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#1 zoltrix

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Posted 07 April 2021 - 07:11 PM

hello

Can you expect to get a good photometric and spectrography accuracy of bright stars with a newtonian reflector despite its difraction spikes ?

#2 GaryShaw

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Posted 07 April 2021 - 08:56 PM

Works great for photometrics for variable star imaging and analysis - can’t speak for spec 



#3 robin_astro

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Posted 08 April 2021 - 05:13 PM

Newtonians are excellent cost effective light buckets for spectroscopy with tight stars on axis and no chromatism. Diffraction spikes are no problem for slit spectroscopy as the slit isolates the star and for slitless spectroscopy if they are a problem just rotate so the diffraction spikes don't lie in the dispersion direction

 

Cheers

Robin  


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#4 zoltrix

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Posted 09 April 2021 - 05:36 AM

Works great for photometrics for variable star imaging and analysis - can’t speak for spec


thanks Gary

Should the spikes be encompassed for the counting of the photons ?

#5 GaryShaw

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Posted 09 April 2021 - 09:12 AM

thanks Gary

Should the spikes be encompassed for the counting of the photons ?

Once I have the right range of adu values, I have not seen spikes on my target Star or the comparison and check Star. Hence my sampling apertures have never displayed spikes.

 

I hadn’t thought much about this but perhaps the spikes mainly appear when you get to or exceed your sensor’s full well depth. Maybe a smart person will chime in and help me understand if this is true and what conditions, besides using a newtonian scope, cause spikes.

cheers,

Gary 
 



#6 zoltrix

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Posted 09 April 2021 - 10:19 AM

hi Gary

You did not see spikes with your newtonian scope
Even with bright stars,do you mean ?

#7 robin_astro

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Posted 09 April 2021 - 10:56 AM

 The spikes are caused by diffraction from the secondary mirror spider support and will always be there even on faint stars. They contain very little light though compared to the main star image so are only really obvious when the star is very over exposed, for example in stretched images of long exposures containing bright stars. As long as you use the same aperture for target and comparison stars their small contribution will be the same in any case and will not affect the photometry.

 

Cheers

Robin 


Edited by robin_astro, 09 April 2021 - 11:13 AM.

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#8 robin_astro

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Posted 09 April 2021 - 11:03 AM

 

 

I hadn’t thought much about this but perhaps the spikes mainly appear when you get to or exceed your sensor’s full well depth. 

 

You can get vertical streaks (blooming) if you exceed the full depth on non anti blooming CCD  and the charge spills over into neighbouring pixels but this looks quite different. Saturation (or even exposing beyond the linear response of the sensor) is definitely to be avoided though for photometry (and spectroscopy)  of course

 

Cheers

Robin



#9 GaryShaw

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Posted 09 April 2021 - 11:34 AM

hi Gary

You did not see spikes with your newtonian scope
Even with bright stars,do you mean ?

Robin summarized it well above. 

All I was saying in my prior post is that, once I've adjusted my exposure/gain settings for variable star imaging, to be less than the full well depth of my sensor's pixels, I do not 'see' the diffraction spikes on the stars of interest. Those stars include the target 'variable' star, the 'check' star and all the 'comparison' stars that I'm using in my magnitude analysis.

 

In my EAA observing, most stars do not display spikes. The brightest ones are the only ones that routinely display spikes. Here's an example of a typical view where one star is displaying spikes. Often there are several with spikes but the vast majority do not.

 

Hope this helps,

Gary

 

ps: if you blow up this image you will see another star in the upper right corner with some short spikes.

 

Flame Stack_24frames_359s_WithDisplayStretch.jpg


Edited by GaryShaw, 09 April 2021 - 11:35 AM.


#10 zoltrix

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Posted 09 April 2021 - 06:10 PM

The spikes are caused by diffraction from the secondary mirror spider support and will always be there even on faint stars. They contain very little light though compared to the main star image so are only really obvious when the star is very over exposed, for example in stretched images of long exposures containing bright stars. As long as you use the same aperture for target and comparison stars their small contribution will be the same in any case and will not affect the photometry.
 
Cheers
Robin


Hello Robin

So, with the stacking of short exposures,it should be possible to tune the linear streching of the image in order to make spikes invisible

#11 robin_astro

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Posted 10 April 2021 - 06:20 AM

Stacking short exposures can be used to avoid blooming due to saturation in non ABG CCD eg as shown here

 

https://en.wikipedia...tical_smear.jpg

 

Stacking will not make any difference to the intensity of diffraction spikes though. Their visibility in an image does depend on the contrast stretch (the setting of maximum and minimum levels in the image)  but for photometry it is important to use the full range in the image without any clipping of values at the top or bottom end.

 

Cheers

Robin


Edited by robin_astro, 10 April 2021 - 06:21 AM.


#12 robin_astro

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Posted 10 April 2021 - 06:30 AM

Stacking short exposures can be used to avoid blooming due to saturation in non ABG CCD eg as shown here

 

https://en.wikipedia...tical_smear.jpg

 

 

Note I believe the accompanying text to this image is incorrect/out of date.  Most CCD sensors have anti blooming gates (ABG) to avoid this problem. In astronomy, professionals and some amateurs prefer to use sensors without ABG for photometry as they allow a larger linear range. With modern CCD design though the advantage is not that great and sensors with ABG work fine for photometry/spectroscopy. I am not an expert on CMOS sensors though. I know they can be used for photometry but  I don't know how they behave when saturated. (They will still show diffraction spikes though of course as these are inherent in the design of the telescope optics independent of the sensor used.

 

Cheers

Robin



#13 zoltrix

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Posted 10 April 2021 - 12:18 PM

Hi Gary

I see that you own the iOptron AZ mount and the ASI 290 mono diagonal 6.2 mm
I am going to purchase a similar set up
Also the latitude in Massachussets is about the same as the one in Milano (46°)

What is about the max elevation (no field rotation) of north/south oriented objects with about 30 sec short exposures ?

Thanks in advance for your help

Bye


Alberto

#14 GaryShaw

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Posted 10 April 2021 - 01:32 PM

Alberto

I am an EAA observer and don’t do astrophotography. The longest exposures I’ve needed to do were 20 seconds. Also, the ASI290 mini is used as a digital finder so it’s never been my main viewing camera.

 

Regarding field rotation, I’ve attached a chart prepared by Niels Haagh, of TracktheStars.com. It gives you a rough idea of the allowable exposure times. I have some doubts about the exposure times listed for the “E/W” direction.
 

Here’s the chart from Niels Haagh:

 

DD1355A2-0218-4134-9C5B-DADCCF7DD32B.jpeg

 

Here’s a link to some more reliable charts relating to field rotation and exposure times:

 

https://www.cloudyni...ttach_id=665796

 

i hope this helps...
Gary

 

 

 

 

 




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