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Old 28-06-2019, 09:57 PM
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The_bluester (Paul)
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The_bluester is offline
Join Date: Feb 2011
Location: Kilmore, Australia
Posts: 3,016
OSC cams and filters

I have been using a ZWO ASI294 for some six months as my first toe in the water for AP, firstly with my existing SCT (A non Edge HD Celestron C925 that I have been using visually for almost ten years)

A few months ago I bought a Skywatcher Evostar 72mm to gain some widefield experience and have something easier to guide than a 2200mm focal length SCT. Results in fainter objects were pretty pleasing at my experience level but bright stars in frame were giving me some spectacular reflection problems.

I had read that cameras like the ASI295 are still quite sensitive into the infra red and that this can play hob with focusing with programs like SGP as the poorly focused IR or NIR component will impact HFR measurement and even manually focusing using HFR you can find yourself focusing in part on information that you do not want, and even if you get the focus right, you may find the poorly focused (In the case of the refractor in particular, obviously) IR/NIR data producing bloated stars. For that reason there are recommendations out there to use an IR cut filter with OSC cameras. Background sky brightness is also supposed to benefit due to atmospheric heat being re radiated in the infra red.

Long story short, I was buying a focus motor to enable automated focus runs on the 72mm and grabbed an IR filter while I was at it. And the difference is gobsmacking.

The two images here are basic integrations, 2 subs each, 120 second subs without the IR filter, 300 seconds with. Stacked, background neutralised and star colour calibrated and stretched using the same settings in APP. Photoshop used only to reduce them down to a size I can post here. The star at the bottom left is Gacrux, I got a similar improvement on Antares. Aside from the background brightness reduction I have to assume that the AR window coating is not effective in the infra red due to the other obvious reflections. The reflection blobs calculate out by diameter in pixels and F ratio to be multiples of the distance between the sensor and the window at the front of the camera. I am reasonably sure that the diffraction like pattern around Gacrux is reflections from micro lenses adjacent to the pixels that should be lit. The "Ghost" diagonally opposite is a similar reflection and stays opposite in the frame from wherever the star in question is.

TL;DR? If you are using an OSC astro cam, particularly with a frac, seriously consider using an IR filter!
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Click for full-size image (Gacrux-with-IR-cut.jpg)
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Last edited by The_bluester; 29-06-2019 at 09:46 AM.
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