is good?
[Re: freestar8n]
#2979359 - 03/12/09 10:11 AM
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The problem is not your mount, it is the scope.
If you want smaller stars get a Refractor...
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Jaxdialation
Post Laureate
Reged: 04/01/07
Loc: Northeast, FL
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Re: Autoguided PE -- how good is good?
[Re: TxStars]
#2979377 - 03/12/09 10:27 AM
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All the guys with RCOSs would probably take exception to your recommendation.
Which refractor would you suggest to replace a 9.25" at f6? Would it drop right into his existing fork mount?
I don't know that there is a "problem".
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Miguel Lopes
scholastic sledgehammer
Reged: 01/04/07
Loc: Portugal
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Re: Autoguided PE -- how good is good?
[Re: TxStars]
#2979648 - 03/12/09 01:08 PM
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The problem is not your mout, it is the scope.
If you want smaller stars get a Refractor...
This guy (spoiler: great images!) might disagree with you:
http://blog.deepskycolors.com/gear.html
Also a SCT is very good for photometric stuff.
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TxStars
Pooh-Bah
Reged: 10/01/05
Loc: Lost In Space
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Re: Autoguided PE -- how good is good?
[Re: Miguel Lopes]
#2982007 - 03/13/09 05:08 PM
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This is what I was looking at...
OP " I wonder if my stars could be made smaller by better guiding?"
1)Find a mountain top and still dry air
2)Faster f/ ratio scope
3)___________
Fill in the blank..
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Jaxdialation
Post Laureate
Reged: 04/01/07
Loc: Northeast, FL
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Re: Autoguided PE -- how good is good?
[Re: TxStars]
#2982591 - 03/13/09 11:26 PM
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So, after repeating his question, you didn't answer it? 
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Strgazr27
Reged: 10/04/04
Loc: StonyHill Observatory
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Re: Autoguided PE -- how good is good?
[Re: TxStars]
#2982695 - 03/14/09 12:16 AM
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Quote:
This is what I was looking at...
OP " I wonder if my stars could be made smaller by better guiding?"
1)Find a mountain top and still dry air
2)Faster f/ ratio scope
3)___________
Fill in the blank..
1. Some of the best seeing in the US is 3' Above sea level and about 20 yds from the gulf of Mexico
2. Although this will allow for shorter exposures it does NOT guarantee smaller stars
3. Hmmm.....Some of the best images I have seen taken have come from mirror based systems so there are several choices I could use to fill in #3.
I don't see how this helps the OP.
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idahoman
professor emeritus
Reged: 11/25/05
Loc: Idaho, USA
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Re: Autoguided PE -- how good is good?
[Re: Strgazr27]
#2986832 - 03/16/09 11:07 AM
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Thanks for all the really good input. One thing about my setup is that I am imaging at something like 1600mm focal length. Which is more demanding and will lead to "bigger" stars than imaging at, for instance, 800mm. Nonetheless, I've considered swapping the 9.25 out and mounting one of those new, cheap, RCs in the forks instead. But, I am not convinced if I will see a noticeable improvement since the focal length will be about the same.
I think where I am at is:
1.) I may be able to slightly improve my focusing
2.) Up my AO-8 guide rate by using better guide stars (if I can find them!)
3.) Wait until Jared sends me his Mach1 (I haven't given up on this one, yet)
4.) Live with what I've got until I can make a significant upgrade (not certain this can happen).
This upgrade would have to be more than just a CGE or Losmandy mount. My guess would be that I would have to consider an AP mount and possibly different optics as well.
The worse thing I could have done, at this point, is shell out $3K for a CGE or Losmandy, modify my pier, defork my C9.25, debug the new system, and have no noticeable difference other than meridian flips to deal with.
I've learned a lot. But, as with everything else in this hobby, it is a frustrating learning process.
Maybe a new high quality mount will appear out of China?
Oh -- I will check with some of my astro buddies to see what fwhm's they are getting.
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Miguel Lopes
scholastic sledgehammer
Reged: 01/04/07
Loc: Portugal
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Re: Autoguided PE -- how good is good?
[Re: idahoman]
#2987262 - 03/16/09 04:01 PM
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Forgot to add this:
a color camera allways has worst FWHM than a mono. Not only because of lack of resolution but also because you need more time to get the same quantity of photons...
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Psyire
Pooh-Bah
Reged: 06/24/07
Loc: 55* North
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Re: Autoguided PE -- how good is good?
[Re: Miguel Lopes]
#2987332 - 03/16/09 04:39 PM
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idahoman, I think you nailed it. If I were in your shoes, I think a CGE-PRO might be the answer... but the jury is still out on those. You'll have to wait and see I guess.
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Strgazr27
Reged: 10/04/04
Loc: StonyHill Observatory
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Re: Autoguided PE -- how good is good?
[Re: Miguel Lopes]
#2987661 - 03/16/09 07:40 PM
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Forgot to add this:
a color camera allways has worst FWHM than a mono. Not only because of lack of resolution but also because you need more time to get the same quantity of photons...
A OSC camera produces a MONO image until it is debayered so how does this increase the FWHM? Wouldn't pixel size play a larger role than the fact your using a OSC camera.
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Miguel Lopes
scholastic sledgehammer
Reged: 01/04/07
Loc: Portugal
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Re: Autoguided PE -- how good is good?
[Re: Strgazr27]
#2987765 - 03/16/09 08:32 PM
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Quote:
Quote:
Forgot to add this:
a color camera allways has worst FWHM than a mono. Not only because of lack of resolution but also because you need more time to get the same quantity of photons...
A OSC camera produces a MONO image until it is debayered so how does this increase the FWHM? Wouldn't pixel size play a larger role than the fact your using a OSC camera.
Well, FWHM, as the name says, is the full width at half maximum of a star bell-shape. Due to resolution loss from the debayering (compared to a mono camera), the star will have a smaller SNR, the peak value will be minor, therefore the FWHM will be higher.
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Strgazr27
Reged: 10/04/04
Loc: StonyHill Observatory
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Re: Autoguided PE -- how good is good?
[Re: Miguel Lopes]
#2987828 - 03/16/09 09:07 PM
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But for arguments sake most stars are composed of mainly "White" light. in other words, they are an RGB object. That being true, am I not using all of the pixels in the image of my OSC to compute my FWHM? You would be correct if we were shooting through a single color filter but we're not. Correct me if I'm wrong but a 6 MP mono camera and a 6 MP OSC camera should have the same resolution if the pixels are the same size and we're shooting an RGB object no?
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jrcrilly
Refractor wienie no more
Reged: 04/30/03
Loc: NE Ohio
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Re: Autoguided PE -- how good is good?
[Re: Strgazr27]
#2987835 - 03/16/09 09:11 PM
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But for arguments sake most stars are composed of mainly "White" light. in other words, they are an RGB object. That being true, am I not using all of the pixels in the image of my OSC to compute my FWHM? You would be correct if we were shooting through a single color filter but we're not. Correct me if I'm wrong but a 6 MP mono camera and a 6 MP OSC camera should have the same resolution if the pixels are the same size and we're shooting an RGB object no?
They would, if you were looking at the raw data from the chip instead of the interpolated data that is actually used.
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Strgazr27
Reged: 10/04/04
Loc: StonyHill Observatory
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Re: Autoguided PE -- how good is good?
[Re: jrcrilly]
#2988038 - 03/16/09 11:06 PM
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So resolution is lost in the interpolation John? And by that I guess you mean the debayering process? When we speak of OSC cameras are we talking about strictly OSC CCD cameras or are we grouping DSLR's in that group also? I ask as I would think the AA filter in todays DSLR's would have more of an impact on FWHM numbers due to the slight blurring of the image. What if one were to take the FWHM before debayering? I know this is possible in ImagesPlus V3
I would think that image scale would play a bigger part. If my image scale is 2.5 AS/Pixel. If I'm testing on a good night and my seeing is at or better than that my stars would fit on a single pixel making an accurate FWHM reading inaccurate correct?
Sorry if I got off track I'm trying to digest all the info.
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Strgazr27
Reged: 10/04/04
Loc: StonyHill Observatory
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Re: Autoguided PE -- how good is good?
[Re: freestar8n]
#2988042 - 03/16/09 11:08 PM
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Frank,
Can METAGUIDE be made to work with an Orion Starshoot? It looks incredibly promising for guiding and was curious.
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Miguel Lopes
scholastic sledgehammer
Reged: 01/04/07
Loc: Portugal
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Re: Autoguided PE -- how good is good?
[Re: jrcrilly]
#2988317 - 03/17/09 05:31 AM
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Quote:
Quote:
But for arguments sake most stars are composed of mainly "White" light. in other words, they are an RGB object. That being true, am I not using all of the pixels in the image of my OSC to compute my FWHM? You would be correct if we were shooting through a single color filter but we're not. Correct me if I'm wrong but a 6 MP mono camera and a 6 MP OSC camera should have the same resolution if the pixels are the same size and we're shooting an RGB object no?
They would, if you were looking at the raw data from the chip instead of the interpolated data that is actually used.
IMO the problem is not in the debayering, it's just because you have less resolution. Interpolation tries to compensate that.
You are right, for a pure white star, if the FWHM algorithm only looks at luminance data, it should be very similar.
But then when we look at the small details, it is not the same.
For example, a mono camera without IR block will be very sensitive in the IR, which is less affected by turbulence.
Also, as you can take a sub frame in 1/3 of the time, the PSF will be smaller.
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freestar8n
Carpal Tunnel
Reged: 10/12/07
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Re: Autoguided PE -- how good is good?
[Re: Strgazr27]
#2988426 - 03/17/09 08:20 AM
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Can METAGUIDE be made to work with an Orion Starshoot? It looks incredibly promising for guiding and was curious.
Hi-
Sorry - but it is video-based and only works with cameras that have a DirectShow driver. The centroiding it uses is very different from other apps, and it requires realtime processing of several successive frames to take advantage. My standard guide camera is a Lumenera, and I guide with OAG at 8-30 frames per second on stars down to mag 10 with a C11.
On the topic of fwhm - in order to get the smallest fwhm in the image, all errors will contribute - and I find there is enough error in the guide star centroiding that there is benefit in doing things differently. So focus, guide tuning, latency, etc. all need to be addressed - and even then the centroiding algorithm can be improved, which is why I wrote MetaGuide.
Thanks,
Frank
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