Skewed stars on image capture

[Ian Gillingham]

I've caught the astrophotography bug and after several months of trying 1" exposures on a fixed tripod, I bought a HEQ5 Pro mount, on which to fix my Nikon D90 DSLR. Presently I'm using my 105-300mm Nikon zoom lens, which seems to do an 'alright' job and have captured a couple of 'not bad' images of M42 (the usual). My biggest issue at the moment is that, even with best focus using live-view and a bahtinov mask and a well aligned mount, the stars in my images all have a wierd conical shape. I'm wondering whether anyone might be able to diagnose the cause please? Could it be that the 300mm dslr zoom lens just isn't up to the job? Could it be spherical aberration? I don't want to have to buy another lens or astrograph *just* yet, if I can avoid it.

I don't think it's a focus issue. I don't think it's a tracking issue either, because the conical shapes are the same at 5 second and 300 seconds. I have attached images of both scenarios, just to show the star shapes - simple OSC single frame, no processing at all, so ignore the sky glow in the 300 sec exposure. These are screen shots from "Raw Therapy", as my images are raw DNG files and I wanted to zoom in on the star shapes.

Thanks,

Ian

[rickwayne]

Sure looks like coma to me. Pretty common with zoom lenses. The good news is that you don't need anything modern or fancy. Autofocus, zooming, instant diaphragm stop-down -- eh, who needs 'em?

So you can scour Ebay or other used markets and pick up some much-less expensive single-focal-length glass. They made perfectly good optics back in the 1970s, and since you don't need any of the automation contacts, you can even use other lens brands, so long as you have an adapter that lets you reach infinity focus. The old Pentax screw-mount 200mm f/4 has launched many an astro career!

[michael8554]

Hi Ian

This coma might be unnoticeable in "daytime snaps", but is all too obvious in star shots.

You could try setting the lens to different focal lengths, and stopping down the f/ number. 

Better results will be with the lenses RickWayne suggests.

Michael

[Ian Gillingham]

Thanks for the helpful comments! I have tested with 2 stops down (f7.1) and sure enough, there is a noticable improvement. The stars in centre frame are circular, but still some coma showing heading out to the edges. I guess it's the best I can expect from this lens. I shall now go and look for second hand manual prime lenses, see if I can pick one up at a decent price.

I've attached one of last night's subs to show the coma improvement. No metrics, so it's more perceptual at the moment.

Cheers,

Ian

[ollypenrice]

It was ever thus. The good news is that the astrophotographer wants what the regular photographer does not, which is fixed focal length, manual focus and no stabilization. For once, the market delivers nice prices for the astrophotographer.

Olly

Edited April 4, 2023 by ollypenrice
typo

[gorann]

Even expensive zoom lenses can produce terrible stars (like my Sigma 70 - 200 f/2.8), so fixed focal length is the way to go. Samyang (aka Rokinon in the US) makes excellent ones. Old Pentax Takumars also have a good reputation.

[rickwayne]

Fotodiox has a Pentax 6x7-to-Nikon adapter for US$65, and if you're willing to wait for shipping, Ebay has 6x7 Pentax 200mm f/4s from Japan for well under US$100.

AFAIK the 6x7 lenses don't mount directly on any digital bodies, so they are going for a song. Certainly going to have a big enough image circle!