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Getting Started in Variable Star Photometry

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#1 btschumy

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Posted 10 February 2022 - 12:21 PM

I have been gearing up to start doing variable star photometry.  I've been imaging a mixed assortment of variable stars in the eastern evening sky at my location.  I am starting out using a DSLR controlled by an ASIAir Pro.  For image analysis I'm using ASTAP (thanks Han).  I extract the green channel and work with that, reporting my observations as TG to the AAVSO.  I've only done 5 stars so far but my measurements seem in line with what others are getting.

 

I'm interested in your thoughts about how to generate a collection of stars to observe.  AAVSO says this is one common problem with beginners, but I didn't find their suggestions too helpful.  What is a good strategy for picking stars to measure other than looking at a list and picking something that is in my sky at the time I'm out.  I don't have access to the entire sky due to obstructions and currently my observing is limited to times before around 11 PM.

 

Thanks,

Bill


Edited by btschumy, 10 February 2022 - 12:49 PM.

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#2 DHEB

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Posted 10 February 2022 - 02:20 PM

Bill,

 

Variable star photometry is fun and useful. I am a visual observer and know very little on how to do it with cameras and so on. But I do know something about developing an observing program.

 

You may want to first think about observability (a star should be conveniently visible from the desired location, not too low above the horizon), feasibility (the star should be within the reach of our instruments), suitability (our efforts must be worthwhile, for example the magnitude range should be large enough such that our own intrinsic inaccuracy matters relatively little to the long term evolution along the light cycle), and your own personal interest. You may start by choosing some famous stars (for example SS Cyg, T CrB, khi Cyg, Mira, R Leo, or Algol), or may go and get lists of stars from VSX according to criteria that define which stars are useful to you.

 

Whatever you do, I hope you enjoy the journey of variable star observing smile.png

 

 

May be you find something useful in these posts :

 

Towards a variable star observing program: 1. general considerations

 

Towards a variable star observing program: 2. choosing and selecting stars


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#3 GaryShaw

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Posted 10 February 2022 - 02:36 PM

If you want to make a direct and useful contribution to research, consider participating in one or more of the ongoing ‘Alerts’. These are calls for observations from professionals who have specific projects that benefit from observations from the qualified amateur community. 
 

As an example, I’m currently providing data in support of Alerts 754 and 758. Both of these Alerts request that we provide variable star data to researchers who have observation time scheduled on the Hubble Space Telescope (HST). HST observation time is costly and precious but the HST instrumentation is sensitive so researchers need to verify the ‘state’ of the targets before the HST team will make final approval for its use by the researchers. In the case of Alert 754, where dwarf novas are being studied, it is vital that the nova be at its lowest ‘state’ during the observing run so that good data from the associated dwarf star and accretion disk, can be captured. Amateurs provide the observational confirmations that trigger the final approval and scheduling of the HST observation time window. 
 

Doing these amateur observations correctly and submitting quality data provides a real and valuable contribution to the research  team. While they are specifically requesting observations in V and B bandpasses, your TG submission, if properly captured and carefully processed and analyzed in AAVSO’s VPHOT application, could be helpful data. I’d encourage you to acquire a proper photometric filter as soon as you can. A V filter is the most important followed by a B filter. Having both for your observations allows you to ‘transform’ your data and make it more consistent with established standards. You probably are aware of the problem with there being nearly “0” availability of Johnson/Sloan filters. Chroma Company (Vermont USA) has just submitted new Bessel photometric filters to the AAVSO for testing and approval for use by our community. Their prior version is available for purchase and a Chroma V filter is what I’m using. If the new version is accepted by AAVSO, I’ll purchase both V and B filters and begin submitting ‘transformed’ data. 
 

So, there are many types of ‘alerts’ and these provide great opportunities for making immediate and valuable contributions to our professional colleagues. Check out the various alerts and notices that AAVSO issues and get yourself on the notification list for new ones. If you haven’t already done so, I’d also recommend you get yourself an AAVSO ‘Mentor’ and get some correct training and coaching on how to capture, process, analyze and submit Variable Star data. The mentor is provided free to AAVSO Members. Finally, plan to purchase a modern cmos (CCD is ok but it’s ‘old tech’) astro camera - mono and cooled. The DSLR is fine for learning but move beyond it as soon ad you can. Your data will be more credible and your contributions to science will be more ‘real’. Finally, if you’re interested, sign up to get weekly email notices telling you how your uploaded ( to AAVSO) data has been used and whether by students or professionals. 
Hope this helps. 
Gary


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#4 btschumy

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Posted 12 February 2022 - 11:23 AM

DHEB,

 

Thanks for the thoughts and the links.  They look useful.

 

Gary,

 

Most of the Alerts I've seen are for stars beyond my range (around 13th mag).  I will keep watching for ones I can actually do.

 

I did sign up for a mentor and hopefully I will hear something in a day or two.  From the map they displayed, it didn't look like there are that many mentors.

 

A dedicated astro camera is in my plans.  Currently researching which might work well.

 

I did finally get VPhot to work.  I was having problems uploading images.  The "wizard" said everything was fine, but they would never appear in my "available images".  No errors, just didn't appear.  I finally found that you have to have the EQUINOX key in the header.  My reading of the spec says the default is J2000 so it should not be a required field.  I had to manually add that and then it worked.

 

I do like VPhot's photometry features and UI a bit better than ASTAP.  Right now my plan is to use ASTAP for extracting the green channel, calibration, and stacking.  Then I will switch to VPhot for the photometry.  The only thing I don't like about VPhot is sometimes there is a long submission queue and you have to wait 30 minutes or so before you can start working with the images.

 

I wasn't aware that photometric filters were in short supply.  It does seem like Chroma is currently selling them.  Do you think there is a real chance AAVSO will say they are not acceptable.  I also see the Baader is selling UBVRI filters.  Have you heard anything about those?



#5 Ed Wiley

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Posted 12 February 2022 - 12:34 PM

AAVSO has a DLSR photometry course that also would cover, I think, OSC cameras, cooled and uncooled. I am teaching the AAVSO CCD/CMOS photometry course starting next month, but it is only for cooled mono cameras and requires at least two photometric filters for the total experience.

 

One thing to think about: AAVSO really needs more visual photometry observers. No camera required, only eyeballs and a scope (even a 60mm) or even binoculars. Visual fills an important gap covering brighter parts of the light curve for stars like Mira-class variables that go through large magnitude changes. Wide FOVs is frequently a must for comp stars. Visual is perfect for this.

 

Ed


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#6 GaryShaw

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Posted 12 February 2022 - 01:17 PM

Hi Bill

 

VPHOT can be flukey with uploads and even finding prior uploads is sometimes a bit of a pain. It’s old legacy sw and seems like it might benefit from a refresh to handle the greater user load. 
 

I don’t know what you’re using for gear, other than the DSLR, but magnitude 14-16 is totally doable with my modest 8” F4 Newtonian. I’m only 18 months into variable star imaging so I should probably stick with brighter stars too but I’m anxious to refine my skills to be useful to the ‘Pros’ so I’ve maybe jumped in a bit ahead of my capabilities. Having a great Mentor really helped me understand how to produce, process and analyze the TG data. He helped me ramp up on the statistical aspects regarding SNR, and understanding different techniques and for single captures versus time series work. Ed Wiley’s course last Winter was a big help and you should consider it.
 

Before I bought the Chroma V filter, I asked AAVSO whether data from that filter could be accepted by AAVSO for V submissions. The folks there are in the midst of dealing with both the filter shortage and also trying to figure out CMOS tech compares versus the older CCD cameras. My question was referred to Richard Berry, a true Pro, who responded that the Chroma Bessel filters should be fine. After purchasing and using the Chroma V for 6 months, I contacted Chroma to buy the B. They told me that they have developed a new series of photometric filters that more closely align with the prior JS filters and that they had recently provided these new filters to AAVSO for testing. So, I’m holding off on buying any more of the version that I have. If AAVSO endorses the new ones more formally than Richards ‘should be fine’ approval, I’ll purchase new versions of both V and B. 
 

If you’re pretty committed to getting into all this, you might want to keep in touch with them and see how the Chroma testing is going. I don’t think AAVSO has ever tested the Baaders but you might ask them. Part of the issue is that all the established variable star Guru’s already have JC filters and CCD cameras so ‘next generation’ observers face a bit of an ‘inertia factor’ - since we’re mainly using CMOS cameras and have no access to JC filters. Even the great variety of JC ‘type’ filters out there and the differing optics they get combined with, results in variations in the optical behavior of the whole package. Hence, AAVSO, instituted the ‘transformation’ process to try to achieve commonality and alignment of the data coming from a vey diverse array of optics. 
 

I hope some of the above is helpful. 
cheers,

Gary 


Edited by GaryShaw, 12 February 2022 - 01:19 PM.

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#7 pbealo

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Posted 12 February 2022 - 05:03 PM

One of the things that keeps me interested are those few stars I observe that vary considerably on a relatively short timescale almost unpredictably. Its like every time I get an image I think "what now??".

 

If you enjoy things like that, try V1117 HER. This is visible now and for the next 4+ months. Check it out in LCG, look at the last year and how it compares to the last 5 years. Something happened last year.

 

In V, over 1/2 the time its brighter than 14mag, and can hit 12.5.

 

This is a UXOR type variable: a star with a dusty ring that's lumpy...

 

Peter


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#8 Ed Wiley

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Posted 13 February 2022 - 10:27 AM

Just a note about filters and CMOS cameras.

 

For those with CMOS OSC or DSLR cameras who get seriously interested in variable star photometry I would encourage them to transition to a monochrome camera. Of course, the major problem with that transition is the photometric filter problem. 

 

There is lots of talk on the AAVSO forums about filters. You don't have to be a member to read the forum posts on Equipment that contains these threads. Some point to specific companies and their filters, but the actual testing of some of the new generation filters seems to be lagging. This is frustrating, but we have to understand that all of this evaluation is being done by volunteers. And, as I understand it, at least some of the companies are responsive to feedback and some filters now available are fine, including Johnson V. My take: attend to the forum talk, see if some of the talk concludes that a particular companies' filters conform the the JC standard* (I think you will find some do) and then put in an order for a Johnson V to start. (Or to end for that matter, many AAVSO members use only V).

 

There is no need to spend a lot of money on a mono CMOS camera for the transition. An inexpensive 12-bit 183mm-class CMOS is quite capable of producing good photometry so long as its well-depth limitations are respected and you keep the images in the linear range of the camera (this based on tests that will appear in the JAAVSO). I have one myself and it yields good photometry against tests with standard stars as targets.

 

Ed

 

*That standard is not simply the transmission coverage, it is also the shape of the coverage. 


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#9 Ikon2k

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Posted 12 March 2022 - 06:23 AM

Bill,

Try the AAVSO list of easy-to-observe variable stars at https://www.aavso.org/easy-stars .  It includes variable stars of several types/magnitude ranges/periods/spectral classes, for which AAVSO provides standard comparison stars.  Another suggestion (that I have not yet tried) is running photometric analysis on the M67 cluster for which AAVSO provides a chart of calibrated comp stars. 

 

At this point I am working on comparison of my measurements to known stars -- I'm four weeks into the photometry learning curve, and also transitioning from OSC to mono camera.  Tweaking settings, learning AstroImageJ, and refining my objectives and workflow. 

 

Dave  


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#10 Degen1103

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Posted 21 June 2022 - 07:38 AM

Could you please recommend, which planetary camera will be better for variable stars photometry - ZWO ASI178MM (mono) or new ASI585MC (color)? New camera has deeper full well and lower noise, but I'm not shure in its sensitivity...



#11 Ed Wiley

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Posted 21 June 2022 - 09:27 AM

Cameras with tiny sensors, so valuable for planetary work, are usually too small (FOV tiny) to be of much use in photometry. As I said above, the 183M-based sensor cameras are OK for a variety of photometric tasks and their capabilities has been empirically tested against standard stars. My recommendation is the 183-based sensor and a V-filter that can be screwed onto the nose piece for a start. Avoid uncooled and color cameras. 

 

Ed

 

ps: I teach CCD and CMOS photometry for the AAVSO. Not tooting my own horn, but I deal with these issues.


Edited by Ed Wiley, 21 June 2022 - 09:32 AM.

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#12 GaryShaw

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Posted 21 June 2022 - 10:09 AM

 

 


 

ps: I teach CCD and CMOS photometry for the AAVSO. Not tooting my own horn, but I deal with these issues.

 

I'll toot his horn for him - Ed is the Guru we all need when starting out in CCD/CMOS photometry...

 

Gary
 


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#13 Degen1103

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Posted 21 June 2022 - 10:34 AM

Thank you for the reply!

Both cameras have comparely broad field (especially with reducer), that's why I began to think about photometry.

Do I understand right that color camera practically does not suite for variables?

 

Cameras with tiny sensors, so valuable for planetary work, are usually too small (FOV tiny) to be of much use in photometry



#14 Ed Wiley

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Posted 21 June 2022 - 01:23 PM

So, a ASI 178 cooled mono ($700) would be a great match for an 80mm refractor (53'x36' ca 1pixel/arcsec) and perhaps useable up to a 8" SCT (12'x8', ca 0.25 pixels/arcsec). Plenty of FOV for the 80, minimal for the 8" where it may be hard to pick up comp stars.  On the other hand a 183-class sensor is almost twice the size. Yes, its ca $300 more expensive.

 

The point is that you need to match camera and scope for two things, FOV and resolution. The AAVSO CCD manual (still relevant on these points to CMOS as well) has some suggestions. I try for an image scale of about 1-1.5"/arcsec at bin 2x2 based on my average seeing. For CCDs it is not recommended to bin more than 2x2. CMOS cameras bin differently and those rules don't apply (some are using CMOS cameras with tiny pixels may bin even 4x4).

 

Small FOVs can be a plus, especially for faint variables and crowded fields; but one presumes that is on a large-ish scope.

 

So, Degen1103; I urge you to look into a cooled camera that matches your scope, If its a smallish refractor you will be in one place, if you have a 14" SCT you will be in a much different place. An, BTW, focal reducers are OK, but only with good flats. You can do work with color cameras, but it is more complicated and since I presume you are looking to buy a camera I recommend against color. Why cooled: in a word, calibration. You need dark, bias and flat frames for calibrating and they need to be at the same temperature as the lights. Cooling vastly simplifies the work load.

 

Ed

 

PS to Gary: Thanks, I appreciate the kind words. My main motivation for the teaching effort is to be pushed to learn more. I have never taught the course without at least one student teaching me something. That includes you.

 

Useful links:

https://astronomy.to.../field_of_view/

https://www.aavso.or...ometryGuide.pdf


Edited by Ed Wiley, 21 June 2022 - 01:33 PM.

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#15 Degen1103

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Posted 22 June 2022 - 12:13 AM

Thank you, Ed, again!

I do not think about cooled cameras, regretfully.

I use 6" Mak F/12, so ASI178MM with 2.4 mcm pixel exactly fits to 1,8 m FL according to  "5x rule" (2.4*5=12). But I hope that a few bigger pixel of 585MC will suite too. I guess, that deeper Full Well of 585MC will be better for photometry, well as larger size of matrix. But I'm not confident in useability of color CMOS for variables. What should I choose from this dilemma - smaller old B/W or bigger new color matrix?


Edited by Degen1103, 22 June 2022 - 01:15 AM.


#16 Ed Wiley

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Posted 22 June 2022 - 04:39 PM

Degen1103:

I never heard of the "5x rule." What is your FOV and resolution (plate scale) of your rig with the 178 camera on you Mak? Since stars are point sources, F12 will not get in the way, but if you don't have a minimally suitable FOV and plate scale then that will get in the way.

 

You can do photometry with OSC cameras, the AAVSO has a manual for DSLRs that can be used to navigate obtaining a kind of V photometry from either a DSLR or a OSC. I have never tried that, I shudder at the complexity, and don't need to learn the ropes as I have mono cameras. But I know people who do this and have had success. I would never recommend a OSC camera as a new purchase IF the motivation was to do variable star photometry. But that is me and I do not speak for anyone but me. I DO think they are great for astrophotography. (And I have one, cooled; I am lazy.) 

 

Photometry with uncooled mono cameras is doable, I would recommend matching the temperature of your darks and lights as closely as possible (I do this with a DSLR by taking darks are various intervals through a run to "average" for temperature).

 

As you are in the "experimental" phase of investigating photometry I say use what you have on hand and give it a try.

 

Ed



#17 Ed Wiley

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Posted 22 June 2022 - 05:07 PM

To all following this thread:

 

Photometry can be done with just about any sensor and any optical train. I know people who do it with telephoto lenses and the AAVSO Bright Star Monitor program does it with CMOS cameras like the ASI183mm on very small telescopes. You don't need fancy equipment, but, you have to follow the rules given by the professional community (not hard to follow at all, a snap compared processing astrophotos) and you need to match your equipment with what you can effectively analyze.

 

Ed


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#18 Degen1103

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Posted 23 June 2022 - 12:20 AM

Indeed, experience is the best teacher.

I hope, even non-cooled planetary camera will bring more relable mag evaluations, than my own myopic eye cool.gif



#19 GaryShaw

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Posted 23 June 2022 - 08:27 AM

Hi

FWIW, I started in photometry with an AAVSO Mentor who taught me how to convert my OSC  images to tri-color and then how to select out and process just the ‘green’ layer to obtain final binned and plate-solved images for analysis in the AAVSO’s VPHOT application.  The workflow I learned was all based on Maximdl but I imagine that the process outlined in the AAVSO DSLR handbook is similar to the method I learned and shouldn’t be overly daunting. 
Good Luck.

Gary


Edited by GaryShaw, 23 June 2022 - 08:29 AM.


#20 Degen1103

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Posted 24 June 2022 - 04:34 AM

High-speed multicolor photometry with CMOS cameras

Performance of commercial CMOS cameras for high-speed multicolor photometry

 

We present the results of testing the commercial digital camera Nikon D90 with a CMOS sensor for high-speed
photometry with a small telescope Celestron 11" on Peak Terskol. CMOS sensor allows to perform photometry
in 3 filters simultaneously that gives a great advantage compared with monochrome CCD detectors. The Bayer
BGR color system of CMOS sensors is close to the Johnson BVR system. The results of testing show that we can
measure the stars up to V ≃ 14 with the precision of 0.01 mag. Stars up to magnitude V ∼ 10 can shoot at 24
frames per second in the video mode.
We present some results of testing of commercial color CMOS cameras for astronomical applications. CMOS
sensors allow to perform photometry in three filters simultaneously that gives a great advantage compared with
monochrome CCD detectors. The Bayer BGR colour system realized in CMOS sensors is close to the Johnson
BVR system. We demonstrate transformation from the Bayer color system to the Johnson one. Our photometric
measurements with color CMOS cameras coupled to small telescopes (11 - 30 inch) reveal that in video mode stars
up to V 9 can be shot at 24 frames per second. Using a high-speed CMOS camera with short exposure times
(10 - 20 ms) we can perform an imaging mode called "lucky imaging". We can pick out high quality frames and
combine them into a single image using "shift-and-add" technique. This allows us obtain an image with much
higher resolution than would be possible shooting a single image with long exposure. For image selection we use
the Strehl-selection method. We demonstrates advantage of the lucky imaging technique in comparison with long
exposure shooting. The FWHM of the blurred image caused by atmosphere turbulence can be decreased twice and
more.

Edited by Degen1103, 24 June 2022 - 05:00 AM.

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#21 robin_astro

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Posted 24 June 2022 - 07:54 AM

The cited papers are looking at a very specific phenomenon (fast variability in chromospherically active stars)

 

The key statement is

 

"The results of testing show that we can measure the stars up to V ≃ 14 with the precision of 0.01 mag."

 

Note the distinction between accuracy and precision ie they are looking for a technique giving precision in simultaneous measurements in different passbands for fast changing targets (ie not accuracy compared with photometric standards, which is an issue with colour cameras as the filter responses do not match the standard photometric filters, although the G channel can be approximately transformed to standard eg Johnson V systems to give reasonable results on specific types of targets)

 

Robin


Edited by robin_astro, 24 June 2022 - 07:56 AM.


#22 robin_astro

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Posted 24 June 2022 - 08:06 AM

Colour cameras can be used for photometry but if you are choosing a camera specifically for best compatibility with  historical and contemporary values measured in standard photometric systems then mono cameras used with standard filters are the way to go


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#23 robin_astro

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Posted 24 June 2022 - 08:20 AM

The quote from the paper that "The Bayer BGR color system of CMOS sensors is close to the Johnson BVR system."  is not correct (Well I suppose it depends on what you consider "close to")

 

Robin



#24 Ed Wiley

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Posted 24 June 2022 - 06:25 PM

I don't know of any paper using a OSC has tested the accuracy (contra precise) of the transformed BGR using standard stars as targets, If there is such a paper, properly peer reviewed, it would be important and I would love to see it..

 

Ed


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#25 Degen1103

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Posted 27 June 2022 - 12:21 AM

Dear Ed, method of "switching" from the RGB colour system to Johnson's RVB colour system is presented there: Meteor colorimetry with CMOS cameras, however without standard stars testing.

 

Naturally, using if DSLR or planetary cameras for photomerty is marginal hobby, but there are some tasks where absolute calibration probably is not so important - minimas of long-periodically variables, for example.




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