500mm Vivitar f/6.3 @ f/8
1/250th
ISO 200

Seems every so slightly out of focus. I might have to adjust the infinity focus a wee bit.

It looks pretty good though. Much better than my steady rain. :P I've done a few of those stop adjustments too. It's just more convincing to be able to jog back and forth across the focus point. Cranking the viewscreen magnification also helps.

Seeing can also be a factor. You might try taking many shots and stacking them to reveal the best, or even short videos.

cheers!

I might try some short videos, along with a program that extracts the best moments from them. Thank you!

Get a bahtinov mask for it and nail the focus.

That's focused to infinity. I can't go further without modification of the lens mechanism. I'm looking at it to see if I can do that.

Might it be focused a bit past infinity then? I don't think I've owned a lens that didn't reach a bit past infinity if it was a native M42 or K mount (or T with appropriate T->M42 adapter).

It's possible, although I could also have a questionable adapter. Are there any good brands that are better, or is it just trial and error?

If you go the Bahtinov route, I just learned a valuable lesson: It's tempting to use the brightest star you can find. But the pattern is actually easier to focus with if you use something dimmer. Best for me is when it's just dim enough that the bases of the spikes break up into little nodules.

Since I have more manual-focus lenses than autofocus ones, I sprung for a KatzEye focusing screen back when those were being made -- it has split-image and microprism focusing aids like an old film SLR's in the Days Before Autofocus. Still a lot of work to nail focus on the 500, but much better than the simple ground-glass field. If you can pick one up used, I highly recommend them.

IMO it's worth making or buying a mask just to check the infinity focus stop on your lens, if nothing else. I don't know of any good guidance on slot width per focal length or aperture, though. Anyone care to chime in? I have one finely-grained one that the seller said was tuned to his StellarVue (identical to mine), and a coarse one. Would love to fine-tune one for my 500.

Getting one for your 500 shouldn't be difficult as that is something that won't have a super fine feature size. Fine tuning will likely solve some of your brightest star problem as I do just point it at the brightest star I can find and haven't had an issue. I have at times not paid close enough attention and used a planet (usually Jupiter or Saturn) which results in a mess when trying to focus with a bahtinov mask.

As far as making masks go I create the template on this site and get an SVG file. I usually try to have as many slots as I can cram in to get more light to the sensor so I play around with it for a while. As far as the range the function over if you have one optmized for one focal length it will work for ones down to about 1/2 that length but the results will gradually get worse. The one I have for my 300mm I can use on my 135mm and it is useable and on my 100mm it is barely functional. After that it is totally worthless. The one for my 400mm I've used on the MTO 1000 f/10 with 2x converter to get the focus correct and going up seems to allow for better leeway of usage. However for small lenses you may find the slot width to be too thin to create with what is available to you. I'm trying to get one made for my 50/1.2 but I need access to a laser cutter.

As far as what I usually do after getting the SVG I typically use this site to cerate an STL file by extruding the SVG file 3 to 5 mm depending on how big I am making it. For my 400/2.8 I needed a big one so made it 5mm thick, but for the others I went with 3mm. Then I have just brought the STL down to the library to use their 3d printer to print one out.

The problem with 3d printers is that any that a normal person has access to won't do well with slots much narrower than 1mm so for masks for short lenses you need some other option. One thing I've thought about is laser printing onto transparencies but transparencies one can send through a laser printer are really expensive for something that I am likely to use just one of so I haven't tried this.

This is my 500 - Vivitar 500mm f/6.3 Compatible T System - Camera-wiki.org - The free camera encyclopedia - what measurements would one use at that website to design a mask? It seems like it's optimal for telescopes.

On the generator site I used these values for that lens as they should produce a pretty good one that fits inside the filter threads:
Focal Length: 500mm
Aperture: 85mm
Edge thickness: 3mm
Horizontal slot count: 13
Angled slot count: 15
Angled slot offset: -3

The slot width for a 500mm lens is 1.7mm so that is easily something that could be 3d printed with good results even with a cheap 3d printer so even if you don't own one a good number of libraries seem to have them that is worth a try. If you don't have access to a 3d printer you could probably print the generated mask on paper and used a drill and saw to cut it out of wood or some other material. I actually did that with my first attempt at creating a mask for my S-M-C 300/4 takumar and used a 4" PVC pipe cap, that fit over the end of the lens, that I cut slots in with a dremmel saw. It worked but once I got my sigma 300/4 it wouldn't fit over that lens so I went and got one 3d printed. With 18 blades that must be pretty close to devoid of any starbursts for astro shots.

The bahtinov masks are really optimal for longer glass and scopes but all that really means is that they become easier to make for longer focal lengths as the slot widths get bigger. Basically the optimal width of the slot goes up by .33mm for every 100mm of focal length so for short lenses you need some rather exotic methods. Like I said I am trying to get one for my SMC A 50/1.2 made but it looks like it needs to be laser cut having a suggested .2mm sloth width (probably .16 or .17 would be more accurate).

Mossy, thank you! At the HS where I teach, the tech teachers are good with 3D printers, and my college has one as well. So, someone ought to be able to do it for me. Thank you!

Knowing people with a 3D printer is good. They seem to always be looking for stuff to 3d print as they soon realize that they don't use it as much as they thought they would. One side note if they use PLA plastic to print it I suggest spray painting with some flat black paint. PLA plastic when doing the finishing (removing the outer edge and cleanup of rough spots) has always seemed more abrasive than ABS. The standard rustoleum paint does a nice job of giving a softer smooth finish over the plastic.

Thanks, Moss!