Poor results with nikon lens and SBIG8300

[Spookyer]

Hi all

I finally got around to trying my Nikon lenses to do some widefield with my SBIG STT8300 camera and filterwheel. To use a lens with this camera you need to use the standard filterwheel cover rather than the self guiding filter wheel cover and you need a lens adapter.

First off I got nothing but stars because the aperture of modern Nikon lenses are actually closed when off the Nikon body. I had to put a little piece of plastic in the aperture slot to keep the aperture wide open on the SBIG camera.

I tried my 24-70 2.8 full frame lens. The image posted is 24mm and is a crop of the bottom left of the image (about 1/8th of the image), the center of the actual full image is better but the rest is a shocker. It is like that all around the outside third of the image. I tried it at 50mm and 70mm and got similar results.

I am not sure what is happening. I believe I am following the recommendations correctly for this camera and I purchased the standard filter cover and the adapter from SBIG for this purpose so perplexed as to what I might be doing wrong. This a full frame camera lens that works on a full frame sensor that is much bigger than the 8300 chip in the SBIG. I would have thought only the centre of the lens would actually be in use.

The images are unguided but the mount is good and the sub short so it is not a guiding issue, at 24mm you can get away with a fair bit and the centre of image is not so distorted.

Any thoughts on what is going on?

Brett

[multiweb]

Looks like tilt to me. Try to square the lens on the camera.

[Spookyer]

It is much the same story in the other 3 corners. The lens locks into the Nikon f mount adapter which is screwed to the filter plate. Quite solid but no way to adjust for tilt.

[RickS]

Can you show a pic of all four corners and the centre, Brett? If you're still using PI then the AberrationInspector script will produce a suitable image.

Cheers,
Rick.

[Spookyer]

Wow, if a script could fix that it would be impressive!

I think I would like to try and find out why it is happening firstly. Do you think also it is tilt Rick?

Brett

[RickS]

Hi Brett,

The script will just show us what is happening in the centre and the corners of the image. It won't fix it but it might provide some clues as to how to do that :)

Cheers,
Rick.

[doppler]

It might be just spherical aberration of the lens. In the old film days we would have stop down a fast lens a couple of f stops to reduce the effect.

The first pic is a 50mm f 1.7 shot and the second is the same lens stopped down to f 2.8, not as bright but much sharper.

[alocky]

As other guys have said, you might want to stop the lens down a little - I've never used my 24-70 @ f2.8 for AP but I know I have to stop the 14-24 down to about f4 to get pinpoint stars at 14mm.
Have you used the lens on a DSLR at that aperture? That way you'd know whether it was lens or something else like spacing.
I'll put mine on the d800 and the Polarie and see how tight it is wide open - if it ever bloody clears up over here....
Cheers,
Andrew.

[Spookyer]

Here is a small version of the entire image.

The lens gives a nice image wide open on the DSLR so I doubt it is the glass itself leading to the problem, maybe a problem with spacings though on the CCD camera.

[Peter Ward]

As CCD's don't use blocking filters (as in DSLR's), you get to see what a camera lens really produces, warts and all, from UV to I/R.

The camera is certainly not at fault, it is simply recording the unfiltered image produced from the lens.

Any off-axis coma, astigmatism and spherical error all become pretty obvious.

Easiest fix is: stop the lens down 2-3 clicks.

[DavidTrap]

You also need to consider the thickness of the filters and it's effect on the effective flange to sensor distance.

A 3mm astrodon filter will effectively shorten the light path by 1mm.

I tried my 14-24 on my QSI and when focused on the stars the focus marker would read ~1m. I got a 1mm shim to increase the distance and it improved things.

My suspicion is that a complex zoom lens will require a very specific flange to sensor distance to work correctly and produce a flat field.

The G series Nikon lenses without aperture rings are also a pain.

I had much better results with a old prime lens. It's field flatness wasn't affected by the altered light path distance.

DT

[Spookyer]

Yeah, given the SBIG can't control the lens there is no way I know of to stop the lens down a bit unfortunately other than trimming my bit of plastic sprue so it doesn't hold the aperture totally open. Bit unscientific?

[Peter.M]

I dont do any work with lenses but I have seen people make a mask to stop a lens down manually. Now that you have it shooting wide open all you need to do is make a piece of card to put over the end of it with a hole cut in it at the diameter you want.

[RickS]

Brett,

You can make a circular aperture mask and attach it to the front of the lens. This will also get rid of the diffraction patterns around bright stars caused by the shape of the internal aperture stop.

Cheers,
Rick.

[Spookyer]

Ok guys, the front element is 77mm across, what sort of size do you suggest for the hole in the aperture mask?

Brett

[Merlin66]

Brett,
You don't mention the distance from the rear camera flange to the CCD....

[sjastro]

First of all I would go to the Photozone website which shows tests on the lens.
http://www.photozone.de/nikon_ff/456-nikkor_afs_2470_28_ff?start=1

If you look at the MTF test you will find the "sweet spot" for lens performance is more or less at f/5.6 over the entire FL range for the 8300 chip.

Hence the the diameter of your aperture mask is simply the focal length divided by 5.6

Regards

Steven

[DavidTrap]

You can buy a filter step down ring to stop down the aperture. Reasonable cheap out of Asia on eBay.

DT

[Spookyer]

Yeah, they are cheap that is for sure. I might try them. I might also get my 50mm 1.4 prime out as well. It has an aperture ring and I can close it down to say f4 and give it a go. Thanks to all for the suggestions so far. When I get some clear skies I will report back the results.

brett

[gregbradley]

I think these lenses are also sensitive to the flange distance. For Nikon I think the flange distance is around 44mm (check that). So you can measure how far your adapter puts the sensor plane to the back of the lens.

Also as mentioned several times many lenses do not perform well wide open especially short focal length lenses. Most need to be stopped down.

My 14-24 works fine at F2.8 on a DSLR but on my Proline 16803 with a Nikon adapter it would not work looking even worse than your sample images. I suspect again the flange distance is off. The Nikon 50mm F1.8g though was super sharp at F2.8.

You can jam a tooth pick in the little spring loaded aperture control at the back of the lens.

Now you see why lenses with aperture rings are better suited to astro work.

Also 24mm wide open is supposed to be a bit weak on the 24-70 although I must admit I did not notice it on my copy and a D800e.

Try different lenses. Zooms are usually a compromise so stick to primes unless its the Nikon 14-24 which is a super lens.

Pentax 67 lenses are quite forgiving and work up to a 16803 chip very well. They are also cheap. The 75 F4.5 and the 165mm F2.8 wide open work well.

I think you will find not many camera lenses work well wide open for astro work especially fast lenses (F1.8 or less). An eye opener really about lens performance. 50mm is a bit of a sweet spot as manufacturers seem to be able to make them high quality quite easily and most 50mm lenses I have tried worked well (Pentax 55mm 67 lens not being so good).

The best lenses I have used are:

Pentax 67 165 F2.8 - really sharp and pinpoint no chromatic aberration or very little at F2.8 - rare performance.
Pentax 67 300 F4 hard to mount as its a heavy and large lens but it works well.
Pentax 67 75mm F4.5 not quite as good as the 165 but really very good overall.

Nikon 14-24 really a DSLR lens and have not been able to mount it on CCD so far with success.

Nikon 50mm F1.8g F1.8D. Both are superb. I would go the 1.8D as it has a metal aperture ring making aperture control simple whereas the G lens has no aperture ring.
Nikon 85mm F1.8g looks good around F2.8 better at F4.
Samyang 24 F1.8 - nice at F2.8 and above.
Fuji 14mm F2.8 wow what a lens, maybe a better alternative to the Nikon 14-24 for an 8300 chipped camera as it has an aperture ring, is light and is super sharp and much cheaper. Haven't tried it on a CCD but I suspect it would be good. Shorter flange distance of around 28mm may be an issue for adapters.
Fuji 18-55 F2.8 zoom - very nice not used on a CCD though.
Contax Yashica 28-85mm - worked well at about F4.
Minolta Rokkor 28mm F2.8 - not bad a bit of chromatic aberration but has an aperture ring, is cheap and has a solid reputation as a good lens.

I am keen to try my Zeiss 55 F1.8 which is the 2nd highest rated lens on DXOMark. On a Sony A7r its sensational. Whether it performs for astro remains to be seen. Again hard to adapt to a CCD as its 27mm flange distance versus around 44 for Nikon and Canon.

Greg.

[DavidTrap]

Greg - I think you'll find the aperture rings on the Fuji lenses are merely a selector switch, rather than a mechanical linkage that adjusts the aperture diaphragm.

Haven't tried the Canon trick of depth of field preview while removing a lens from the body - don't think there is such a button on my X-E2.

I did get some improvement by using a 1mm shim to adjust the flange to sensor distance with my 14-24, but there was still a lot of distortion in the corners.

DT

[Spookyer]

Yeah, I experimented with more lenses including primes like the 50mm 1.4 that does have an aperture ring. My conclusions are that the STT8300 is not really suitable to use with DSLR lenses. There is nothing wrong with my lenses, they work fine on a DSLR. My STT8300 works fine with my scope. But they don't work fine together. I got some average sort of results with some work but I would guess the spacings and angles are not quite right, as a result it is hard to focus and even stepping down the lenses a fair bit you don't get great results.

A bit disappointing as I bought the recommended stuff to use lenses with the CCD but looks like I will be back to the CCD on the scope and if I want to do widefields it will have to be with the lenses on the DSLR.

Thanks all for your comments and suggestions.

Brett

[alocky]

The odd thing is the 8300 chip is smaller than a full frame sensor, and it's a flat surface , and it's got larger pixels than a D800, so it has to be a spacing or tilt problem since that is the only difference there could be between the systems.
Right? Or am I missing something?
My point being, in that case you have something that was not fit for purpose as supplied and it should be something the vendor would be obliged to resolve?
Cheers,
Andrew.

[gregbradley]

Yes you are abosolutely right. I just went and checked. I did not know that.

Perhaps exposure lock then release the lens may leave it at the last selected aperture?

Greg.

[gregbradley]

I have a FLI Microline 8300 and the Nikon adapter that fits in the filter wheel. I'll try that out at some point.

Martin Pugh did some widefields with an SBIG8300 camera and lenses. Have a look at his site to see which lens he used.

Greg.

[Garbz]

Are you sure you have the spacing exactly right? I've experienced similar on my Nikon adapters. It's not sufficient to be in the focal range, but rather if your lens isn't focusing very close to it's infinity mark then it makes things much worse around the image. This is worse on wide angle lenses than telephotos in my experience.

If you have the spacing right then it likely really is just a sharpness issue. I typically shoot at f/4