Wed, 09/21/2022 - 10:09
Hello! Just curious. The Celestron Rowe Ackermann cannot use a filter wheel. As a result, it is limited to a single filter, like a V filter, or a one shot camera. In the latter case, the G channel would be most commonly used for photometry.
Just curious - are there objects that cannot be imaged with a G filter but can be imaged with a V filter? Thank you and best regards.
Mike
Hi Mike,
Figure 1 in this publication:
https://www.semanticscholar.org/paper/A-Demonstration-of-Accurate-Wide-field-V-band-Using-Kloppenborg-Pieri/542d03f2dd40777c9c4ade534f555ed2436cde83/figure/1
(yes, our Brian Kloppenborg!) shows the difference between the Johnson V filter and the DSLR/OSC G "filter". Note that they are very similar - Johnson V might be a bit redder and a bit narrower than tricolor G. So photometry using either bandpass seems like it would work. Using a one-shot-color camera on the RASA makes sense, as you get three simultaneous bandpasses without having to deal with a filter wheel or buying a separate filter/filter wheel. You also get the preferred bandpasses for the deep sky imagers, and so can make nice pictures just like anyone else.
However, you rarely get something for free. Here are some negatives, especially as related to photometry:
- the bandpasses are similar, but not exactly the same. If you look at any light curve that has both V and TG (tricolor G) data plotted on it, you will see an offset between the measures. For strict comparison, it is essential that you transform your data. This is difficult with the (B-G) color index, since those bandpasses overlap, but it is not impossible.
- the dye coatings over the front of a color sensor usually have red leaks. These can be compensated for by using an IR-cut entrance window into the camera, but be careful of this red leak. For example, the IR-cut filters that are used often remove Halpha and make the tricolor R bandpass very different than the Cousins R.
- the OSC camera uses a Bayer masked sensor. This means that each "color pixel" is formed from two G, one B and one R native pixel. So by using this, you automatically cut the throughput of your camera by half, since only half of the pixels are G pixels. You can recover some of this by using all 4 native pixels to form an "unfiltered" result, often producing photometry that is useful for projects like Joe Patterson's CBA.
- an OSC camera decreases the spatial resolution, since instead of using the smaller native pixels, each output pixel is effectively like a binned 2x2 pixel. For RASAs and their fast f/ratio, this may degrade the ability to split close stars for photometry using a circular measuring aperture. In addition, because the Bayer masking means there is structure in the color pixel (only half of it is sensitive to G), you need to oversample the seeing disk, which further reduces the resolution.
IF your main goal is photometry, you are probably better off using a monochrome camera with a V filter in its nosepiece. Or, maybe buying two RASAs and putting them on the same mount, so that you can do simultaneous B and V, or V and Ic, photometry. If your main goal is a general-purpose system for photometry and other imaging projects, then the OSC might be a better choice.
So like everything in astronomy, you have to make tradeoffs. G is not inherently better than V.
Arne
Thank you!
Mike