Difficulty in imaging the IFN

[cfinn]

1 hour ago, LeftyAstro said:

Really interesting! I've now moved onto M63, attempting to pull out the tidal streams from the same location. I don't know if I have enough time left to finish the project before I run into some obstructions, but at 6 hours I already see a bit of structure. Another variable that may be worth looking into for imaging from this location is the UV/IR cut rather than a light pollution filter combined with a mono camera. I began the IFN project using the Optolong L-Pro and at 10 hours, there was a bit of IFN present as well as much cleaner data. However, there was clearly a lot of good signal cut out when compared to the Baader UV/IR cut at 10 hours. The 294 Mono + the UV/IR cut filter made such a huge difference. The one shot color data was actually pretty nasty for M81/M82, even with a LP filter, so I am reluctant to switch to UV/IR with the color cam (unless I use short subs to keep out LP) but I will have a look at it for some smaller projects.

Astrobin makes it pretty easy to miss, but I use a 2 camera setup (294MM and MC) because it made it a bit easier to not have to switch between individual filters for the mono. I'm sure that the 90% QE also helped a lot with the LUM data

Welcome to SGL! Your findings make a lot of sense. The chart I made based on your setup/location was assuming the use of the ASI 294MC as you say, which has an effective bandpass of roughly 100nm since every pixel essentially has either a red, green or blue filter over it. Using the mono variant with a luminance filter effectively increases the bandwidth to 300nm and you benefit from higher QE as well, so for broadband targets (galaxies, reflection nebulae) you can get to the same SNR in less than 1/3 of the time. For these kinds of targets I don't think there is benefit to be had using a light pollution filter because you will cut out almost as much light from the target as you do from the sky, particularly as street lights move to LEDs. Different story of course for emission nebulae. I think your experiments show that the best way is to go mono to maximise bandwidth and QE and get as much integration time as possible. Sub-exposure length is probably best kept quite short as well, so that you don't saturate pixels with the sky glow and preserve dynamic range.

[Knight of Clear Skies]

On 18/05/2021 at 09:14, gorann said:

Alternatives to the RASA 8 are fast and short FL camera lenses like the Samyang 135 f/2 or similar.

Yes, the Samyang can pick up a fair bit of IFN from a dark site without too much difficulty.

This is about 3 hours of data with a Canon 6d, using 2 minute unguided subs. From the edge of Bodmin Moor, which is typically around SQM 21.2-21.7.

[gorann]

6 hours ago, Knight of Clear Skies said:

Yes, the Samyang can pick up a fair bit of IFN from a dark site without too much difficulty.

This is about 3 hours of data with a Canon 6d, using 2 minute unguided subs. From the edge of Bodmin Moor, which is typically around SQM 21.2-21.7.

IFN is clearly there but without colour and structure. The SQM is virtually the same as mine so my first thought was a surprise that it did not pick up more in 3 hours at f/2. I know that aperture helps in some odd way (67 mm vs 200 mm so 9 times more photons with the RASA), but f/2 is f/2, and on a RASA8 and the photons are more spread out. @ollypenrice may want to have a comment. So my thought now is that you also need a very low noise cooled camera like the ASI2600MC - a DSLR just cannot make it.

Edited May 20, 2021 by gorann

[ollypenrice]

4 minutes ago, gorann said:

IFN is clearly there but without colour and structure. The SQM is virtually the same as mine so my first thought was a surprise that it did not pick up more in 3 hours at f/2. I know that aperture helps in some odd way (67 mm vs 200 mm so 9 times more photons with the RASA), but f/2 is f/2, and on a RASA8 and the photons are more spread out. @ollypenrice may want to have a comment. So my thought now is that you also need a very low noise cooled camera like the ASI2600MC - a DSLR just cannot make it.

All I would say is that the Samyang has done well and that F ratio is meaningful if you don't want to crop and concentrate on a region of interest - which you don't in the case of the IFN.

Olly

[Adreneline]

I am pretty satisfied with the way my Samyang 135mm and ASI294MC performed imaging IFN from Bortle 3/4 site.

A Samyang 135mm costs a lot less than a RASA 8" and an ASI294MC is a lot less than an ASI2600 so it goes without saying the images won't be anything like as good.

[gorann]

When astrodarkness arrives in late August I am certainly going to try my Samyang 135 with an ASI2600MC.