I have BVIc filters and I am thinking to add a red one. Given that APASS uses BV in Johnson and the Sloan g', r' and i'; would my best choice be a r' filter rather than a Rc filter?
That is a tough question, with my usual "it depends" answer. I know several astronomers who think BVIc is all you need, and that the Rc filter just adds redundant information. Most of the time you are on the Rayleigh Jeans tail of the spectral energy distribution when dealing with red filters, so one filter gives you almost as much information as two.
That said, I'd either use Rc/Ic or r'/i', but not r'/Ic. That means in your case that I would buy Rc. I'd use U or u' first, if you have a blue-sensitive sensor, as the U bandpass contains a lot of astrophysical information. At the same time, it is harder to work with, both due to the extra atmospheric absorption as well as the decreasing sensitivity of the sensor. If you had a thinned, backside illuminated CCD, I'd definitely choose U in preference to Rc.
The advantage of Rc or r' in my mind is that it is sensitive to Halpha, and so can give you some information as to the strength of that line in objects like novae.
So part of the answer has to be: what kind of stars do you like to observe? What sensor are you using? How many slots do you have in your filter wheel, and how are they populated now? What is your elevation, and what kind of telescope are you using?
I am using an ATIK 4000 with the Kodak 4022 sensor. Definately not backlit or thinned and with a peak in the 450 nm range (ca 55% QE). OTA is a C11 Edge (with or without 0.7x reducer) and the observatory is in a black zone in the Texas Hill Country at Stellar Skies (544 meters). I am just finishing up the new observatory; previously I had concentrated on LPVs with a bit of work on Z cams and EBs.
I guess my motivation was to take advantage of what amounts to secondary standards provided by APASS as there are data for r' but not for Ic. But I see your reasoning, I think I will stick with BVIc for now and think about projects that might benefit from r' and i'.
Many thanks, Ed
The KAI-4022 sensor is a nice 2kx2k device with 7.4micron pixels. As is typical with interline transfer sensors, it doesn't have great QE throughout its range, but is more blue-sensitive than red-sensitive. It still has 20% QE at 350nm, which isn't shabby! In general, I find that you work about 5-6 magnitudes brighter at U-band than at V-band. So if you were doing a CV at 15th magnitude easily with your V filter, than you could do a 9-10mag CV with the U filter. This means in general that you are limited to the brighter CVs in outburst, like SS Cyg, or transient objects like novae. Still lots of stars, but you have to think differently.
Right now, where most of the r'/i' work is being done is when professionals want to monitor variable stars with both a national facility telescope as well as collaborating amateur photometry. Then they want the same filter to be used at both sites. The use is limited right now, but will only increase in the future. In the meantime, getting Rc helps to match archival data and is currently useful for most of the AAVSO projects. You can't go wrong.
There are transformations to take the APASS r'/i' and produce Cousins Rc/ic with decent reliability. Those are what we use for VSP.