Great image! It is always amazing what you and others achieve in your darkside imaging.
Is there any modest price commercial filter/combination that could achieve similar results? Another basic question, for Venus beginners, three filters at the one time seems a bit less standard to fit in an optical train, I am used to at times having 2 filters, one the camera and one on the barlow, do you need an additional spacer or some such to fit a third?
Thanks Ross! And also thanks for the 'likes' folks!
Ross, your filter question opens up a can of worms as there really isn't a simple answer--- at least for similar results with many current cameras. I'll try to explain:
The first night side thermal imaging I did was back in 2009 with a fairly cheap (at the time, $25 USD) Schott RG1000 colored glass filter. I believe the same filter type was used by Christophe Pellier back in 2004 when he became the first amateur to image the dark side--- he also recorded features. At the time, camera sensors that worked well for this included the monochrome versions of the Sony ICX098 and later the ICX618 CCDs and these sensors seemed well suited to the task as they kept problems with the glare and reflections from the sunlit phase of Venus from overwhelming the night side.
In recent years many of the current CMOS sensors are plagued with multiple sensor artifact 'ghosts' of the sunlit portion of Venus that ruin getting a clean shot of the night side. It seems that while all sensors have some issues with this problem, the Pregius sensor models from Sony have them under reasonable control--- those are all global shutter sensors, and I'm using a Pregius IMX265 sensor. Of the popular rolling shutter sensors I've seen, all of them have a strong case of the artifact problem. I've attached a shot of Venus taken on Dec 12th while testing the currently popular ASI462 using the near-IR sensitive IMX462 rolling shutter Sony sensor--- plenty of crescent ghosting artifact overlap the night side!
Point is, even if you have best filters available, if you have a camera with these artifacts it can be frustrating. In part, my 3 filter stack was an attempt to get better control of these artifacts. While that didn't work (at least enough to be truly noticeable), I think I've gained a bit of contrast. The thermal emission window bandpass near 1 micron is actually centered around the 1.01 micron point, and at it's strongest zone the band covers from about 0.995 to 1.025 micron. Any light passed on either side of that bandpass window risks capturing enough of the sunlit part of Venus and can hurt contrast. For that reason, it's best to image when the crescent is as thin as possible, but that limits the range of opportunities where one can image with Venus in a dark enough sky.
So for filter types, the RG1000 colored glass is by far the cheapest--- but it's very gentile cut-on wavelength slope means a lot of transmission from the sunlit phase gets through that can lower contrast--- but at least you should be able to record the night side and at a very reasonable price. There are nice 1 micron longpass filters with dichroic coatings with a sharper cut-on available from Edmund, Thorlabs, Asahi Spectra and others that offer better performance. Those longpass filters can also be used along with a shortpass filter to create a bandpass that frames the thermal window nicely. Potential stand alone filters exist like my Edmund 1025/50, there is an interesting Edmund 1030/10 used by Joe Izen (banjo1000 here on CN), and some have had success with the Edmund 1000/25 although I think tilting that one will shift it's bandpass too short. But these filters can get expensive--- a good longpass starts at ~ $130 USD, and the better bandpass filters can easily hit $300 USD. One of the filters that Wesley and Miles used in 2020 for their excellent results lists at nearly $500 USD!
My current 3 filter stack is in a tube over 80mm long, with an Edmund 1025nm shortpass up from front, about 30mm or so behind that is a Thorlabs FELH1000nm longpass (tilted 10°), and lastly about 30mm behind the Thorlabs is the Edmund 1025/50nm bandpass filter (tilted 20°). I've attached a photo of that setup--- it uses a main tube that the 1025 SP filter cells screws onto the scope end of, and the other filters fit snugly in, and are held at the tilt angles I want by angled spacing tubes that fit snugly inside the main tube. Back in 2017 I bought the Edmund 1025/50 to use as a stand alone filter for this imaging and it worked well enough, but I eventually realized that I could tilt the filter to effectively shift it's bandpass CWL to the 1.01 micron CWL of the thermal emission window. When I finally tried that in 2020 I found that I had also eliminated a reflection issue between the sensor and filter. So that was a plus!
The shortpass and longpass filters frame the desired effective bandpass nicely. In the past year or two I had come to the conclusion that the Edmund 1025/50's 50nm bandpass was wider than it needed to be, so adding the other filters eliminate that as an issue.