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iEQ45 PE reduced to 4.5 arcsec peak to peak!!

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#26 freestar8n

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Posted 28 February 2013 - 08:25 AM

Hi Alfredo-

Most image processing tools provide a way to measure it. You just need to take a guided image of perhaps 5-10 minutes that is focused as well as you can. You need to know the plate scale, arc-seconds per pixel - which you can estimate using the focal length and the pixel size - or use plate solving software. It should be a raw or calibrated image that has not been stretched or manipulated at all.

As long as the stars aren't saturated, they should all have the same width when measured at half their height (fwhm). So the brighter ones look wider - but they are also "taller" - and when you measure them half way up to their peaks - the width will be about the same for all.

The fwhm in arc-seconds is an indication of how well you are imaging. Extreme professional results at excellent sites can be as small as 1", while a typical result with a guidescope is around 4" perhaps. I consider a good target to be 2".

So if you use Maxim or Images Plus or something - you can just enter the plate scale (arc-sec per pixel) and point at a star and read the fwhm.

If you get a big number it could be due to seeing or guiding, but I think a common limitation is just making sure the focus is excellent. Anyway - it serves as a quantitative measure of how good your imaging is - so you can measure it and try to improve it.

Frank

#27 Alfredo Beltran

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Posted 28 February 2013 - 09:29 AM

Hi Frank

It seems very simple! I will try it next time I'm outside and see what I can find.

Best regards

Alfredo

#28 freestar8n

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Posted 28 February 2013 - 09:39 AM

Hi-

I just noticed you may be using a DSLR. In that case you can't measure with the raw images very well and you need to debayer them first. So I would calibrate them with darks/flats/bias - or however you do it - then debayer - then measure fwhm.

Frank

#29 Alfredo Beltran

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Posted 28 February 2013 - 09:46 AM

Hi-

I just noticed you may be using a DSLR. In that case you can't measure with the raw images very well and you need to debayer them first. So I would calibrate them with darks/flats/bias - or however you do it - then debayer - then measure fwhm.

Frank


Very good suggestion Frank, thank you!

Alfredo

#30 Phillip Easton

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Posted 28 February 2013 - 11:40 AM

Alfredo,

You could also get a trial of CCD Inspector. You can easily measure FWHM of several images. It is a good tool for analyzing your images to help troubleshoot issues with your imaging system.

http://www.ccdware.c...s/ccdinspector/

I would take images with short exposure 15-30 sec and then at times you normally image at. This way you can get a better indication of performance of the mount. If the two sets are close then I would think the mount is doing what you want. If the short subs have large FWHM then of course there may be other issuse like colimation, focus, etc. affecting your performance.

Here are the results from my last imaging session. I am imaging at 0.89 arc-sec/pix with my Canon 60D and have adapted a Hotech T-adapter to keep the camera centered. From an analysis of 10 files while guiding for 5 mins each, I neglected to get shorter exposures for analysis last time to compare.

Min FWHM: 2.08"
Max FWHM: 3.50"
Curvature: 31.6%
Tilt in X: -0.1"
Tilt in Y: -0.3"
Total Tilt: 9% @107 deg
Collimation 3.7"
Stars Used: 2483

The numbers don't tell the whole story. When you look at a plot you see how the FWHM changes across your image. Mine is fairly flat but I get curvature in the extreme corners so that is were the stars start getting fat and the max goes to 3.5. The average value was around 2.5.

The rest of the information tells you about colimation and whether your camera is tilted.

Cheers!

#31 Alfredo Beltran

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Posted 28 February 2013 - 12:31 PM

Alfredo,

You could also get a trial of CCD Inspector. You can easily measure FWHM of several images. It is a good tool for analyzing your images to help troubleshoot issues with your imaging system.

http://www.ccdware.c...s/ccdinspector/


Hi Phillip

Thanks for the suggestion. I will take a look. Seems very promising.

Best regards

Alfredo

#32 freestar8n

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Posted 28 February 2013 - 01:52 PM

Short exposures 1-15 seconds can tell you about the optics - but for assessing the autoguiding of the mount it would be better to go for many minutes - preferably at least one worm period. That's what you do when you do deep sky imaging - so it makes sense to use the mount to do what you need to do - and then measure it to see how well you did.

If someone gets small fwhm with a mount - that is a direct indication the mount is capable of achieving that fwhm in a deep sky image - because it did.

Frank






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