Affiliation
American Association of Variable Star Observers (AAVSO)
Thu, 06/04/2020 - 20:50
Hello,
Sorry for the potentially controversial question. I have learned last month that time is now over for our beloved KAF sensors, including the ones that were mostly recommended for photometry (0402, 1603 and 3200). Considering what is available today, what sensors would be best for photometry for a reasonable price? Any suggestion?
Thanks
Normand
Buy up a couple of used ST-8's or ST-10's or even ST-402's, and take good care of them.
Phil
Last time I checked, Moravian still offered their G2 1600 NABG camera.
Ed
Hi Ed. Yes I saw that too, also SBIG is still selling 2 models based on 3200.
Normand
Hi Normand--
I don't think I'd call it a controversial question. It's more a technical question that poeple have strong opinions about <grin>. I'm inclined to agree with the previous comments: look for one of the smaller CCD cameras on the used market. Now that big CMOS arrays have become so incredibly inexpenssive, I bet you'll find former pretty-picture,observers "trading up" and selling their old KAF-based cameras. I will never give up my QSI 532 camera. A truly fine sensor!
--Richard
Hello Richard,
thank you and don't worry, I am keeping an eye on 3 or 4 web sites selling used equipment, just in case. Haven't seen many 1603ME so far, but I don't give up.
AFAIK, Sony continues to make their CCD sensors, so if you want CCD and the Kodak/OnSemi sensors have reached the end of their production life, you might look into Sony. Starlight Xpress uses lots of Sony sensors, and they are available from other vendors as well. These have high sensitivity, but are usually ABG and quite blue-sensitive compared with the KAF sensors. Which sensor/camera is best depends on your use, telescope, and finances. I know that several vendors stocked up on the KAF sensors in anticipation of their production demise, and so they may still be available in cameras for a while. There is some risk buying a camera with a KAF sensor today, but most cameras work reliably for many years, and even when needing repair, the sensor is seldom the problem.
AAVSOnet is switching over to CMOS for the most part, driven by lifetime failures of our existing SBIG cameras and lack of repair parts (we may have a few ST8/ST10 cameras for sale one of these days). The best of the current CMOS sensor crop is the Sony IMX455, used in the QHY600 and ASI6200 cameras. While inexpensive for the size of the sensor, they do cost more than many smaller CCD sensor cameras. We're doing pretty well with the ZWO ASI183mm-pro (essentially same physical size as the KAF1603 in the ST8), and I know Roy has been using a ZWO ASI1600 effectively.
Arne
Thanks Arne,
you brought an interesting point. I do use a discontinued Sony-based camera (ICX285AL) from Orion, but because it has an ABG and mostly because it is not thermo-regulated (only "cooled"), I have the impression I am pretty much at the limit of what I can expect from it to do serious photometry work. I am not familiar with all Sony's sensor models, but which one(s) would be good for photometry, considering my scope has around 2000mm FL?
I didn't follow all the messages in the thread on CMOS chips, but wouldn't their photosite small size be a problem for a Casgrain-type telescope?
Hi Normand,
To determine whether a certain sensor is optimal for your telescope, I highly recommend experimenting with a CCD pixel scale calculator, such as
http://celestialwonders.com/tools/imageScaleCalc.html
there are several equivalent calculators out there (perhaps others can recommend their favorite!). Such a calculator shows that a 9 micron pixel matches nicely with your 2000mm focal length, yielding 0.928 arcsec/pixel. As most sea-level locations have typical seeing in the 3arcsec range, this pixel scale will rarely result in undersampled images.
It is true that most CMOS sensors have small pixels which natively don't match longer focal lengths very well. (An example of a bigger one is the KL400, found in the FLI Kepler camera, which has 11 micron pixels.) Typically, CMOS sensors in commercial cameras either have 2.4micron or 3.76micron pixels. If you bin a 3.76micron pixel camera 2x2, you would have 0.776arcsec/pixel with your focal length, not a bad match. You would get something equivalent by binning the 2.4micron pixel camera 3x3. I usually shy away from binning, but in this case most CMOS cameras have lots of pixels, so binning helps in image storage and analysis, and the smaller pixels have a smaller full well and so binning yields better use of a 16-bit FITS image space. With CMOS, however, binning does mean increased readnoise. For the ZWO ASI183 camera, binning 2x2 brings the readnoise from ~3electrons to ~5 electrons, an increase but still lower than most CCD cameras in native 1x1 binning mode.
If dark current is a major contributor to your photometric error, then using a cooled camera is a good first step, and temperature regulation is even better. Usually the average dark current for modern sensors is low even at room temperature, but you have hot pixels that are more temperature-sensitive and can make an uncooled image pretty noisy, especially for faint targets or long exposures.
Anti-Blooming-Gate (AGB) technology is not a show-stopper. APASS, a widely accepted photometric survey of the sky, uses AGB sensors, for example. Almost all CMOS sensors are AGB.
I would rather not recommend a particular sensor, as it depends on your system, price range and vendor. For example, a Starlight Xpress Trius Pro 694 has 6.1Mpix with 4.54micron pixels, about the same physical size of a KAF1603 sensor. With your 2000mm focal length, it would yield 0.47arcsec pixels and a field of view of 21.5x17.2arcmin. With its included 7-position filter wheel, the price is around US$3600. Is this too expensive for you? Do you want to cover a wider field of view? Would you rather use a different vendor, or a different sensor? Instead, I recommend starting a conversation with a vendor to see what they would recommend for your requirements and matching it with their camera line.
Arne
I just found this web page and it is an interesting take on extending the lifetime of a CCD sensor:
https://diffractionlimited.com/product/sensor-transplant/
As Arne points out, if the CCD sensor is seldom the main failure mode of a CCD camera, then transplanting the sensor into a completely new and up to date camera model may be a solution.
NBL Bob N.
Do you know if SBIG will transplant a KAF-3200 sensor into the older ST-7E?
Tim,
Why not ask SBIG yourself? There is a phone number and an email contact form in the SBIG link Bob posted.
If they say no, you might want to ask the SBIG camera technician directly. He might do it for you privately. I'll send you his email address.
Phil
What about ZWO Optical ASI183MC Pro? I have recently seen that VerifiedCouponCode is selling CC sensors. I am thinking to purchase ZWO Optical ASI183MC Pro , Its reviews are positive and this has all the features that I am looking for.