Hello! I may be looking to upgrade my system. I am wondering if folks using a Newtonian scope could share how often they obtain flat fields.
I have an 8" SCT that works well. I obtain sky flats monthly. This works well since the closed system prevents dust buildup near the sensor that would need more frequent flats, according to the guidance I received when I first established my work flow.
A Newtonian is an open system so may require a different flat field work flow.
Thank you for your assistance. Best regards.
Flat-fields shouldn’t depend on telescope type especially. Now with that typed, I’ve seen open-truss telescopes, recently a 24-inch on UVAs Fan Mtn. Observatory had some off axis light issues creating hot spots. Easy to fix by adding one of those large Dobsonian light baffle sleeves around the truss. They also painted an internal baffle… but now I random-walk.
The best thing to do at a remote observatory is to take “twilight” flats. I presume the shelter roof is rolled off after sunset to allow the telescopes to cool to ambient temperature. That is the time to get sky flat frames.
Others with scopes at remote observatories or AAVSOnet may have additional info on what they do.
I was thinking more about dust problems that are different between Newtonians/SCT/refractors rather than system aberrations such as vignetting. I’m curious how often folks with Newtonians obtain flats to correct possible changes in dust donuts. Best regards.
Since you don’t know when dust motes will appear, or when the optics will next be cleaned, in an open optics tube like a Newt that would suggest you take flats say in a round-robin cycle each night. Get one or two filter flats each night or however many you can fit into twilight then use the latest, rinse and repeat.
When I took Arne's best practices course, if I remember correctly, he said that flats for closed tube systems (like SCT) last for a good period of time since dust cannot get close to the sensor, so do not need the same frequency of flats as an open system, like Newtonians.
Additionally, because the moon acts like a point source, flats with the moon in the sky are not ideal.
I may have misunderstood him, but he seemed to suggest that Newtonians need more frequent flats because of dust buildup. I'm wondering what frequency folks have found in practice.
Thank you and best regards.
I agree with the point about stability. In my experience, the frequency of flats often depends on the F-ratio of the Newtonian. Faster systems are much more sensitive to even microscopic tilt or dust movement. I’ve switched to using a dedicated flat panel integrated into the observatory roof, which makes taking daily flats less of a chore. If you’re serious about your light curves, ‘reusing’ flats from a week ago is always a gamble with a Newtonian.
Thanks! Have you found a light panel that provides wavelength in for an I filter? I had to add incandescent bulbs to my lightbox since the bright white LEDs didn’t do the trick. Mike
The short answer is no — I haven’t found a commercial off-the-shelf panel that works for the I-band. As you noticed, standard white LEDs drop off a cliff after 700nm and are practically useless for near-IR
Thanks! I looked at the 850nm and 940nm. The FWHM was listed as 40 to 50 nm, so for the 850nm, that gave a range of 825-875, which seemed like it would miss a lot of the Johnson-Cousins I filter range that an incandescent bulb gets.
Did you find the LEDs coverage greater than the listed range? Or did the narrower range not impact the flats? Best regards.