Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 10/11/2012 - 23:37
Hi
The video tutorial mentions that the ensemble method can "improve accuracy but increase the error rate".
Could someone please elaborate a bit on the subject? Under what circumstances is either method preferable?
Thanks
Juan
Hi Juan,
what video tutorial are you refering to? Cheers,
Mark
At the bottom of the page you'll find 8 video tutorials on how to use VPHOT (Astrometrica)
http://www.aavso.org/vphot
Your question is a very good one. When writing the tutorial, teaching how and when to use ensemble versus a single comp star was beyond the ken of the tutorials time limits and focus. So the disclaimer you mentioned was added so that it would alert you to seek more knowledge on this subject, which you have correctly done!
I'm sure there are many who can provide better details on resources to give a full explanation of this issue and a detailed answer to the many things to consider.
For the purposes of VPHOT, we thought it was necessary to point out that while the ensemble method is ideally preferable, it is not always practicle, and that always blindly applying it can cause more harm than good So the short answer to the question is that what to do depends, among other things, upon the quality and type of the comp stars available.
For example if there are ten comps in the field, but nine of them have a large error value or are very far from the magnitude of the target--whereas one comp which has a low error and is in the same ball park magnitude wise as the target--then that is a case where using just the one comp would yield a more accurate result in some ways--but there is always the risk that other things might bring down the overall confidence level of the measurement (not really quantifiable, though I may be incorrect).
On the other hand, when more than one to many comps are available with qualities such as a low error value, where they are close to the target as opposed to near the edge of the frame--which have ADU, SNR and magnitude values within certain parameters which others or I can describe later in more detail--then it is most always better to use an ensemble.
This is the fun part which requires some discretion and which requires a bit of experience.
So to begin with, never just blindly use all the comps in the field under the assumption the ensemble method is better If there are many comps from which to choose, they should ideally have an error of less than .01-.02 and certainly not more than .05 to .1, an ADU of at least 10,000 but not more than, say 30,000, an SNR of at least 100 as well as a magnitude comparable to the target's, (for example if the target is around 120 then try to avoid using a comp of 150).
I think I will stop there for now as a general point of departure.
We are just starting to work on a follow up tutorial to address enhancements to VPHOT as well as a manual, so I am glad you raise the issue as I am still not sure if we should cover questions like this in the second tutorial in more detail. On the other hand there is so much material to cram into the tutorial just on how to use the software, that I tend to think how to actually do photometry--which your point addresses, should be addressed in other materials.
But please everyone, let us know what you think. Also I would be interested to know if people will prefer a voice over tutorial such in the same style as exists now, or whether showing the same type video with instructions and commentary written in printed captions is more desirable.
Ken Mogul (MQE)
That was
Thanks a lot Ken !
That was most enlightening and the tutorial are not only helpful but indispensable for someone like me. As for VPHOT it is a fantastic piece of software. It allows the beginner to start gathering good data before fully understanding a complex technical subject.
I love the current format and structure of the video tutorials. You can't go too deep at the beginning or the challenge will seem monumental and you'll loose mere mortals like me. In your more advanced series of tutorial. I hope that you'll cover the determination of transformation coefficients. Can't wait for M67 to show it self up.
Juan
M67 is now up at elevation 50 deg. before morning twilight starts.
But there are Landolt fields all over the sky.
Attached is screen shot of 27 x 17 arcmin. CCD FOV centered on part of field SA-110...evening sky...get it right after sunset. The red numbers by the Landolt standard stars are B-V color indices.
If your FOV is smaller...cover the stars in upper right corner of this field so that you cover a large range of B-V values.
Attached is a spreadsheet of Landolt stars. Get fancy...export to a .CSV file or other formatted text file...import into your favorite planetarium software. (I then display only the data on star name, such as SA-110-502, the V mag, and B-V color index...that's enough info to choose fields that your CCD FOV can cover.)
Now you can choose fields for any time of year, or time of night. Not every field will cover a very big range of B-V values. Some fields are faint stars for big scopes. Choose carefully.
Automation software makes it easy if you must re-do the images (tweak pointing, exposure, etc.)
Keep the questions coming.
Do transform coeff's keep long or are they better fresh?
Once or twice a year (to determine, or check the value of transform coeff's) is good for most scenarios...
...e.g. the transform coeff's such as Tv and Tb-v. Nightly changes in the atmosphere typically don't have much impact on them.
But zero point and extinction values change from night to night. But if you don't do all-sky photometry, you really don't need them.
I recommend you just determine those coeff's that are useful for differential (i.e. all stars in one CCD FOV) photometry.
But before we determine transform coeff's, we'll sanity check your filters to make sure they are performing as they are supposed to.
Your first run on various Landolt fields (or M67) will probably be practice to make sure you get proper exposure values...the brightest star should be close to the top of your linear range...take several images per filter so you can average measures for even greater confidence. Choose stars/fields that allow not-long exposures, if possible...such as 20 - 60 seconds.
This is much more fun
Boy !
This is much more fun than pretty pictures !!!
Juan
The Bright Star Monitor is the telescope for which we have the best long-term coefficient information. This is a refractor with colored-glass filters. Over a period of 3 years, you can see definite trends at the few percent level for each coefficient (especially blue). Within a given year, trends are hard to see. So if you determine your coefficients several times within a year, and take the mean value (each night of determination will yield a slightly different value, so averaging them is the correct procedure; don't trust just a single night), then that value should be ok for the season or year.
Zeropoints change more rapidly (you can follow the variation during a season); extinction will be different for each night. So if you are doing all-sky photometry, you need to calculate coefficients often. If you are just doing differential photometry, using mean extinction and mean transformation coefficients will usually suffice.
This holds for the BSM system; I haven't done the same kinds of tests for reflectors, SCTs, or dielectric filters, at least for CCDs (there is information in my old photometry book for changes seen with a Cassegrain telescope+PMT). Ulisse Munari has that information for the ANS consortium and will be submitting a paper sometime soon detailing what he has found.
Arne
[quote=astrolopitec]
Thanks a lot Ken !
That was most enlightening and the tutorial are not only helpful but indispensable for someone like me. As for VPHOT it is a fantastic piece of software. It allows the beginner to start gathering good data before fully understanding a complex technical subject.
I love the current format and structure of the video tutorials. You can't go too deep at the beginning or the challenge will seem monumental and you'll loose mere mortals like me. In your more advanced series of tutorial. I hope that you'll cover the determination of transformation coefficients. Can't wait for M67 to show it self up.
Juan
[/quote]
Juan
Don,t bother waiting for M67 to get high in the sky. Just pick any of the standard fields that are near the zenith. There is a list of them here
http://www.aavso.org/standard-stars-vsp
Just put the coordinates into the VSP and all the standard star values are in VSP. Makes it very simple.
Cheers
Terry
Juan,
No one mentioned NGC 7790, which is an exceptionally well positioned standard star field right now, for someone in Quebec since the center of the field is at 23:58:23.20 +61:12:25.0 J2000. Terry would never think of it since he is in the southern hemisphere. Attached is a sequence Arne published for it.
The star labels may be difficult to see in the PDF. I downloaded this years ago from AAVSO. I f I recall correctly, I obtained this from a link in a version of the CCD photometry manual that was in use in 2005.
If you can't read the ID numbers in the PDF of the star field, perhaps Arne or someone at AAVSO can point you to a URL to download the file from which I made the PDF. I can't attach the original HTML file that I downloaded in 2005.
Brad Walter
WBY
and you can capture 3 Cepheids (CEa+b Cas and CF Cas) if you take another set of exposures that ensure that they aren't saturated!
If you have other questions regarding VPHOT, I do mentoring, sometimes via telephone...feel free to contact me off line if I can help:)
My initial contact address for this is klm@hush.com
Welcome to the club and glad to see that this is interesting to you!!
Ken