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Photometry with a ZWO ASI120MM

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Justin Fuller
Photometry with a ZWO ASI120MM

Hi everyone,

I'll be getting a ZWO ASI120M monochrome CMOS imager soon, I'll be mostly using it as an autoguider (for my DSLR) and planetary imager, as well as maybe some deep sky imaging. I'm interested in photometry and did a little searching, but couldn't find anything on whether or not anybody had done photometry with it. It seems to me like if you can do photometry with a DSLR's bayer matrix CMOS, you could do it with monochrome CMOS, but I'd like to hear your advice. I'd be using the camera on a 5" Celestron SCT, with a f/3.3 focal reducer, giving me about a 1/2 degree square imaging area, with about a 1.9 arc second/ pixel resolution. Is this a worthy setup for doing valuable photometry, and if so, should I invest in a quality V filter to get started, or can I get away with using no filter at first, and still get useful data? Thanks for reading and for any advice you can give!



hambsch's picture
Should work out fine


your CMOS should work out fine. After checking the internet it seems the camera has a 12 bit pixel depth which translates to 4096 ADUs, hence exposure for brighter stars need to be limited not to saturate the chip.

It can handle longer exposures (up to 1000s), so also for fainter stars it should work.

Your combo has a large field of view so you could go for brighter targets but also ones in the probably mag 12 region. You could start unfiltered but if you want to do decent work you should go for a V filter as well.

Just try it out on a medium bright target say an eclipsing binary or RR Lyr star where you see within hours strong changes in the light curve.


Josch (HMB)

Justin Fuller
Thanks for your input

Thanks for your input hambsch! I'll definitely put a "V" filter on the wish list then. I was looking at Aperture Photometry Tool, or LesvePhotometry for software, is there an analysis software you recommend? Thanks again.

HBB's picture
Your camera




Ditto what Josch is saying.   Even though your camera should saturate at 4096, like any camera doing photometry you should do linearity testing to make sure that it is linear up until 4096 and doesn't level off well before that.  

You can also use your DSLR for photometry.   If you do then like your other camera it needs linearity testing also. At least you can image targets with this without a V filter.

You definitely will expand your choices of targets with a V filter but one class of targets that you can do without a V filter are eclipsing binaries.  If you are imaging an EB for a light curve, the interest is more in the timing of minimum rather than the absolute magnitude of the star.


hhu's picture
Justin, You also can observe


You also can observe High Amplitude Delta Scuti stars without filter. Timings are the most important issue here, just like with the RR Lyra stars.


Photometry software

Hello Justin,

I can really recommend APT as a great tool (with OK documentation) till you have two following "things":

  • Data frames with high-quality world coordinate system (WCS)
  • Limited number of frames

Why those two points? When you have good WCS, you can measure your sources based on coordinates. Of course this is a Good Thing. Even when your imaging software can create WCS, you should check that it can create a TAN-SIP format, because it is not possible to cover your (relatively) wide FOV by ordinary linear WCS function. E.g. MaximDL/Pinpoint creates high-qualituy WCS but not for tools used by professional astronomers - Bob Denny has not published Pinpoint's algorithms as far I know, so e.g. APT/Aladin/DS9/IRAF/... doesn't support that implementation of WCS "tweaks".

While APT can be easily automated by scripting, it is rather slow to execute. I mean SLOW! Even on decent computer. Measuring hundreds of frames using scripting can take many hours, because you have to execute APT again for every frame. When APT is already running, it's speed is OK.

If you feel yourself comfortable with command line tools and are Mac or Linux user, there is (almost) nothing better than having sex :-) SExtractor has fairly good documentation (e.g. SExtractor for Dummies), and more important, can create good quality photometry. It is used in many sky surveys nowadays. It's performance is also excellent. Downside is - you can run it easily on Mac and Linux and while there is patch for Windows, I'm not sure how easy it is to compile. Maybe there are "third-party" installation packages for Windows - Google may know.

There is also Munipack or C-Munipack ( but I haven't used them by myself lately. Second one runs on Windows, too.

Best wishes,

hambsch's picture
Photometry software

Hi Justin,

I can not comment on APT as I use Windoofs based PCs.I use LesvePhotometry as it is the best software for my myriads of data. You would need the full version of Pinpoint though. IT is about 150 USD if I remember correctly. An investment only useful if you get much data and you will do phtotometry as a main part of your hobby.

Otherwise I would recommend softwre which outputs AAVSO format (MAXIM, AIP fo Win, ....). You could try with MUNIWIN as it is free and has the advantage to measure all stars in the field, so you could look for other variables in the FOV. It is a neat program to learn the basics and generate lightcurves. Easy to learn and quite fastdefinitely with the size of chip you use.

Even though others say you could do without V filter but to be taken serious you should use a V filter vor high amplitude Delta Scuti and RR Lyr stars.

Only cataclysmic variables are normally observed unfiltered due to their faintness (typically 13-18 mag).

Hope that helps,



APT works well under Windows, it just requries Java JRE installed. Probably you have JRE already.


MJB's picture

I also use Lesve the only downside to which is the requirement to have pinpoint liscense.  A local professor that works a lot with amateurs though uses Cmuniwin.  It is free and will follow your target star and comps through a large stack of images just like Lesve.  What I am not sure of is how friendly it is for producing standard format aavso reports.  Just never have looked that hard at it. 


hambsch's picture


I have used CMUNIWIN in the past a lot to look also for new variable in the field. However since I get too many observations I only use LesvePhotometry as it is based on a database for variables and comp stars. It is just great because of this and the ease of use with tools where you can choose comp stars based on AAVSO sequences and store them in the format for LesveP.

To me it is the best software and deserves more attention definitely for power users like me.

I am not sure if CMUNIWIN is generating in the meantime AAVSO compatible formats. I guess it is not.


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