Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 09/26/2012 - 16:23
I am just getting started using a CCD for photometry using VPHOT. The software that came with my camera does not have a plate solving feature. What do you recommend that is compatible with VPHOT? Will something like astrometry.net be sufficient or do I need to purchase some more software?
Thanks, Blake
What are your performance/speed goals, what is the budget?
Astrometry.net is free, but you need to manually upload/interface with it, yes?
Some folks may be developing a more automated way to use it, but I'm not following that closely.
More expensive automation/telescope control/plate solve software will do it all for you on your observatory computer...for every image...in a couple seconds. Maxim/Pinpoint is one setup, CCDSoft/TheSky is another. There are probably others, such as Elbrus Star Locator, or PlateSolveXP...which may be free...but do they interface with your scope/mount control software?
What rig do you currently have? (Hardware and software).
Lower end rigs often do not write scope RA/Dec info to the FITS header...which can hamper/slow plate solving attempts.
I hope this helps.
I have learned that with typical amateur rigs/catalogs/software...if the CCD FOV is smaller than about 150 square arc minutes...the odds of a successful plate solve get lower...to the point that it gets frustrating with so many failed plate solve attempts.
You don't need to plate solve your images before uploading to VPHOT - VPHOT will do it for you. You just have to make sure they are fully calibrated (BDF) and have your FITS header values correct.
Jim Roe [ROE}
[quote=roe]
You don't need to plate solve your images before uploading to VPHOT - VPHOT will do it for you. You just have to make sure they are fully calibrated (BDF) and have your FITS header values correct.
Jim Roe [ROE}
[/quote]
[quote=roe]
You don't need to plate solve your images before uploading to VPHOT - VPHOT will do it for you. You just have to make sure they are fully calibrated (BDF) and have your FITS header values correct.
Jim Roe [ROE}
[/quote]
Correct - no plate solve needed. But you need a starting RA/Dec in the FITS header. Some low end software/scope control rigs don't do that at all. Gotta find out what his rig entails....
Here is my what my FITS header looks like when coming into VPHOT (you may recognize it from an earlier post when I wasn't able to upload it):
Key
Value
Comment
SIMPLE
T
C# FITS: 9/26/2012 12:02:51 AM
BITPIX
-32
NAXIS
2
Dimensionality
NAXIS1
752
NAXIS2
582
INSTRUME
Orion G3 Deep Sky Monochromatic Camera
Camera used
SWCREATE
Orion Camera Studio
Name of the software that created the image
IMAGTYP
Light Frame
Type of image
CCD-TEMP
16.6035897435897
CCD temperature at start of exposure in C
ENCODING
RAW
Encoding type: RAW/RGB/YCbCr/Multichannel
PEDESTAL
0
Pedestal
SET-TEMP
16.951282051282
CCD temperature setpoint in C
YBINNING
1
Binning factor in height
EXPTIME
10
Exposure time in seconds
DATE-OBS
2012-09-26T12:02:51
Date
EXPOSURE
10
Exposure time in seconds
XBINNING
1
Binning factor in width
YPIXSZ
8.30000019073486
Pixel Height in microns (after binning)
XPIXSZ
8.60000038146973
Pixel Width in microns (after binning)
HISTORY
File modified by user 'Blake' with fv on 2012-09-26T01:39:04
File modified by user 'Blake' with fv on 2012-09-26T01:39:04
HISTORY
File modified by user 'Blake' with fv on 2012-09-26T10:07:09
File modified by user 'Blake' with fv on 2012-09-26T10:07:09
HISTORY
File modified by user 'Blake' with fv on 2012-09-26T11:36:55
File modified by user 'Blake' with fv on 2012-09-26T11:36:55
It does not include RA/Dec, but VPHOT did prompt me for that information before letting me upload the image. I will admit that this was the first image I took with this camera and I did it rather hastily just to see if I could get it into the VPHOT system. I did a very rough telescope alignment and the actual RA/Dec (as opposed to what I entered) is probably way off. Can you explain what is meant by "fully calibrated (BDF)"?
Thanks,
Blake
BDF means the bias (B), and dark (D) have been subtracted from the image and that a flat (F) correction has been applied. As all images contain a bias, a dark image taken at the same temperature and the same exposure as the light image will contain both B and D will suffice for that part of the calibration. A flat correction involves subtracting a dark (BD) from an image of an evenly illuminated source, scaling each pixel with the mean of all the pixels and dividing the dark (BD)-subtracted light image pixel-by-pixel with the resulting flat frame. You may have to acquire some software to do this for you as it is not for the faint of heart.
Jim Roe [ROE]
It would be good to get your rig/software to put RA/Dec in your FITS header.
Such items as:
SWCREATE Orion Camera Studio
INSTRUME Orion G3 Deep Sky Monochromatic Camera
...leads me to believe that it may be a bit challenging to automatically write RA/Dec to your images...but I don't know until I know more about your:
-mount
- mount control software
- CCD and CCD control software
...and maybe some other details, like your use of the ASCOM platform and drivers.
I have no experience with that CCD and control software. You may need to contact some folks in forums dedicated to that camera/software...and learn what you can and can't do.
...more questions/comments:
CCD-TEMP 16.6035897435897
- is your camera cooled? if yes, is it regulated/repeatable/settable/consistent in cooling? (or is it only on/off for cooling, and you just get a delta T based on ambient air temp?)
PEDESTAL 0
- I'm used to seeing a pedastal of 100 in SBIG cameras...to make sure that you correctly capture all pixel values from your imager. A pedastal may mean that truncation of some pixel values was made...but I don't know that unless I can examine some of your raw images (and calibration frames, and calibrated images)
Thanks in advance.
I am using a Celestron Nexstar controlled by Stellarium. The CCD is a very basic Orion Starshoot G3 monochrome. It came with Orion Camera Studio software that controls the camera and an ASCOM driver. After reading everyone's responses, last night I downloaded a trial version of MaximDL to see if I could integrate everything into a more user friendly package.
The camera is cooled, but I will have to get you the details later as I don't have them in front of me right now.
Thanks, Blake
When I do a search on Celestron NexStar...it's a very diverse and wide field.
What is your aperture and focal length?
What is your mount? Is it configured alt-az or equatorial?
Your camera appears to have regulated cooling. That will make the job of dark frame calibration easier...take darks on a cloudy night (so that you don't waste a clear night)...take light frames of the same temperature...and you can calibrate well. You need to determine what delta T your cooler can deliver...and then adjust your cooling setpoint as the seasons change average temperature...and take appropriate bias/dark frames.
If you are serious about photometry with this camera, I recommend you test it to verify gain, read noise, dark current, and linearity. For that you need a stable, faint light source. An LED (green is fine) and a regulated voltage supply is all you need...and set up the CCD in a closet/basement. We can go over details if you want to do the testing.
If you have a scope with a longer focal length (8 inch, f/10)...then this not-large-chip CCD needs a focal reducer...or you will have low odds of getting a successful plate solve.
Maxim is a powerful Swiss army knife piece of software:
- control the CCD (and filter wheel if you get one)
- control the mount (ASCOM driver)
- plate solving
- calibration
- scripted operation (as one example, for CCD linearity testing...you could write a script to take exposures all night long of your stable light source...of many different exposure times...to build a dense/confident map of CCD response)
Regarding your scope and mount - if you start using Maxim for camera/scope (and dome and coffee maker) control...you will probably find that you need a pointing model so that your mount points well enough to always get the target 'close enough'...no matter now long your slew. (Close enough...to at least plate solve and reslew...which could be 1 or 2 CCD FOV's away...or preferably to always put the target near the center of the CCD frame) In that case you would also need MaxPoint. I use a pointing model (TPoint for those rigs that run CCDSoft/TheSky from Bisque) on every rig here...I consider it of vital importance in automated CCD/telescope ops.
What kind of analysis have you done of your mount's pointing? Periodic drive error? Focus/collimation stability as you slew all over the sky....and so on....
Take this stuff one step at a time. Keep the questions coming.
I have the baby version:
102 mm aperture
1325 mm focal length
I use the mount in equatorial mode.
I haven't performed any of the tests you mentioned (periodic drive error, etc.)
Sounds like the NexStar 4SE. This is considered a beginner scope, yes? If that's the case, there may be some performance issues with the drive, to include backlash. I'm not sure, but I think the drive uses spur gears, not worm gears.
Looking at the manual for this scope...no mention of periodic error correction capability. It may be difficult to get good, long, unguided exposures. (And I don't see in photos, or mentioned in the manual...an autoguider port.)
f/ratio is slow...Maksutov optical tube assembly. You may find a focal reducer helpful, but I don't know if one can be added to that system.
It will be informative to learn what results you get from testing things like backlash (in both axes), pointing, mirror flop, etc.
Hi Blake,
Just catching up on this thread. I just wanted to say what everyone has contributed here is very valid. However, just take it one step at a time. It can be a little overwhelming being bombarded with all the information at once. If you know the rough coordinates of where your image was taken, put them into the header manually in the proper format and see if VPHOT can solve it.
If it solves and you shot a variable star field, load the AAVSO comparison stars from the menu and watch them "magically" get plotted on the image. Again, call up the GCVS or the VSX on the image and watch them be plotted. Take a look at the photometry report. Now things can get exciting!
My point is that if you just solve and do this for that first image, you will want to make all the improvements and efficiciency upgrades that have been talked about in this thread and you will be on your way. Get that first one under your belt, keep asking questions and have fun.
....Tim
Thanks to everyone for all the advice. I have spent the past few days tinkering with my setup and running a lot of numbers through a CCD calculator. Obviously, the easiest solution to my problem is a bigger scope with a better mount. I have come across a deal on a Celestron C8 XLT on a CG5 mount. Specs = aperture 203.2 mm, focal length 2032 mm. Does anyone have any experience with this setup? The reviews I am reading are pretty favorable.
If I got this scope and used a focal reducer to get down to f6.3, I would be able to get a field of view of 13.2 x 17.6 arcmin. This seems to be a decent improvement, but the main benefit would probably be the better tracking with the mount. Also, if I upgrade to an ATK-314L CCD down the line, I could get a big jump in the FOV.
Do you think this would be a reasonable setup to keep me busy for awhile? I have also considered the Celestron CPC 800 or the CGEM 800, with the only difference between the three being the mount.
Not-high-end, mass-produced telescope gear...there can be wide variation in quality of assembly/adjustment.
Testing, and probably tweaking are important.
Every rig I take apart has different problems that usually can be fixed, or at least improved.
You can probably do semi-decent work with the CG5 or other mounts. But make sure you are protected from the wind...mount not very beefy/stiff.
I prefer fork mounts to avoid meridian flip...but German mounts can do good work. Master your rig and you'll get usable results.
A focal reducer/extension tube/filter wheel/CCD can be a tall 'instrument stack' that prevents a fork rig to 'swing the pole', unless you are careful in component selection.
I can't give additional/specific advice because much depends on the results of testing/evaluation.
I would say that some one starting out in photometry couldn't go wrong by getting http://www.willbell.com/aip/index.htm . The book explains what's needed for good images
and it comes with all the software needed to do image processing and photometry, no plate solving needed.
CMP