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Is one way significantly better than another for photometry?

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#1 m1thumb

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Posted 10 December 2023 - 03:09 PM

I am using C6 and 178mm Cool and filter wheel. Plan to do variable star and secondarily dabble in exoplanets.

What would be a better set up? I need to get some adapters and spacers either way.

1. Meade .33 reducer at around 1 arc sec/pix

2. Celestron .63 which i can do .47 arc sec/pixels bin 1 or .94 bin 2.



I like the field of view with the .33 and with the small sensor of the 178, but a little better image at least on the edges with the .63.

Seems like f3.6 would be an advantage with smaller aperture and photometric filters.


Thanks,

Terry

Edited for more clarity, I hope.

Edited by m1thumb, 11 December 2023 - 10:38 AM.


#2 Ed Wiley

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Posted 11 December 2023 - 11:00 AM

Have a look at the discussion linked below and download the AAVSO manual. There are also discussions that suggest that going faster than F4 effect photometry negatively with certain filters, but I can't remember where they are. Beware and do your homework.

 

https://www.aavso.org/focal-reducer-0

 

Ed


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#3 m1thumb

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Posted 12 December 2023 - 04:29 PM

Thanks Ed,

I haven't looked at the AAVSO manual since your class this year, will review again. I did find a thread discussing hyperstar which Arne said is unsuitable. The idea of f4 was discussed as a low limit.

Nothing is as simple as it seems in first light!

Terry

#4 Xilman

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Posted 16 January 2024 - 01:46 PM

My experience, but YMMV.

 

The star images needs to have a FWHM of at least 5 pixels and preferably 8 or more. Attaining this is clearly a matter of your camera, telescope aperture and seeing. In my case, seeing can be anywhere between 2 arcsec and 20 arcsec, though the latter nights are generally useless. Quite often photometrists defocus their equipment to make their image large enough.

 

The larger the FOV the better, as sequence stars may be quite a distance from the target.

 

By far the best advice, IMAO, is to suck it and see.  The only true way to become an expert is through experience.

 

Sorry if this is not the response you were expecting.


Edited by Xilman, 16 January 2024 - 01:48 PM.


#5 rutherfordt

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Posted 17 January 2024 - 09:36 AM



 


The larger the FOV the better, as sequence stars may be quite a distance from the target.

.

I think this is true only up to a point-- with a really large field of view, its possible that the comp star(s) will be far enough from the target star that there could be different amounts of atmospheric extinction between them (more critical if they were low than if they were high in the sky), especially if they were of different colors.

 

 

 

 


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#6 Ed Wiley

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Posted 17 January 2024 - 10:03 AM

Like Tom said.

 

As FOV increases from something like 30x30 arcminutes, air mass differences and the need to consider extinction become critical. Better for the vast majority of we amateur photometrists using CCD and CMOS cameras to stay within the 30' distance (and above 30 degrees altitude) to avoid the need to take into account extinction coefficients. In particular when dealing with red stars and Johnson B and V image as high as possible. Since I do long time series of eclipsing binaries I usually shoot in V and Ic rather than B and V.

 

Larger FOVs are OK, but keep your comps closer than 30' within that FOV or you might need to account for extinction, depending on the filter used.

 

Ed


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#7 Xilman

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Posted 17 January 2024 - 12:16 PM

Both of the above two responses are correct, of course, in that too wide a FOV can be detrimental.

 

My view is coloured by having a FOV of roughly 15x10 arcmin. It is not unusual for one or more comparisons to be outside the image when using an AAVSO standard F-scale (18.5') chart.

 

Atmospheric extinction is relatively unimportant when measuring exoplanetary transits, something I do occasionally, where accurate timings of ingress and egress are much more valuable than the depth of the transit and where the out-of-transit magnitude is essentially unimportant. Indeed, although Rc or r' filters are generally recommended quite often the photometry is unfiltered so as to maximize cadency for a SNR of at least 500.


Edited by Xilman, 17 January 2024 - 12:17 PM.


#8 m1thumb

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Posted 17 January 2024 - 01:29 PM

Thanks everyone,

I recently got my filters and as luck would have it, weather hasn't cooperated, I have to travel the next 5 weeks for work and will be relocating from Kansas to South Carolina shortly after.

Work is not kind to this hobby!

I will get out and gather some data when I can and get some feedback to you.

I agree with an earlier comment there is no better way to figure it out but to do it.

#9 Xilman

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Posted 17 January 2024 - 01:40 PM

Work is not kind to this hobby!

I will get out and gather some data when I can and get some feedback to you.

I agree with an earlier comment there is no better way to figure it out but to do it.

Work is a four-letter word.  :(

 

Good luck.

 

It won't take long before you are doing good science - if, and only if, you submit your results to the likes of AAVSO or BAA-VSS. Hidden work might be personally satisfying but it doesn't do much for anyone else.

 

Paul



#10 m1thumb

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Posted 17 January 2024 - 01:53 PM

It's my intention to submit data.
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#11 Xilman

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Posted 17 January 2024 - 01:54 PM

One final thing: you mention having a  (relatively) small aperture.

 

How big it is is not as important as what you do with it.

 

Four times the length of exposure with half the aperture will collect the same number of photons. Please always keep this in mind.

 

For example, a 24", which is vastly bigger than most amateur equipment, has 16 times the collecting area of your 6" scope. Roughly speaking, a minute on the giant scope is equivalent to a quarter hour on yours. You lose out only on very faint objects. You should be able to do good work on exoplanets down to magnitude 11 or thereabouts, and on VS to around 17. These numbers are based on my experience with a 0.4m (~16") where I can reach 20.0 magnitude with 0.1m accuracy through a V filter with a 4 hour exposure.




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