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I am wondering about the common wisdom of always using a UV/IR cut filter if shooting broadband OSC. I can understand its use with refractors that might not be well corrected at the extremes of a cameras sensitivity range but why do it with a reflector which brings all wavelengths to the same focus. I can see that it would effect having a "natural color balance" whatever that means these days but wouldn't it be better to get as much signal as the sensor can detect?
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7.22
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you could maybe get a decent image, if you use a small sensor, remember to use SPCC in your processing, don't use a coma corrector, and if you don't go near really bright stars but I really don't think it's worth it; the days I didn't use a UV-IR cut, I had slightly bloated stars all the time, not sure why, I guess it's the UV; UV-IR cut reduced down the star size, and also brought out a lot of strong RGB color that I previously couldn't get (as strong), because UV was overwhelming the image in my case, I used a 533mc |
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Oscar: What was your OTA? |
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7.22
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Tony Gondola:Oscar: a Celestron C8-newtonian, was my first telescope before moving to my current small petzval |
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8.11
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I shoot with an F/6 Newtonian, no corrector and 585 sensors. I really need to do some test shots to try and understand this a bit better.
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I tested using R610 Long Pass with 183MM instead of using a regular R filter. When photographing distant galaxies, the difference is significant. Much more signal. The overall color is reddish, but for distant galaxies, like 600 mLy, it would be reddish anyway. |
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I recently purchased a UK IMX571 Altair Hypercam 26C which is fitted with a heated UV-IR window. So this implies Altair thinks it's a good idea to use this all the time. I am fairly new to astrophotography with 2 years expeirence so I cannot say at the moment how good or bad images using this astrocamera. Just thought I'd mention that some suppliers do sell camera's with UV-IR filters fitted and wondered whether you should be swapping them out when necessary.
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Rogers Pereira: I've found the same when using an IR pass filter. I think part of the reason the signal is so strong is because at very long wavelengths the filters in the bayer matrix become almost transparent. It's almost like working with a mono camera at that point if chip has usable sensitivity down there which most do these days. |
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Gary Warnes: I'm sure that is to produce a decent color balance out of the box, similar to using a DSLR. It can be a limitation though in certain applications. My little QHY715C has a window that's user removable, it's a nice feature that I really like. It came with a AR coated window which I just removed and run it with no cover filter at all. Removing one set of air to glass surfaces is never a bad thing. Of course, that wouldn't work with a cooled camera. |
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Thank you for my second reply on Astrobin. Yes Altair sell the glass option so that you can switch when you need too. I might do that in the future as my astro journey progresses.
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I can hardly see any reason to remove the protective window from a planetary camera, and I tested and used loads of them. For straight RGB imaging AR windows is an annoyance as some filters, notably NB ones, have a leak in the near IR. Either way, not worth the hassle to replace the window.
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andrea tasselli: Andrea, It wasen't a hassle, took about 30 seconds. Every air to glass surface scatters a bit of light and degrades the signal. I know it's a tiny thing but in this case, it was so easy, there was no reason not to do it. |
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Tony Gondola: Whenever I shoot full-spectrum I find that my colors are significantly washed out-- whiter to be exact. The more data I get, the more white my stars appear. When I put on my UV IR window, this issue disappears, but then so do any background galaxies. My get-around will eventually be going mono (mono modded 2400?  ) with Bessel UVBRI Photometric filters- the colors from those are immaculate. Other than that, with a reflector, the UV and IR not being cut gives more signal to a target, and allows you to see many stars and galaxies that would normally go unnoticed. You are also right about the bayer transparency, most modern sensors go fully transparent just after 720nm. I do it all the time, and I get pretty decent results, so get to imaging!
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Based on comments here and my own tests, that's the conclusion I'm coming around to as well. Shooting in near IR can be a very viable way of imaging galaxies if your OSC camera has good sensitivity down there. With some sensors, fully half of the total signal is beyond 600 nanometers with the highest QE in that area as well due to filter transparency. I suspect that the general rule about uv/ir cut filter use doesn't always apply, especial with reflecting systems and OSC sensors with good IR sensitivity. In my case of OSC cameras, reflector and bortle 8 skies, IR pass works very well and the full spectrum/no filter approach is worth exploring for the vary faintest subjects. something I haven't tried for brighter targets is to shoot IR pass for the bulk of the data and add Ha/Oiii dual band to bring in some color information. I suspect that could be a very effective combination. |
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8.11
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Here's a full spectrum image with no filter of M-51. It was a very bad night with constant thin high clouds and breezes nocking around the guiding around so I was only able to get 4.5 hours of pretty poor data. This is 1085x15 at F/6, Bortle-8, cropped from a 585 sensor. I could have gone a lot deeper with the stretch has it not been for the clouds. Clear nights are few right now so I'll try and get a uv/ir version as soon as I can.  |
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Are the stars white non-gaussian disks because they were saturated because of the IR, or from post processing?
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8.11
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Rick Krejci: It's from post processing, trying to get as much as I can from bad data. This was a quick go at it so the stars where not removed for separate processing. In the raw stacked .fit before processing, only one star was clipped. |
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Oscar: I have also a ASI 533MC Pro and use it along with a Askar SQA 55. Being based in outskirts of a city the light pollution levels are very high. Hence am using a Optolong UV-IR cut filter to get a good result. The stars are good in this case. Normal CLS filters dont work well especially when most City lights are LED based and cut across the entire spectrum. Using Background extraction tools will help clearing some unwanted light noise |
2.11
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Tony Gondola: Worth keeping in mind that you’ll still be using refractive optics in most cases (e.g. a corrector on a Newtonian). There aren’t many fully mirror-based imaging setups being used for deep space imaging. And even in those cases, a UV/IR cut can help considerably to refine detail (e.g. as applied to planetary imaging with a color sensor) for a number of reasons (e.g. light scatter relative to wavelength in the atmosphere). You also tend to get better-controlled stars, which can emit a lot of light in infrared, which compete less with nebulae and galaxies. |
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James Peirce:Tony Gondola: Actually no, I do not use a coma corrector. |
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Tony Gondola:James Peirce:Tony Gondola: With the ASI585 it seems? This would be a case of why I wrote “most cases.” The rest would still apply, however. |
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Yes, I agree. I'm only really talking about when shooting with a all reflecting system and no corrector. In that case, there might be something to be gained by opening up the system to all the wavelengths the sensor can detect.
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2.11
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Tony Gondola: You might attempt some proper side-by-side comparisons to get a sense of how the abstractions being discussed (and other considerations not mentioned, such as potential contribution to reflections) affect images from your perspective. Especially where there’s a reasonably dense star field. You will always get more signal, of course, from extending a bit more beyond the visual spectrum, but it generally comes at an appreciable cost to detail/potential resolution, even when not incorporating refractive optics (wavelength and concerns like Rayleigh scattering). And the extra prominence stars receive relative to features in deep space objects also tends to be problematic in post-processing, as it affects the amount of detail which can be retained near stars when stars are extracted. For my case, I have found that imaging without the UV/IR cut is essentially never worth it. In fact, the UV/IR cut has been, for me, much more valuable than collecting a bit of extra signal from the extended spectrum. But if I’m imaging something like a galaxy my primary interest tends to be resolving as much detail as I can manage. |
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Yes, it's mostly in terms of imaging galaxies that I'm interested in this. I've posted a 4.5 hour example of M51 in the thread above. I'm just waiting to get a clear night to get another 4.5 hours with the uv/ir cut.
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2.11
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Tony Gondola: If you want to do a comparison, you’ll want to do it in sequence under the same conditions, because concerns like atmospheric turbulence/seeing can vary dramatically night to night, as well as position in the horizon, etc. Along with the usual concerns of needing to be careful with things like focus (as it will shift with the introduction of a filter). It is hard to read much into a processed image as you can do a lot in post-processing, and this is about detail and signal at acquisition. |
