Horsehead with nasty reflections

[Atmos]

Haven’t been out under dark skies for a few months so it was nice to get a clear moonless night for a change. Mozzie season is starting so I got a few bites but survived the night.

I’ve never properly imaged this target before and I was curious as to how the RH200 and ASI094 combination would handle a crazy bright star in the field. Better than I was expecting although there are some other quite nasty reflections I want to try to track down. Given their hard stop they could be from adapters, I’m guessing anyway.

Besides the reflections I’m happy with the result from 3.4 hours data.

For Pixel Peepers ( https://www.astrobin.com/full/npr2g6/0/)

[multiweb]

Top shot Colin. Beautiful colours and details. :thumbsup::thumbsup::thumbsup:

[strongmanmike]

Lovely field that Col, the internal reflections are annoying but it still looks good :thumbsup: the edge/corner stars are kinda double and wobbly but I am thinking some image tilt is at play, as your rig tracks across the sky..?

Mike

[gregbradley]

A nice wide field view. Reflections like that are probably best handled by moving the scope a bit. Often they drop out. Chances are this is from the scope not the adapters but yeah check out your adapters. Anodised aluminium reflects IR. Keep in mind Alnitak is about the worst test out there for reflections.

Adjusting the tilt adjuster on your camera should reduce those corner aberrations. Top left are out of focus in the corners and the bottom right shows the most tilt. Bottom left corner looks quite good so that is the standard you need to adjust for all 4 corners.

I think you'll find that from the viewpoint of standing behind the camera the bottom left equates to the top right of you camera.
Top and bottom and left and right are reversed in these compound scopes.

With a methodical approach it can only take 10 minutes to adjust. Not methodical can take a few weeks hehehe.

Greg.

[Stefan Buda]

Colin,
The f/3 version of the RH design is extremely prone to reflections caused by off the field stars, to the point of being totally unusable without the front baffle, which is integral part of the design.
It took me a couple of extra iterations of the baffles to get my own design cured of such reflections. The extremely short tube of the RH at f/3 makes this design different to any other common astrograph. With this design you may have perfect cutoff for stars that are one to five degrees off the field and then another star that is say 6 degrees off the field, will cause a stray reflection. Very tricky instrument.
I used the Moon and an eyepiece to track down track down internal reflections. Keep moving the Moon away from the visible field and look for any bright arcs.

[Placidus]

A lovely composition and for me the two reflections don't especially detract.

We've taken to recentering up, down, left, right by say 5% of the field width for each sub, in a random pattern, and then using statistical rejection during stacking. It's astonishingly good at handling not just cosmic rays and satellite trails but things that would not otherwise move between frames: bad pixels, tiny specks of dust, bunnies, dodgy columns. Our guess is that these reflections would also move, but you might need to move more like 20% of the field to make sure that they don't appear in the same place. Need at least 3 subs for it to work.

There is a feeling of overall glare in the image. I guess it's probably a genuine astronomical feature, but perhaps there is some grounds for subduing it.

A lovely image.

Best,
Mike

[Atmos]

Thanks Marc, except for the reflections I’m pretty happy with the results. Stars aren’t quite as perfect as your FSQ though ;)



Thanks Mike, I was pretty happy with the resolution from a wide field setup. It is tilt that’s the issue. Even without tilt the stars won’t be perfect in the corners but I can get 3 of the corners better with some work. Don’t waste dark skies though! Given how late it is when it finally gets dark, I think fixing the tilt will be a winter project :lol:



Thanks Greg, the FOV is a bit wider than most. There are some blue ghostly regions (only quite small) that don’t show up in most images which I think is quite novel.

You are definitely correct in regards to tilt, I’ve put a little time into it but I’ve been a bit lazy in trying to narrow the rest of it down. The ASI-094 has a tilt adapter in the design which helps as long as I don’t rotate the camera. It is a bit fiddly not only because of location but also the critical focus zone is tiny haha


I do remember you discussing the reflections in your build thread a while back. This is the V2 version of their RH200 that’s apparently less prone to reflections than their first so I do wonder what that would have been like then. I might have to check out a planetarium to see where these ones could be coming from. I’ve pushed the saturation and counted 5-6 of them.

I have also discovered that you were right about the trefoil, I think it is temperature related! It was there in the cooler weather but not on the warmer night last week.



Thanks MnT, one of the reflections is pretty prominent but the other 5 aren’t that noticeable :)
Using really large dithers isn’t something I’d considered as a way of removing those kinds of reflections. At the moment I’m working with 0.05% of field (2-3 pixels) which seems to work well for cosmic rays, hot pixels and FPN but you do make a good point. Given that I’m only doing 2 minute exposures, what I could do is a mosaic with 75% overlap surrounding the centre coordinates and use that for tidying up :)

I see what you mean by the glare, it could be caused by the RH design but Stefan would know a lot more than I about that. Possibly even more exposure time could help increase contrast which may remove some “glare” when there is better separation :)