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How to Handle Images that are Significanlty Brighter than Comps?

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How to Handle Images that are Significanlty Brighter than Comps?

Hello! I've been following a couple of LPVs in which I band reaches about 6 while the comps in my 10 x 16 minute field are about 13 (R LMI and R HER). With R LMI, I can offset the image to get the 116 comp, but even so, the small exposure to keep the variable from saturating means that no comps are in the image.

    I tried stack/average 50 images. However, since the noise is rather low in the chip, this did not improve the SNR much.

    Are there any other techniques that might help when there is such a large difference between comps and target? Would stack/sum work rather than stack/average? Otherwise, I'll have to wait several months for the variables to dim so that I can image them along with the comps in I band.

    Thank you and best regards.


Bikeman's picture
Telephoto lenses?

Stack sum vs Stack average won't make any difference at all, the difference is just a factor applied to the pixel values and if that could help it would be great, but no, there's no such thing as a free lunch in optics:-)

Mag 6 is rather bright, have you conseridered adapting DSLR-style telephoto lenses on you astro-camera that would give you a sufficiently wide field to include brighter comp stars? There are adapters (make sure they allow combination with filters) on the market, and if don't already have a telephoto lens form a DSLR or some such, used legacy telephoto lenses are also a great choice.

Your finder/guide scope might be working as well. 



Sorry, I did not list my

Sorry, I did not list my equipment. I use an 8"LX200 with SBIG ST402 with BVIC filters set up in an unattended observatory in my back yard.

    I suspected that I would not be able to image in I filter when there is such a large difference between the magnitude of the target and comp, since the FOV of my system is too small to include brighter comps, but I thought I would ask. Best regards.


CTX's picture
Bright Target and Fainter Comps

Well Mike, you are sure generating very hands on experience of the difficulties of attempts similar to yours. 

Not trying to sound to unkind but there is no real solution to your dilemma except to choose another target that fits your fov and offers comp stars within ~ 2 magnitudes of your target or, as you have already suggested, wait to observe them when they are fainter.

No magic wand for this problem, that I am aware of.

But no harm in asking!  :-)

Good Observing & Ad Astra,

Tim Crawford


Bright Target and Faint Comps



In general, what Tim wrote is

In general, what Tim wrote is a very good advice :-)

However, there are few options if you really-really want to do such observations:

  • Use "classical PEP approach" - target in one and compstar in another frame, repeat the sequence of variable-comp (per filter of course) 3..5 times, variable always(!) bracketed by comp measurements. Reduce the data as it would be done in PEP case. The only downside is, that you need stable transparency for that and of course, data reduction is slightly more unconvenient (but hey, astronomy was done decades like that by professionals). But your telescope is computer controllable and could do very well when pointing between variable and good compstar. I have done what I described above, using a manually pointable 60-cm telescope and results were definitely acceptable.
  • Your camera has "photometric" shutter and frames are small, so in principle you can collect many short exposure frames (so short that your main target is never saturated) and shift-n-stack them after preprocessing. If you have some amount of signal from your compstar, summing many frames helps to collect more photons and while your target star may have insanely high SNR (at least from photons point of view), you're after acceptable SNR of compstar (40 stacked frames "broadens" your magnitude range roughly by 4 magnitudes). AND acceptable level of exposure time to beat scincillation (30+ seconds?).

I personally would try bright stars with an used photographic telephoto lens attached to the main telescope and a very low noise modern high-frame rate CMOS camera (e.g. ASI ones with filters) and live calibration+stacking. Those cameras have 12-bit dynamic range, to get effectively 16-bit one, one has to stack anyway 16 (2^N for N extra bits) frames... Actually I will try that out in August when nights return in Estonia... ;-)

Best wishes,

Bikeman's picture
There are also 14 bit CMOS

There are also 14 bit CMOS cameras for this kind of project, e.g. the ZWO ASI 178mm (or similar one from other vendors using the same Sony sensor).

I think a challenge is how to squeeze a filter (wheel) between the camera and the telephoto lens. A single filter is no problem, I have seen adapters that have a thread for a 1.25" filter inside, e.g. for Caneon EF lenses. But the space between the sensor and the lens flange seems a bit small to have a a filter wheel etc.

But even with a fixed V-filter this could be useful I guess, piggy-backed on a bigger scope.



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