HELP PLEASE! I am fairly new to Photometry so any help will be highly appreciated.
I just started doing this couple of weeks back and have successfully processed 4 targets which are easier to recognize, for example, GAM CAS, BET LYR, DEL CEP, etc.
However for fainter and more complicated targets I am finding it difficult to recognize correct stars.
Many times I am 99% sure but not 100% sure! This is not acceptable in any scientific work.
I am using MINIWIN and I have 3 specific questions:
1) How do I download an Atlas for Muniwin so I can recognize/search stars?
2) If I have RA/DEC position of a star can I enter it in Muniwin to make sure that the star I have picked is correct?
3) Muniwin gives you many options of Apertures #1(2.00), #2(2.73), etc. before it outputs the results curve based on your selection of Aperture. What Aperture should I use? (Normally, lower apertures are giving me more accurate numbers, I think)
Thanks again in advance for your help. Your guidance will shorten my learning curve and prepare me for the big SS CYG event coming up in November! And most importanly it will give me the confidence of accuracy of my results.
-Vinay Bhatt, Austin, Texas
I’m sorry to say that I’m completely unfamiliar with the software package you’re using, and I’m unable to answer your specific questions.
I’ve been making DSLR observations for a couple of years, and it’s a very rewarding way to observe. It does require some effort to achieve accurate results. I strongly recommend that you download and read the AAVSO DSLR Observing Guide, which can be found on the Observing tab under Observing Manuals. It’s an excellent resource that will help you get off on the right foot. I’m using AIP for Windows, primarily because I’m familiar with it from years of nova searching. I’m not comfortable writing computer code , so I make my own computations rather than let the software do it all. You might consider using the AAVSO VStar software, which might be easier and has a large community of users. It’s also available free. Others can give more information on that than I can.
Comparison stars are best found using the Variable Star Plotter. You will want to ask for a table of photometry, then plot and print the charts in the size and format matching your images. The AAVSO has a wealth of resources and expertise. I strongly encourage you to take advantage of all it has to offer.
I hope this has been helpful.
Best wishes for your endeavors,
I use Muniwin and I am happy with it. (A little downside, Muniwin has no PSF function i've read..)
Now I am finding the stars without problems, but that took a while. (Hmm.. I also like to have an star atlas, but this needs platesolving I quess...)
Finding stars: so what you can do is:
1) Attach your DSLR in a way that North is up. Therefore you have the same orientation like the VSP chart!
2) Determine your Field of View (FOV) See: https://astronomy.tools/
With your telescope focal length and cameras pixelsize and sensor size...
3) Plot VSP charts with the corresponding FOV.
4) I have with the AZ-EQ6 Pro mount. After some month of testing, now the GOTO is very accurate, so the variable is always in the center of the frame! (If you have a place where you can park the mount permanent, it will be benefitting.. (you can resume from the park position...)
I use the ensemble option. (2-6) compare stars and at least 2 check stars.. here I get 3-5 mmag error with good seeing..
If the "Wachstumskurve" - growth - curve is looking like this:
the compare + check stars were all good. Select the aperture with the lowest possible error (Std.dev.).
If the curve is jumping wild, better select other stars.
Muniwin reducing steps:
The reducing steps are:
1) Convert the raw files to fits file format, using TG filter (tri color green), which is close to Johnson V
2) Time correction to UTC
3) Apply masterdark and 4) masterflat
5) Do photometry.. Here you have to look how big the aperture (star + sky) must be.. Do not have other stars within the variables aperture!
6) Match all frames (search for a master frame, by whom you have the most star matches)
7) Plot a lightcurve ..differential photometry, the "Ensemble Option" with more compare stars provides more accuracy in mmag.
8) Save data, with the Aavso extended file format, you have to select the correct magnitudes of the comparison stars.
PS: How can i upload a picture?? Its on the server but the picture button is not there anymore???
This is so great about Aavso. Its is a tightly coupled scientific community that shares common goals.
Peaple ready to help new comers is a great tradition and AAVSO's mentor program is Awesome.
I have a great mentor in Dr. Barbara Harris.
I am a member of Slooh remote telescopes and there also I have enjoyed the same spirit.
I love Muniwin's workflow. I do my stacking, alignment, calibration in Muniwin and the software does a great job in cleaning up my stacked image.
Now, coming back to your response in detail, Thank you so much.
Ever since I asked this question to AAVSO members, I have downloaded MUNIWIN user's manual and read more. I got some new hints from your response as well. It has given me confidence in my work accuracy.
Yesterday night, I did an experiment on Aperture selection process. What I found is that if variable star's app. magnitude increases, std deviation decreases.
This are my results for BET LYR:
Aperture #1 (2.00), App. magnitude V=3.45072; Std. Deviation (d)=0.03628==> (V=d) = 3.487
Aperture #2 (2.73), V=3.47689, d=.03589==> (V+d) =3.51278
Aperture #3 (3.82), V=3.4992, d=.00533==>(V=d)=3.50453
Aperture #4 (5.27), V=3.48902, d=.02523==>(V+d)=3.51425
My Comp star is GAM LYR (V=3.24), Check star ZET LYR (V=4.36), FWHM=3.31
So, if I select Aperture #3 with lowest std. deviation (.005), and report my results as 3.5 (3.4992) I would be more accurate than simply taking Aperture #1 (2.0) value as 3.45072. I always report std. deviation error magnitude along with my readings as a good habit.
I am still working on plate solving issue for SS CYG. My image has more than 300 stars. To identify SS SYG is difficult! If you have any idea about this partcular target please let me know. As I said before, I come very close to SS SYG and say this has to be it. But how I can be 100% sure for a faint target like this star? The funny thing is, Muniwin has a feature called "Find Variable Stars". When it plots all the variable stars in FOV, it presents them in a weird way!
Thanks again for your help and I am sure in future we will have more fruitful conversation.
We have two things in common! Passion for Variable stars and we use the same software!
In post #1 you wrote: “…for fainter and more complicated targets I am finding it difficult to recognize correct stars.”
And in post #5 “I am still working on plate solving issue for SS CYG. My image has more than 300 stars. To identify SS SYG is difficult!”
You have not said what lens or telescope you are imaging through. Your equipment will of course determine the width of your field of view. In crowded fields, and particularly with faint stars, identification of targets can be difficult with wide fields of view. Furthermore, DSLR photometry with shorter focal lengths is not optimal for faint targets.
It is not absolutely clear what your problem is. Is it that you are not sure you have found the region of the target? Or is it that you know exactly where the region of the target is, but you cannot see an image of the star? If the latter is your problem, then you cannot perform aperture photometry, and you will need to change the focal length of your lens/telescope and/or the exposure time of your images.
I hope I explain better this time.
I am using no telescope.
Its is all pure DSLR Photometry.
I aim my Canon DSLR to the sky, take a series of 10 sec. exposures and then use Muniwin to stack, calibrate them and do Photometry. I am using a 35 mm Prime F/2 lense for this purpose so my FOV is 36.6x24.4 degrees according to Astrometry.net (quite wide FOV).
My real question is can I use this procedure to capture and do photometry of SS CYG (which is lately 11.7 mag). I understand that this Dwarf Nova is going to be active soon so I am interested in this target.
Shall I use zoom lens (like 70 mm-100mm) or take exposure for 15-20 sec so that I will have more photons and I can capture faint star like SS CYG?
Ever since posting this blog, I fould out that I can change a star detection threshold in Muniwin and now the software detects many more (>1000) fainter stars. This makes my life even more difficult to pick SS CYG. How can I be 100% sure that I have selected correct star?
Hi again, Vinay.
The equipment you’re using is best suited for the brightest variables, such as Betelgeuse. The wide field is needed to include the bright comparison stars required. SS Cyg would be an extremely difficult target, if at all possible.
I’ve been working with standard camera lenses since I started. I began working on the stars in the AAVSO binocular program, which typically range from mag 5 to 9. I was getting my best results by stacking 30 second exposures with a 105mm f2.5 portrait lens. I’m at a moderately dark site southeast of Tucson.
I’m currently using a 180mm f2.8 telephoto, and getting down to mag 12 with 30 seconds at f2.8. This rig is too heavy and unwieldy to point with a camera tracker mount, so I had to invest in a Celestron CG5 equatorial mount. It’s computerized, and allows me to accurately find my stars once it’s been properly aligned.
I hope this helps
Thanks, John, for your suggestion again.
I agree, I will have to try out my higher focal length lenses.
And as you suggested I may need to track my camera.
I agree with John. I use a 200mm f/2.8 prime Canon tele lens at f/3.5 with a Canon EOS 500D DSLR (I think it used to be sold as the EOS Rebel T1i in the USA), in a fixed position on a tripod. I take 15 exposures each of 10 second exposures. I do not stack them, but of course that could be done. The benefit of not stacking is that I can calculate standard deviations for the results on the check stars. I'm comfortable with precision down to about 8th mag. See the recent part of the light curve on T Cen. Most of the DSLR obs there are mine.
Clearly, for much fainter stars, a tracking mount would be needed.
I would like to mention the importance of knowing the Linearity Curve of the camera for a correct photometry. All sensors have a point where they "stop knowing how to count", this point must be taken into consideration to make a correct calculation. Not only must we not saturate the images, but we must also know where this ADU`S limit of our sensor is.
I use a Nikon d3200 (12bit - 4096 adu's), the ADU`S limit is around 3600 (RAW not debayer).
I can understand your interest in SS Cyg, but there are plenty of stars needing observations, all with interesting stories.
There are three good stars in Auriga for your current setup. I’d like to suggest you check out Psi 1 Aur, UU Aur and AB Aur. The first two are almost naked eye semiregulars, and AB is a star in the YSO program. It’s usually about mag 7, so it will serve as a good test for your equipment, in addition to providing data on a unique class of variable. (It’s also too bright for my setup. YSOs are my new obsession.)
I am glad that a retired sheriff is guiding me in my journey of exploring Variable Stars.
Per your suggestion, I will try out the proposed targets in Constellation AURIGA.
So far I have never paid close attention to this constellation. This will give me opportunity to knw it better!
Vinay, I have just noticed the following, which you posted above:
"I fould out that I can change a star detection threshold in Muniwin and now the software detects many more (>1000) fainter stars. This makes my life even more difficult to pick SS CYG. How can I be 100% sure that I have selected correct star?"
To find the correct star it must be visible in your image, and you must be able to recognize it by comparing your image with a chart in which the target has been clearly identified. It helps if the scale of your image is similar to the scale of the chart, and if the number of stars in the chart is about the same as the number of stars in your image. To put it another way, it makes it easier if the range of stellar magnitudes in your image is about the same as the range of stellar magnitudes in the chart.
I completely agree with you, Roy. Thanks for your comment.
Lets say I found a star detection threshold which gives me a similar star pattern in my image as compared to my finder chart.
My question is" "visual plate solving is the only method in detecting your target star?"
No software does Astrometry for you and confirm that the variable star you have selected for photometry is this name or ID?
"No software does Astrometry for you and confirm that the variable star you have selected for photometry is this name or ID?"
I can only help you a little with this. Others may have more info. AstroimageJ has the capability of plate solving, using the Astrometry.net Web Portal. See the PDF describing AIJ at https://arxiv.org/abs/1701.04817
I use AIJ, but not the astrometry features. My understanding is that astrometry will attach World Coordinate System (WCS) information to the image. Some stars will be labelled with their catalogue numbers (e.g., Henry Draper). I don't know that the target will necessarily be labelled. However, with WCS information attached to the image, the RA and DEC corresponding to the position of the cursor will be displayed in a panel above the image. Since AIJ allows a wide range of zooming, you could presumably obtain pretty accurate coordinates using the above method.
If any other software does anything like the above, I am not familiar with it. However, I seem to remember that Aladin Sky Atlas (https://aladin.u-strasbg.fr/) allows importation of user images.
I perform Astrometry.net on a routine basis but many times Astrometry.net does not identify fainter stars. Let me try out AIJ and see if its workflow is helpful in the identification process. Also I will check out Aladin as you mentioned.
Hi again, Vinay
I think you should really check out VStar. I have no experience with it, but I reviewed a bit of the Introductory tutorial, and it seems to have everything you’ve been looking for with automatic comparison star plotting. In fact, it seems to allow an observer to do everything needed for photometry.
I would love to use it, but all my internet access is through my IPhone hotspot.
Can anyone give us a better description of its capabilities for DSLR observations?
I am looking into VStar. It has two versions. One can be downloaded to your desktop. Second version is Java based and it runs online so you dont need to download anything. You should also give it a try. As it is Java based, it runs very efficiently.
My apologies to Vinay and readers of this thread.
I was confusing VStar with VPhot, which is my actual recommendation for Vinay to investigate.
Last night I looked into VStar. I figured it out.
Interesting program but does not do any Photometry or Astrometry.
It is purely for Light Curve plotting and data analysis.