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need help with CCD photometry question

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#1 ASTERON

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Posted 20 September 2020 - 03:08 AM

I am a newbie interested in starting doing stellar photometry.  Sorry about the dumb questions but I need to make a decision about  buying a used camera or not

I need help with the following questions:

 

1) How critical is the age and pixel column defect status of a CCD imager to the accuracy of the measured results ?

2) If the sensor has a large number of hot pixels, would this affect the measured results.

3) Can I use bias and dark frame subtraction methods successfully to correct for pixel defects ?

4) How many pixels are typically involved with measuring the flux of a single star and how will one or more hot pixels degrade the results to uselessness ?

5) How do you deal with column defects in photometry?

6) In principle would I be OK buying an old ( Ca. 10 years or more) CCD camera with some ( limited) sensor defects, or should I break the bank and go for a new class 1 sensor camera  at the risk of annoying the CFO ?

7) I will be using a AT8RC F/9  ( possibly with the dedicated Astro-tech FR to achieve about F/6)  with the camera, what is the best sensor size and type for such a scope.

8) Are CCD cameras worse or better than the modern CMOS sensor cameras on the market for photometry ?

9) Do newer CMOS based sensors have sufficiently linear response near the full well capacity ?

10) how critical is the antiblooming feature of  CCD sensors to successful photometry, am I truely better of with a sensor with no antiblooming or is it possible to use a CCD with antiblooming  by avoiding using the full well capacity and staying in the linear range of the CCD ?

 

Any help, ideas, suggestions are humbly sought and will be greatly appreciatedbow.gif

 



#2 pbealo

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Posted 20 September 2020 - 08:22 AM

I would go to https://www.aavso.org/ and go through forums and their getting started documents


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#3 ASTERON

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Posted 20 September 2020 - 03:33 PM

I would go to https://www.aavso.org/ and go through forums and their getting started documents

Tried to, I cannot find any way of posting a question there ( not a member of AAVSO).

I need the info for making a buying decision on an old camera



#4 ButterFly

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Posted 20 September 2020 - 04:59 PM

As mentioned in the duplicate post, there are also great manuals on AAVSO.  Most of your questions are covered there.  Read the CCD one and the DSLR one if using CMOS.

 

In many ways, a mono cmos will make your life much easier with calibrations.  It's not impossible with other setups, just harder.  Sensor size is not very relevant for photometry - pixel scale is.


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#5 ASTERON

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Posted 20 September 2020 - 05:50 PM

Thanks for your help,

I have read the CCD manual but still cannot get a definite understanding if bad (hot or dead) pixels/columns may be overcome by avoiding your target star being imaged on them or by using calibration procedures ( Bias, dark, flat).



#6 StarmanDan

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Posted 20 September 2020 - 07:43 PM

Thanks for your help,
I have read the CCD manual but still cannot get a definite understanding if bad (hot or dead) pixels/columns may be overcome by avoiding your target star being imaged on them or by using calibration procedures ( Bias, dark, flat).


Since you are doing photometry, you will be required to process your images with calibration frames. This will deal with hot pixels and column defects. Yes, if you know where a defective pixel or column is do your best to avoid placing the target and comparison stars over them. As long as the image sensor remains linear over the exposure times used, you can get away with a few defects in the sensor.
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#7 RussD

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Posted 20 September 2020 - 09:14 PM

Hello Asteron!

1) Typically this is not a problem as defects care taken care of by dark subtraction.  I am talking about a few column defects and a smattering of bad pixels.

2) It can be annoying but not generally a problem. Unless it is like one of my old cameras where there was a hot pixel directly in the center of the frame.  That is bad luck.  

3) Yes , typically.  But is not perfect for hot pixels I have found.

4) Depends on the focal length and pixel scale.  

5) I dont worry about them.

6) Yes, that is a great way to get started. There are incredible deals out there on used cameras. CCDs are my preference.  The ST10XME is a great choice for low noise and inexpensive used camera.

7) Lookup pixel scale calculator and see what pixel size fits that focal length to give you a pixel scale range somewhere between 1 and 2 arcsec/pixel.  It is better to be a little oversampled, meaning on the lower end of that range.  I have used everywhere from 0.94 arcse/pixel to 1.8 arcsec/pixel under typical conditions with 3 arcsec-4 arcsec seeing and had very good results.  I am presently operating at 1.2 arcsec/pixel with about 2.0-3 arcsec seeing typically.

8) I have used CCDs and checked them for linearity.  That usually means NABG cameras.  CMOS can be good as long as they are linear. I have not used them for photometry...

9) Dont know, depends on camera I would imagine.

10) Antiblooming could be a pain if the camera goes nonlinear say at half well capacity.  You dont want that.  But you can make it work if you are careful.  I would rather not have to worry, so I have just used NABG cameras.

 

Get a camera, try it out. Take one of the AAVSO photometry courses!  

 

Regards,

Russ



#8 SeymoreStars

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Posted 20 September 2020 - 09:31 PM

What's the FL of your telescope and what the pixel size of the camera (or model-make)?



#9 ASTERON

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Posted 21 September 2020 - 06:10 AM

 Hello Russ,

Thank you very much for chipping in.  You have addressed almost all of my concerns. This is extremely helpfulbow.gif



#10 ASTERON

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Posted 21 September 2020 - 07:07 AM

Hi Seimore,

 

I have several scopes:

 

1) My first choice would be my AT8RC  with f=1624mm ;F/8 native  and with the CCTD67   focal reducer it reaches f=1088mm and F/5.5 - the only thing I am worried about with this configuration is the spider spikes on brighter stars  ( I have no idea how they might affect photometry)hmm.gif

 

2) Second choice 12"Meade ACF f=3000mm  F/10, with the same reducer I'm looking at  F/6.7 ( if it works) , I also have a 2" x0.5 Agena Astro focal reducer for  reaching F/5 but I don't know yet if it will work properly with the ACF optics.

 

3 ) APM/ LZOs 130mm refractor  f=780  F/6, which I can use natively with a Field flatener if necessary.

 

4) APM Apo Doublet 152mm refractor f=1200  F/7.9, which I can use natively or further reduce to around F/5.3.

 

regards

 

Lihu


Edited by ASTERON, 21 September 2020 - 07:08 AM.


#11 ASTERON

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Posted 21 September 2020 - 07:11 AM

Since you are doing photometry, you will be required to process your images with calibration frames. This will deal with hot pixels and column defects. Yes, if you know where a defective pixel or column is do your best to avoid placing the target and comparison stars over them. As long as the image sensor remains linear over the exposure times used, you can get away with a few defects in the sensor.

Thanks Dan,

This is more reassuring.  I may go for an older used camera.

Regards,

 

Lihu



#12 RussD

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Posted 21 September 2020 - 03:08 PM

You are welcome!

 

For photometry you probably want the larger scope.  That makes everything so much easier as your signal will be higher.  But a larger scope may need a better mount.  The focal reducer may work or it may not.  When I imaged with a C11 and C14 I found I could use the f6.7 reducer no problem to get the right pixel scale.  I guess the best thing to do is avoid the reducer if you can to eliminate losses in signal.  But sometimes it is the best option.

 

What mount do you have?  

 

On the other hand the refractors have no mirror flop and will not lose collimation so they are attractive as well.  

 

Either case, go for the highest sensitivity and lowest noise camera you can find.  That is one thing CMOS has going for it over many CCDs.  

 

Russ



#13 ASTERON

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Posted 22 September 2020 - 02:26 AM

You are welcome!

 

For photometry you probably want the larger scope.  That makes everything so much easier as your signal will be higher.  But a larger scope may need a better mount.  The focal reducer may work or it may not.  When I imaged with a C11 and C14 I found I could use the f6.7 reducer no problem to get the right pixel scale.  I guess the best thing to do is avoid the reducer if you can to eliminate losses in signal.  But sometimes it is the best option.

 

What mount do you have?  

 

On the other hand the refractors have no mirror flop and will not lose collimation so they are attractive as well.  

 

Either case, go for the highest sensitivity and lowest noise camera you can find.  That is one thing CMOS has going for it over many CCDs.  

 

Russ

 Hi Russ,

I think I am going to order the 10 micron HPS1000 soon. Nominal capacity for AP is 55 Lbs.  So I will be within spec even with the 12"ACF.

Do you think the spider induced spikes of the AT8RC will interfere with photometry ??  is there any workaround  for such spikes ?

Thanks

LK



#14 RussD

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Posted 22 September 2020 - 07:43 PM

Lihu,

I suspect the spikes are going to have a small effect, but not enough to worry about.  On my Planewave 20" it does not seem to be an issue and I regularly do better than 0.01 precision photometry.   But I have not used the AT8RC.  

 

You will really have a nice setup with the 10 micron.  

 

Regards,

 

Russ



#15 ButterFly

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Posted 23 September 2020 - 01:42 AM


You will really have a nice setup with the 10 micron.  

 

Seriously though.  An old used camera on that mount and those scopes is like wearing a tux and seal top hat with flip-flops.  Just get a modern cooled mono cmos with Johnson-Cousin's filters.  Money seems to not be an issue, so why waste in on an older used camera?
 


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#16 ASTERON

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Posted 23 September 2020 - 03:24 PM

Seriously though.  An old used camera on that mount and those scopes is like wearing a tux and seal top hat with flip-flops.  Just get a modern cooled mono cmos with Johnson-Cousin's filters.  Money seems to not be an issue, so why waste in on an older used camera?
 

What cooled CMOS camera would you choose ButterFly ?



#17 ButterFly

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Posted 23 September 2020 - 04:41 PM

What cooled CMOS camera would you choose ButterFly ?

It depends on which scope you choose for what and your typical seeing conditions.  The mount won't be blurring the stars, but the seeing will.  Oversampling by a factor of four is quite good for photometry.  For regular AP, a factor of 2-3 is sufficient.  You can just bin for AP.  Choose a camera with a pixel scale at one-fourth your typical good seeing.  Let's say 1" is your typical good seeing, and excellent seeing there is 0.75".  A choice of 0.25"/pixel is a safe bet.  An exact match is unlikely because pixels come in certain sizes.

 

Pixel scale in arcseconds = 206 * (pixel size in microns) / (scope focal length in mm)

 

One camera for more than one scope will be a compromise.  Assume 1" seeing and make a table for all your scopes' focal lengths.  Have columns for 4, 3, and 2x oversampling showing the micron size for each scope to get that.  If your seeing is 1.5" instead of the assumed 1", each entry gets multiplied by 1.5.  For example, for f=3000, the 2x entry becomes (1/2) * 3000 / 206, for a 7.3 micron pixel.  The row becomes:

 

f            2x    3x    4x    2xbin2   3xbin2  4xbin2

3000     7.3   4.9   3.6   3.7        2.5         1.8

 

Gauging your typical good seeing can take some time.  Looking at double stars can help.  Asking others who image in your area is faster.

 

The 12" is the best photometry candidate just by aperture.  The other scopes have great imaging potential.  Finding a good fit for as many of the scopes starts with finding a good pixel size to match your seeing.  Find some imagers nearby and think about how you want to use each scope.  An astronomy department at a nearby university is also an excellent resource.  They would love your data and will likely help you get up and running.



#18 ASTERON

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Posted 24 September 2020 - 02:12 AM

It depends on which scope you choose for what and your typical seeing conditions.  The mount won't be blurring the stars, but the seeing will.  Oversampling by a factor of four is quite good for photometry.  For regular AP, a factor of 2-3 is sufficient.  You can just bin for AP.  Choose a camera with a pixel scale at one-fourth your typical good seeing.  Let's say 1" is your typical good seeing, and excellent seeing there is 0.75".  A choice of 0.25"/pixel is a safe bet.  An exact match is unlikely because pixels come in certain sizes.

 

Pixel scale in arcseconds = 206 * (pixel size in microns) / (scope focal length in mm)

 

One camera for more than one scope will be a compromise.  Assume 1" seeing and make a table for all your scopes' focal lengths.  Have columns for 4, 3, and 2x oversampling showing the micron size for each scope to get that.  If your seeing is 1.5" instead of the assumed 1", each entry gets multiplied by 1.5.  For example, for f=3000, the 2x entry becomes (1/2) * 3000 / 206, for a 7.3 micron pixel.  The row becomes:

 

f            2x    3x    4x    2xbin2   3xbin2  4xbin2

3000     7.3   4.9   3.6   3.7        2.5         1.8

 

Gauging your typical good seeing can take some time.  Looking at double stars can help.  Asking others who image in your area is faster.

 

The 12" is the best photometry candidate just by aperture.  The other scopes have great imaging potential.  Finding a good fit for as many of the scopes starts with finding a good pixel size to match your seeing.  Find some imagers nearby and think about how you want to use each scope.  An astronomy department at a nearby university is also an excellent resource.  They would love your data and will likely help you get up and running.

Hi ButterFly,

Thank you for your detailed answer.  This is real help !. When I was considering CMOS cameras I had two major concerns.  I could not find much data on their sensor linearity. Probably didn't dig deep enough ( I am sure Sony published this data somewhere).  I  was wondering if CMOS sensors come with ABG  or NABG architecture and how this affects sensor linearity.  Having no previous serious experience in AP let alone photometry, I was trying to figure out what would be a good camera with large enough full well capacity and also reasonable anti blooming, because I may also want to use the camera for regular AP at one stage or another.   While it looks like money is no issue  (it is), I still have to consider the CFO's opinion because it may seem excessive to splurge on a good mount and then buy a new camera with adequate filters for a hefty sum.  The good mount is a must because I do not wish to, or have the inclination to tinker with faulty or sub par equipment ( an Orion EQG almost ruined my appetite for AP in the past).  I may eventually Go for a cooled ZWO or a medium cost similar camera.



#19 ASTERON

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Posted 24 September 2020 - 02:15 AM

P.S.,

What are the alternatives for UBVRI filters ? I know Baader sells them, is there any other recommended manufacturer for such a set of filters ?



#20 ButterFly

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Posted 24 September 2020 - 03:23 AM

Google photometric systems site:cloudynights.com and go through some of the posts.  There are many different systems, some more specific than others.  UVBRI has been around for while and is rather general.  The coatings on the refractors and the SCT are optimized for visual, so far infrared won't be the best choice.  Any filter set tends to be expensive, so you can hold off on that for quite a while.  There is no rush at all.  There is plenty to learn and do along the way.

 

Any sensor (and filter) is going to have some defects.  The calibrations are about finding out what those are.  Even if a sensor is not perfectly linear, that's fine as long as it's consistent.  You will measure that as part of the calibration process.  The cooled cmos has the benefit of consistency in its dark frames.  The same frames can be used for months at a time.  With uncooled systems, darks need to be taken often, while the sensor is cooling and warming.  That's time you can spend imaging or collecting data.  Used cameras come up all the time.  If the pixel size doesn't work out for want you want to do, you can sell it again close to cost.  New or used, there will be some defects you need to learn about.  If you any camera at all right now, start measuring its linearity.

 

Even without filters, differential photometry can be used for things like eclipsing binaries and variable stars.  The AAVSO courses will give you a good foundation.  Maybe someday you'll decide you like supernova light curves and follow a few for months at a time.  Plenty of exoplanets are within reach of a 12".  Take it one step at a time so you have fun along the way.  Basic photometry is one of the most valuable contributions of amateur astronomers to astrophysics.  AAVSO really wants your data, so they want to help you every step of the way.

 

Have fun!


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#21 gregj888

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Posted 25 September 2020 - 07:46 PM

As mentioned earlier the AAVSO has some good information on this topic as wee and some recorded discussion and tutorials.

 

https://aavso.org/ph...lter-selections

 

https://www.youtube....twToO-pY9A6NpMQ

 

 

Greg



#22 RussD

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Posted 30 September 2020 - 08:36 AM

One thing you need to investigate is whether you even need a full set of photometric filters.  If I were you I would start with a single V filter.  Photometric filters are expensive and doing calibrated photometry is tricky and may be impactable if your local conditions do not allow it.   I would highly recommend taking one of the AAVSo photometry courses.  In fact I should take that advice myself!

 

I do unfiltered differential photometry primarily.  That is useful for things like asteroid light curves and some variable stars when you dont need to have your observations on a standard system.   But when I do AAVSO work with my students I use a Johnson/Cousins V filter.  The last V filter I purchased from Chroma and they delivered it fast.

 

Regards,

 

Russ


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#23 robin_astro

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Posted 30 September 2020 - 04:20 PM

 I use a Johnson/Cousins V filter.  The last V filter I purchased from Chroma and they delivered it fast.

 

 

The published Chroma filter passbands are very different from the standard formulation though

https://www.chroma.c...3-bessell-ubvri

 

Robin


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#24 gregj888

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Posted 01 October 2020 - 01:04 AM

The published Chroma filter passbands are very different from the standard formulation though

https://www.chroma.c...3-bessell-ubvri

 

Robin

The individual filters look reasonable but the combined look more like Sloan filters.

 

https://www.chroma.c...in-ubvri-series


Edited by gregj888, 01 October 2020 - 01:06 AM.

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#25 ASTERON

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Posted 01 October 2020 - 02:39 PM

One thing you need to investigate is whether you even need a full set of photometric filters.  If I were you I would start with a single V filter.  Photometric filters are expensive and doing calibrated photometry is tricky and may be impactable if your local conditions do not allow it.   I would highly recommend taking one of the AAVSo photometry courses.  In fact I should take that advice myself!

 

I do unfiltered differential photometry primarily.  That is useful for things like asteroid light curves and some variable stars when you dont need to have your observations on a standard system.   But when I do AAVSO work with my students I use a Johnson/Cousins V filter.  The last V filter I purchased from Chroma and they delivered it fast.

 

Regards,

 

Russ

 Hi Russ,

Yes I understand What you are saying.  I will Delay buying any Filters Until I get the mount and decide on a good CMOs camera.

I think for starters I will definitely restrict myself to differential photometry until I know what I am doing and get the hang of the software and data reduction method ( might take a while lol.gif ).

Once I am more assured of my system and abilities, I will review the sources and start with a V filter.

 

Lots of work for me till i get to  that stage.


Edited by ASTERON, 01 October 2020 - 02:40 PM.

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