Hi all,
I’d like to start getting into the photometry of variable stars. I have a Player One Poseidon M monochrome camera (IMX571M). I need to buy 2" photometric filters. Which standard should I go for for amateur photography? Are Johnson & Cousin filters still the best choice? Also, if I want to spread the cost, which filters should I prioritise? I was thinking of getting the V filter first, then the B, and the others later. Could you advise me?
Best regards
Gianluca
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V is the one to start with, followed by B or R. The “problem” with Sloan filters is that there are few comparison stars across the sky.
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(OK. I figured out why the formatting was so crazy - No “enter” when adding paragraphs! So, I’m re-submitting my post.)
Welcome! I’m a basic amateur, so take my recommendations with a grain of salt since others will weigh in. It is my understanding that most variables in the database have J-C values, so the possible number of targets/comps with J-C filters would be much larger. V is always a good choice to start. Two filters would be needed for transforms, and B is a common second filter.
However, if you are just starting, there are a number of projects that can be performed without a filter. Additionally, photographic green and blue or red can be used to get started since their cost is much cheaper. (The DSLR Handbook provides lots of information for this.) This would give the advantage of experience in gathering and reducing data and give you some experience to decide what type of targets you might be interested in.
Finally, a basic understanding of visual photometry would probably be useful even if you decide to only pursue CMOS photometry. Spending some time with binocular targets and uploading that data would be helpful in getting a feel about CMOS ensemble photometry in which the number of comparison stars would go up significantly compared to visual photometry, as well as how data uploads to the database are handled.
Folks in the forum are very helpful, so ask any questions as you begin your journey.
Welcome again!
Mike
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I agree that starting with a Johnson V filter is the best move because it’s the most requested one for AAVSO programs. If you want to expand later, adding a B or I filter will allow you to calculate color indices, which adds a lot of value to your observations.
A good tip for a beginner is to first test your camera for linearity to make sure your data stays accurate. You can even start practicing with unfiltered photometry (CV) to get used to the software and the whole reduction process before spending money on high-quality filters. This way you’ll be much more confident when you finally start using them
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Hi,
when you talk about unfiltered photometry, do you mean the complete absence of a filter, or is using a luminance filter (UVIRCUT) still acceptable?
I’d like to take this opportunity to thank everyone for the helpful information.
Best regards,
Gianluca
I think that depends on what you are working. Most such targets are interested in a general brightness, such as determining when a star is spiking, like a nova. (See V0844 HER, for example, a star I follow). In such case, unfiltered observations are fine.
However, I would think that you might want to keep a clear filter on the camera in order to protect the CMOS chip. In that case, luminance is fine. All the comps will have the same filter, so you should get the same result to upload.
It is possible to transform clear/luminance filtered images. CV transforms clear to V, for example. However, in that case transformation coefficients would still be needed in V and another filter, like B.
So, if you have a luminance filter, you can try your hand at some cataclysmic variables, like V0844 HER that I mentioned above. The head of that section can give you some pointers on ones to start with, I would think.
Also, if you have a photographic green filter, that is pretty close to J-C V filter, so you can use that in order to try other objects. The DSLR manual cprovides good information about photographic filters. Best regards.
Mike
The “problem” with Sloan filters is that there are few comparison stars across the sky.
Just for the record, this is pretty patently false. Two of the most widely-used, deep, all-sky reference catalogs, Pan-STARRS (PS1) and ATLAS’ refcat2 both use SDSS-like photometry systems (grizy in the case of PS1, and griz for refcat2). Heck, even APASS itself provides gri photometry across the entire sky.
The ugriz response curves overlap significantly less than UBVRI, and the professional world is largely moving away from the Bessell/Johnson/Cousins systems and towards these Sloan/PanSTARRS-like bandpasses for that reason (among others).
I’ll echo @Eric_Dose’s comment at the bottom of this thread from a few years ago:
Which begs the question “why does anyone even bother with BVRI any more?”
Coming from the professional side there are good reasons for Johnson-Cousins or Sloan. A lot of modern work is moving toward Sloan filters as time goes on and it might be good to work in those filters as more large astronomy projects move that direction. However, that doesn’t mean Johnson-Cousins should be thrown away.
There are many stars such as pulsating variables that have been monitored in JC filters for decades. For consistency it is often nice if filters don’t change since the timing of events is slightly impacted. For careful long-term monitoring it is nice to stay in JC filters. The other place I appreciate JC filters is when matching to my work with IR filters in the JHK regions. There are color indicies that use JC filters combine with IR filters for studies. While one can convert between it is often nice just to work in colors like (V-H) or (V-K).
So, the actual answer is that things are moving more toward Sloan filters over time, but Johnson-Cousins aren’t going away tomorrow. In our telescopes I sometimes mix Sloan and JC filters (7 slot filter wheel). We also have Stromgren filters and other specialized narrow filters. If I had to pick one to start with it would be either a V filter or a g filter (Sloan). Although depending on what you want to monitor a Sloan r is also a good starting point. I’m just happy you want to contribute to the world of variable star reseach.
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While I agree there is a lot of Sloan photometry around the sky, the consistency of the zero-points is poor. Each of the SDSS catalogues shift slightly in zero-point, the Pan-STARRS system is different from the Smith et al Sloan ‘standards’, and the southern SkyMapper different again. APASS DR9/10 has serious zero-point issues (a few tenths of a magnitude shifts, zonal errors etc) outlined by Tonry et al in the refcat2 paper, where they re-reduced the southern APASS data in order to make it useful. Oh, and you’ll find almost no reliable Sloan photometry for stars brighter than mag 11 or so.
In addition, when you consider color-indices, the g filter especially is badly placed when it comes to discerning stuff like temperatures, metallicities, and interstellar reddening (B is way better in this regard when used with other colors). Overall the non-overlapping rectangular passbands are actually fairly disastrous when it comes to transformations to a standard system.
If all you care about is differential photometry of variables to get periods (say), then none of this matters (much). But if you want to get some astrophysics out, you need filters (the narrower the better) that are well-defined by standard stars and linkable to the modelling.
Another reminder: Mike Bessell’s last name has two l’s, though it is commonly misspelled.
\Brian
I’m not sure that “disastrous” is the word I would use…
I do agree that there may be reason to continue using UBVRI(JHK) if you care about consistency with specific historical data, but the future is definitely in ugrizy (LSST, PanSTARRS, ZTF, DECaLS, etc.). I also wasn’t necessarily trying to litigate this here, I was mostly just pushing back on the claim that there aren’t many reference stars in the Sloan passbands.
(Thanks for the note about Bessell—I fixed it in my original comment.)
I’ve had to decide what filters to install for my systems a number of times over the years and at least at the moment, my thoughts are…
The easiest, cheapest, most straightforward thing to do that has most value would be to buy a single V filter and screw it into your camera and use just that. No filter wheel. Only one filter to buy. You are as close as you can get to matching a standard and “popular” photometry bandpass without transforming at the least overall cost. You can take the filter out to do unfiltered CV and get maximum S/N and maximum cadence if you want to work on cataclysmic variables.
If you are OK with biting the bullet and getting a filter wheel, then my order of priority for what filters I would populate that with would be…
1st set: B,V, Clear or luminance, and “dark” if you have a camera without a shutter.
2nd set: Add Rc or Ic. Lots of great things about the Sloan filters and it’s where things are going, but it will be easier for the near to medium term for amateurs to stay with J/C bandpasses. There are more reference stars available and it matches most of what amateurs and pros have been doing for a long time.
3rd set: Either Rc or Ic, whichever you didn’t get above. I think most would argue that you should get Ic first.
There are plenty of good arguments to disagree with my thinking above. There isn’t a perfect answer I think.
Clearest skies,
Walt
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