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Thu, 11/25/2021 - 19:32
In an exoplanet plot, you want the minimum amount of scatter in the target star's plot and maximum accuracy. Considering this, what is the better format for Exoplanet work, CCD or CMOS, and why?
Hi Ed,
There is essentially no difference between CCD and CMOS for exoplanet transit photometry. Both work. I've used liquid nitrogen cooled thinned backside illuminated CCDs on large telescopes, and 12-bit CMOS cameras on small telescopes, with equal success. More important is your observing technique. I suggest that you get a copy of Bruce Gary's book "Exoplanet Observing for Amateurs" (http://brucegary.net/book_EOA/x.htm), as it contains lots of information regarding the proper way to get the high signal/noise that is needed for these shallow transits.
Arne
Ed
Thanks Arne.
Ed
Thank you for this response. The book is invaluable.
Hi Arne
I've been using a Chroma V filter for my initial exoplanet imaging but wondered whether the CBB filters, that get mentioned a lot, would be a bit more effective. While I’m waiting for a 12” F4 to arrive, I’m trying to hone other aspects of my gear, acquisition and processing procedures as best I can. Relative to the CMOS / CCD question, I have the ASI294MM cmos camera. On my first transit attempt, I think things went well and the data from it had the second lowest sigma on the transit depth of all 9 observations on the NASA site. I assume that’s a good indication but I’m new to this so maybe it’s not. I believe I may have made a mistake in running it in hardware bin 1, 2.3 micron pixels, rather than its default which is bin 2 at 4.64 pixels - any thoughts on that?
Also, I understand that you’re evaluating the new version of the Chroma Bessel filters. Will you be issuing an opinion on them in the near future? I am planning to order a the new version of V and B so I can transform my data for variables. I put the order on hold pending your evaluation.
Thank you & be well…
Gary
Hi Gary,
The CBB filter is a good choice for Exoplanet work. It is wide, yet removes those wavelengths that are most strongly affected by atmospheric extinction. If you plan on doing lots of transits, it is a wise purchase.
I haven't evaluated a 294mm camera, but in general, most of the CMOS cameras do software binning. There is a way to do pseudo hardware binning, but I don't know if it is implemented in the ZWO camera. The easiest way to find out is to characterize your camera, and see if the readnoise for 2x2 binning is 2 times the readnoise for 1x1 binning (which implies software binning, since readnoise will go up sqrt(npix) or 2 for the 4 binned pixels of 2x2 binning), or is less (which would imply some sort of hardware support). Testing readnoise and gain is particularly easy if you are using a flat panel for taking flats rather than sky flats, since the process just involves getting two flats and two bias frames.
The new Chroma Johnson/Cousins filters (with the standard bandpass shape rather than flat-topped) are being tested. I have the observational data, but have not fully analyzed the images. At first blush, they look very good. I'll give a complete review on the Instruments forum soon.
Arne
Thank you Arne. I’ll look forward to seeing your evaluation of the Chromas!
Regarding the CBB, I noted that Dennis C used a V filter in his tutorial on WASP-12b, so that’s what I’ve used on my transit imaging so far. It’s the Chroma V from their version a year or so ago. Do you have a favorite vendor for the CBB filter?
Regarding the ASI294MM, they ‘unlocked’ the 294MM so the physical pixel at bin 1 is 2.3 microns. The default mode is bin 2 with 4.63 micron pixels - must be software binning for this. The ‘resolution’ at bin 1 doubles from the default bin2 at 4144x2820 and the files are 37 MB. The full well at bin 1 is about 14,000 compared to bin 2 at 66,000. Here’s a link if you’re curious: https://astronomy-imaging-camera.com/product/asi294mm-pro
I was thinking that bin2 would be better for exoplanet imaging with the larger pixels and a deeper well depth but I’m still pretty new to this so I’m always grateful for input and comments from you folks who know what you’re doing.
Thank you,
Gary
Gary:
Edit: Also see Paul Benni's excellent comments about useful exoplanet filters in a post below.
The CBB filter (Bruce Gary's recommendation) was sold by Astrodon, so hard to know how easy (impossible?) it is to buy a new one currently?
The TESS group sticks with standard J/C filters like V,I or Sloan filters like g,i so that chromaticity issues can be addressed.
If you are working with NASA ExoWatch targets and looking for updated Tc in known exoplanets, the CBB or even C filter allow you to gather appropriate timing measurements on fainter targets. The CBB filter does improve mags by reducing extraneous background sky issues, but IMHO does not disqualify C filter use for faint targets. Again, appropriate/best to use a photometric filter to work with targets that are bright enough for your scope.
Ken
Thank you Ken.
As an exoplanet observer for many years, my favorite filters to use are CBB, Sloan r', and Rc. Coupled with the CCD/CMOS spectral sensitivity maximum, which is often in the red wavelengths, these filters give the best SNR.
For TESS observing, the Sloan r' is my favorite as it has a sharply defined (sharp cutoff) wavelength bandwidth, which is good for transit depth consistency and chromaticity checks, especially combined with data from TESS observers with large aperture systems using Sloan or other sharp cutoff filters in the near infrared (i', z', z_s', Y).
For observing known exoplanets to improve upon the ephemeris or for TTV analysis, the CBB may be best for smaller aperture systems as chromaticity checks are no longer needed. CBB filters were available the last time I checked. Sloan r', i' and J/C Rc & Ic are also good choices as the exoplanet transit light curve becomes more "U" shaped compared to bluer wavelengths due to stellar limb darkening effects. Exoplanet transits have more rapid ingress & egress magnitude change vs time as a function of increased wavelength from blue to near IR from limb darkening effects, which makes it easier to extract transit timing parameters.
So overall, using red and near IR filters work best with commercially available CCD/CMOS cameras to achieve many exoplanet observing goals.
-Paul
BPAD
Dr. Antonio Claret and I provided Limb Darkening Coefficients (LDCs) for the CBB filter. These LDCs are very useful in modeling exoplanet light curves using AstroImageJ and similar software. Our work is being published in "Research Notes of the American Astronomical Society (RNAAS)". You can read more about our work here: CBB filter LDCs now available for use with AstroImageJ | aavso . The website that I developed to make these LDCs easily available is here: cbbldc.com
Ed Mullen
thanks for the book
FYI
Farpoint Astro is selling the Astrodon CBB filters. They appear to be in stock.
Gary
I have a…
Hello everyone.
I have a chroma filter......
ET500lp Chroma filter 50mm Round
- 3mm. thickness
- AR coated on both sides
- no halos
- transmission boosted by 4-5%
I have done several tests and it has no halos and the curve is correct for exoplanets.
They made it for me with coatec and the atmospheric extinction is barely noticeable, except very low.
I am happy with the filter
I currently work with a Zambuto f4 12.5 and a CMOS QHY600Mpro.