In the past (from 2011 to 2021) I used a CCD camera with BVRI filters. I imaged standard fields, got transformation parameter for the scope and provided observations (over 10k of them) to the AAVSO database. In 2022 I retired and promptly lost access to the Observatory I built for my dept. over the years (Academia can be brutal at times). I finally have again access to my own telescope (it is a tiny 71mm refractor but I have it remotely located at Starfront Observatory in TX, which has very dark skyes). I’d like to again take measurements but I do not have a monochrome camera, I have a One Shot Color Cmos camera. I understand the set up is not ideal but I wonder if using the following process can get me there. I would like to image standard fields through B and V filters, reduce the frames as usual (Flat, darks and Bias, although modern Cmos don’t really need darks), then extract the Blue channel and the Green Channel from the calibrated images and finally send the Blue and Green images so obtained to Vphot to get the data necessary to run Transform Generator, and from there start again making transformed measurements. The camera has a Sony IMX585 chip (tiny 2.9 micron pixels) so I’d collect data probably binning 2x2 or even 3x3 (although 3x3 would give me a pixel scale of 3.66x3.66 arcsec/pixel which might be not good enough). Can I get decent enough data with this process? Thanks! Marco
Greetings,
You describe the process well. The key with DSLR and CMOS cameras is to be sure you sample the Bayer matrix well enough. Most defocus a bit to spread the star light out. You may not want to bin to help ensure sufficient sampling the psf of the stars you want to measure. I also use the red channel which in most cases transforms well enough. So you should be able to get BVR in one shot as they say!
Btw, my experience is with DSLRs. Others may have opinions about CMOS gain settings to use, etc. if you have questions along those lines.
Give it a try and try using several bin factors.
Jim (DEY)
p.s. I too have lost my main portable observing site recently. After a lifetime of observing it is no more but the urge is still their. I’m now back to using a Pentax Kp DSLR with a 300m f/4 lens (with slight defocus) and an old original iOptron SkyTracker–the flat rectangular box version. Works pretty well still. Glad I learned my constellations ages ago and the AAVSO star-hopping method of finding fields!
Marco,
Did you really mean to write this? If so, it implies you plan to image with your RGB OSC camera through B and V scientific filters.
That is not the way to go. You should, however, consider imaging through a UV/IR Cut filter to restrain the spectrum reaching your sensor.
A uv/ir block filter has, usually, a bandpass between 400 and 700 nm. One could extract the blue and the green channel from the captured frames. But wouldn’t imaging through photometric B and V filters, instead, further reduce the bandpass of the light reaching the camera CMOS sensor? The bandpass of a Photometric B is roughly 390 to 530 nm and the V is 480 to 680 (data from an article by Mike Wiles of AAVO who studied the Baader BVRI Baader BVRI Bessel Filter testing | aavso). Of the frames captured through the B I would extract the Blue channel and, analogously, frames through V I’d extract the green.
Why would you reduce the photons captured by placing unnecessary filters in the path? You already have blue and green filters on the chip. Transforming the data from them is the recommended AAVSO procedure.
Wow, lot of people here losing access to a scope. I too had access to a remote telescope in the mountains of California but lost it when the RASC sold it off. It was a pretty picture scope with LRGBHaOIIISII filters. I of course wanted to do photometry. I measured the RGB images and worked up transformation coefficients to BVRc. The B transformation was very poor but the TB was good enough to use a TB-TG colour to give a very good transformation from TG to V. It looked like the TR to R transformation was also quite good but I lost access to the scope and never refined it. So that’s the way I would go if I was to do it again - just give up on the B and work on V and Rc.
Richard,
I wouldn’t give up on BV transforms from that one experience.
I have a set of RGB filters (in addition to photometric filters) and a monochrome camera. Out of curiosity I determined Tbv and Tv_bv using the G and B filters. Both transforms were good in my opinion.
Transforming RGB from DLSR and color CMOS cameras as well as using pretty picture RGB filters in front of monochrome sensors such as CCDs and CMOS should transform well enough to Johnson-Cousins BVRc with a little transform effort.
If your camera is using a Bayer matrix you need to sample the Bayer matrix well. Slight defocusing is the most common method.
Jim (DEY)
I think I am getting there. I saw a post by MKZ from August 2024 and I followed the step-by-step instructions there and I got some transformation parameters which appear decent. I will take more careful data and see what I get.