Hi,
I've been doing DSO astrophotography for many years and am running out of interesting targets so I decided to try my hand at photometry of variable stars.
Over the last few days I've been reading articles on this web site about photometry and have gathered a lot of good information but there are still a few questions.
Here is the equipment I currently have , Orion HDX110 EQ-G mount, TPO10RC, SN8 and ED80 scopes, a QHY8L (One Shot Color) CCD camera, ZWO ASI120 C (OSC), SBig STF8300 C ( OSC).
Currently the main imaging setup consists of the QHY8L mounted on the 10", the ED80 and ASI120 camera is my guiding setup.
I have read that may observers use DSLR's for photometry using the data of the green channel. I understand that this most closely mimics the V filter, typically used.
- Can I assume that what is written about a DSLR and also be applied to OSC CCD?
- How does one compute the transformation coefficient when using a OSC or DSLR? From what I've read it requires data from different filters.
I plan on using the following method to process the stars using Nebulosity4:
- Capture 5 or so frames RAW
- Apply darks and flats as I normally would
- Debayer the frames.
- combine the frames into one color image.
- split out the Green channel ( this will be B/W)
- Use the green channel for the Photometry. For now I'll use Astrometrica for the data reduction.
Should I find this hobby to my liking, I will probably look for a used B/W Camera and V filter and perhaps invest in API4WIN to do the data reduction.
So there you have my story. If anyone has any suggestions I would love to hear from you.
BTW you can check out my web site at http://njstargazer.org.
Cheers
Nor
DSLR have additional filter that rejects IR and UV but also reshapes the response of the pigment filters that are deposited on individual pixels. The astro cameras using ICX or IMX color sensors, in general, don't have such additional filter.
ICX, IMX sensors, in general, are not designed for regular imaging but for the security application market. By the way their color response is often very different than the DSLR one, following the case the green channel could deliver a questionnable photometry or not.
The IR+UV filter of the DSLR is in fact a color compensation filter that rejects the deep red and IR leakage of the green channel (essential), in addition it strongly reshapes the red response and the red roll-off of the green response. That filter is a combination of interferential layers and tinted glass.
I made photometry simulation of number of these IMX / ICX sensors, in general it's possible to make a good (but strong) transformation of the Vmag below B-V of 1 ~ 1.2 . Above it the errors get very large. Best is to use my VSF technique but it's in any case limited to 1.2 B-V with sensors without color compensation. One example of compensation filter is the X-NiteCC1 (attached). In DSLR that filter , in addition, gets interferential IR-UV rejection layers.
Clear Skies !
Roger
Nor,
I'm not able to give you any advice about how to use a OSC camera for photometry. I'd suggest questions on this subject should go the the DSLR photometry forum.
If you have not already done this, I suggest you read the AAVSO Guide to CCD Photometry. The Guide will give you a very good introduction to the subject of CCD photometry, and will help you make a good decision if you decide to buy a monochrome CCD camera for photometry. Getting the right match for your seeing, the telescope, and the camera is important and mistakes in this first step are common.
Calculating transforms requires at least two filters. People usually start with B and V. I'm not aware of anyone using photometric B and V filters with OSC cameras.
The Berry and Burnell book you mentioned (Handbook of AIP) is an excellent reference, and the included AIP4W software would give you a very good start once you have a monochrome CCD camera. The book is not a manual for the software, but it does contain tutorials for using the software for various tasks, including photometry.
The next step would be taking the AAVSO CHOICE photometry courses early next year.
Phil
THanks Phil for that info. I've been getting some good advice from the AAVSO folks so far and will keep it on hand shour I decide to persur this hobby further. At the moment , if the sky ever clears, I" going to mess with some easy targets and see what photomertry is all abbout.
Cheers
Nor
Hi Nor,
The QHY 8L looks like an interesting camera, with 6M 7.8micron pixels and 16bit ADC. In general, I like monochrome cameras because they are more flexible for doing multi-filter photometry, but one-shot color cameras have some potential. In fact, I've purchased one of the ZWO 294mc cameras for experimenting here.
The main problem with most of the color cameras (and DSLR as well) is the poor response of the colored dyes used to give the RGB bandpasses. Their shape doesn't match the standard photometric bands, and they have red leaks. You can add an IR-cutoff filter to help with the red leaks, but they still cause problems with red stars. That said, the G(green) color is close to the standard Johnson V, and that is why we've asked people to use that bandpass (called tricolor-Green or TG) as their primary submission. You can transform it to the real Johnson V with some effort (but don't get concerned with it at the beginning - you will have enough work ahead of you learning the steps to good photometry. As opposed to deep sky imaging, you have to unlearn a lot of tricks, like image sharpening or gradient removal. Photometry wants plain vanilla images, dark subtracted and flatfielded only. I also recommend starting with CVs, eclipsing binaries, or hotter stars like RR Lyr or Cepheids and staying clear of Miras/LPVs with the OSC camera.
You should read the DSLR and CCD photometry manuals, and consider taking one of the CHOICE courses. If you become an AAVSO member, you can also take advantage of the mentoring program to help along the way. I've heard that people can get hooked for life after their first light curve, so beware!
Arne
HI Arne
I've read thru the PDF on Photometry on the web site and found it very helpful. I do have a Baader UHC-s filter that cuts off the IR and UV which may help in the IR leak, though I'm not sure. FOr now I'll mess with the green and see how it plays.
Yup That is about where I am now, determing what to image if the sky ever clears.
THanks for the info.
Cheers
Nor
Hello Arne,
I agree with the fact the colored dyes (organic pigments in fact) that are deposited on the pixels of the sensors of the DSLR have number of issues in their spectral response. But in regular DSLR this is not all what makes the response. There is a color compensation filter (tinted glass + interferential layers) placed a couple of millimeters in front of the sensor (in non-modified DSLR) that completely reshape the response curve of the RGB channels. I have attached a typical curve of that filter (less the interferential layers) in my message just before yours. After that filter the deep red and IR leaks are rejected, very strongly, that's also somewhat a problem: the red side G roll-off is cut somewhat too short, not the opposite ! Most of the difference between G and Vjohnson is in fact on the blue roll-off side and can be compensated by subtracting some of the B signal from the G one.
The OSC don't have such color compensation filter and without some of it, it's difficult to make a good photometry. The ASI294MC has a very large G response curve, difficult to use. It's so large it's even possible to reduce it using a Vj filter in addition, it works !
The txt file is the result of the simulation of an IMX294 sensor combined with a Vj filter. The photometry deviation (mmag) is in columns Gtm (classical transform) and Gcm (VSF technique). Gm is the uncorrected V error. The corrections are anyhow very small.
Clear Skies !
Roger
Hi Nor,
One easy star for the next few weeks is CY Aqr (Doug Welch's favorite star). It is a high-amplitude delta Scuti star, varying by 0.7mag in 88 minutes, so you can get a complete light curve very quickly. It changes almost every exposure, so is fun to watch. From the AAVSO home page, enter CY Aqr in the "Pick a Star" box and click on Create a Finder Chart to get coordinates and identification. It will be easy with your 10".
Arne
Sounds like a good target. I'll give it a shot when I can.
By the way I had messed around with light curves in the past just for my own curiosity. For a few years I was trying to get light curves of Supernovea, as an example 2011dh ( http://njstargazer.org/SuperNova/SN2011dh.asp ). I used a star in the neighborhood as comparison to calculate the Magnitude. It was fun to do but now the big guys are finding SNe way out of my range down to 19 and 20 mag. and usually in the southern hemisphere.
So all in all potometry is not all that forign to me I just need to get the process right for submission to this group.
I've fairly comfortable writing programs and have built one that tells me what DSO is visible in my part of the sky ast what alt , az and time. I have over 70k objects in my DB . I've added the 500K in the VSX DB to mine and am able to get that info about the variables. With that and a chart from AAVSO, I should be able to find the targets that I'm interested in.
HI Arn,
I took your advise and gave CY Aqr a try the other night and ran for about an hour taking 3 minute shots. Keith Graham has been helping me , off line, to understand what I'm doing with Photometry. I hope to get out tonight and attempt to get a 3 or so hours of the star and build a complete light curve for it. That will give us a better chance to compare what I got with what others have managed to do with this star as well as see how the QHY8L behaves.
CHeers
Nor