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FWHM and Gain ! fat stars...on focus

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#1 starhunter50

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Posted 24 December 2017 - 04:37 PM

so here is my question to you guys..

 

 

if im "frame and focus" in SGP or in Sharpcap and the FWHM says its 3.0 then i increase the gain 10%  so i can see the star better , the FWHM goes up to 6 , 

 

then if i increase the gain even more 30% lets say , now im at 8 FWHM and the sky has not changed in 5 minutes.  to say nothing of bining 1x1 or 2x2 during focus !!!

 

so which one is the right FWHM for my scope and the sky condition.

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Scope=C8 F6.3  Cam=AA183C hypercam.

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once i start imaging with what i though was perfect focus my SGP reports FWHM at 7 or more. 

 

Does the image scale affect the FWHM size of the stars or the Focal length of any scope?

 

Then what the point of using that method to focus ??? just use the Bhatinov mask and hope for the best.??

 

Mitch, confused.

 



#2 vio

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Posted 24 December 2017 - 04:54 PM

Not familiar with your camera, but what you are experiencing may be more to do with pixels being saturated than not being in focus. Focus also could vary in time, and may be subject to having the scope reaching stability in temperature and humidity. If the increase in FWHM is only due to the change in gain, I would suspect the exposure used is too long for the star you are focusing on. Try focusing on a star that’s not saturated or use a shorter exposure time if possible.


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Edited by vio, 24 December 2017 - 04:55 PM.


#3 Peter in Reno

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Posted 24 December 2017 - 05:00 PM

FWHM measurement must be done on unsaturated stars. FWHM is meaningless if stars are saturated. So if you increase gain, you probably either need to reduce exposure time or switch to dimmer stars. Gain should not have any effects on FWHM, just make sure stars are not saturated.

 

Peter



#4 Kaos

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Posted 24 December 2017 - 11:05 PM

so here is my question to you guys..

 

 

if im "frame and focus" in SGP or in Sharpcap and the FWHM says its 3.0 then i increase the gain 10%  so i can see the star better , the FWHM goes up to 6 , 

 

then if i increase the gain even more 30% lets say , now im at 8 FWHM and the sky has not changed in 5 minutes.  to say nothing of bining 1x1 or 2x2 during focus !!!

 

so which one is the right FWHM for my scope and the sky condition.

----------------------------------------------------------------

Scope=C8 F6.3  Cam=AA183C hypercam.

-----------------------------------------------------------------

once i start imaging with what i though was perfect focus my SGP reports FWHM at 7 or more. 

 

Does the image scale affect the FWHM size of the stars or the Focal length of any scope?

 

Then what the point of using that method to focus ??? just use the Bhatinov mask and hope for the best.??

 

Mitch, confused.

Since you are using AGP, I would suggest, if you have the budge for it, to add an electronic focuser and run autofocus in SGP. THat routine uses stars across the entire field to yield the best focus over the entire field.

 

Kaos


Edited by Kaos, 25 December 2017 - 01:31 AM.


#5 rgsalinger

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Posted 25 December 2017 - 11:31 AM

When you change to 2x2 binning you increase the ADU value of the focus star by 4x.  So, you are more and more likely to be saturating the star. That's why most people recommend focusing at 1x1. The image scale doubles but the focal length of the scope is not affected to answer your other question.

 

I don't know why you feel you need to see the star - it's the measurement that counts. What you could do is apply a higher stretch to the image and that would give you a visible star. I use the SKYX and MaximDL and both can do exactly that. That way I can focus without adjusting gain. When you adjust the gain you may well be saturating the star.

 

When you start imaging you have three complicating factors that affect that measurement. First, the FWHM measurement of a frame is some sort of average. So it will differ substantially from what you measure with a single star. Second, there is guiding to consider. Guide error results in fatter stars. Finally, as you change altitude your FWHM changes substantially. I can get under two arc seconds over 60 degrees with my 12.5" reflection pretty consistently At 30 degrees I'm lucky to get 3 arc seconds most nights.

 

Rgrds-Ross



#6 starhunter50

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Posted 25 December 2017 - 03:30 PM

I did not know you could focus with out seeing any stars,      to quote you (( I don't know why you feel you need to see the star)).

the measurement is not accurate because the stars vary in the FOV. some are real faint others are real bright.

all i can think of is set the gain real low ,and look for a brighter star, but then im back to where i started. (bigger brighter star).

 

Mitch



#7 WadeH237

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Posted 25 December 2017 - 05:20 PM

all i can think of is set the gain real low ,and look for a brighter star, but then im back to where i started. (bigger brighter star).

In looking over the thread, it seems like the image on the screen is not stretched properly.  Stars don't need to be anywhere near saturated to be clearly visible.  I use MaxIm/DL and it has a number of stretching options.  If I set it to use "full range", then only saturated stars would be very visible, which is similar to what you describe.  It also has a number of automatic stretching options, most of which would display even fairly dim stars pretty clearly.  When I manually focus by hand, I usually finish up an a star that is less than 10% of the way to saturated (you can't use the same star to go from well out of focus, to properly focused, because as focus gets better, the stars will get closer to saturated; so I start on a bright star and then switch to dimmer ones as the brighter ones start saturating due to better focus).

 

I've never used SGP, but I suspect that it must have something similar.



#8 starhunter50

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Posted 25 December 2017 - 08:03 PM

Thanks Wade, i will give it a try.




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