I’m considering using a lightweight refractor with a 60mm diameter and 300mm focal length for field photometry, but it’s a small achromatic telescope. Since the filters only allow a specific range of wavelengths to pass through, focusing on each filter would cause a very small aberration, which I think could fall within the acceptable instrumental errors for good photometry. Can someone on the forum confirm these ideas? Is a refractor like this suitable for making good photometric measurements?
can you focus for each filter , either manually or electronic focuser? The ideal way is to set focus offsets for each filter in your imaging software.
I tried an 80 mm refractor and focus was a problem that required the offsets. However, the biggest worry was that the FOV was so large that it was troublesome to avoid bright stars when exposing for the dimmer targets. I got rid of the 480 mm FL optic and installed a 1340 mm F.L. optic. Seems to work better all around for targeted work. Meanwhile 300 mm is a bit to long for 5 degree FOVs, which is another project here.
Ray
I use a 102mm f/7 (714mm FL) ED refractor, kind of between a regular achromat and a apochromat. The BVR filters all use the same focus point but when I use the Ic filter refocusing is a necessity. Hope that helps…
My experience with a 90/500mm achromatic refractor is: V is no problem, I and B need some refocussing, but blue never really went sharp, but that was not too much of a problem unless other stars were very close.
During the APASS survey, we had a period of time when one system was on its way to CTIO and the second system had not arrived in NM. We contacted Celestron, and obtained two 150mm f/5 inexpensive achromatic refractors. These worked fine and gave good results over the 3-degree field of view in the B,V,SG,SR,SI filters of the survey. You are correct in that the extra correction provided by apochromats is nice for visual work, but is not terribly important for filter photometry. As others mention, the biggest problem is focus change, and so an electronic focuser is a necessity if you want to use multiple filters.
Arne