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Old 06-09-2014, 04:49 PM
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Spookyer (Brett)
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Poor results with nikon lens and SBIG8300

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
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Old 06-09-2014, 04:52 PM
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multiweb (Marc)
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Looks like tilt to me. Try to square the lens on the camera.
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Old 06-09-2014, 05:52 PM
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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.
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Old 06-09-2014, 06:05 PM
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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.
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Old 06-09-2014, 07:09 PM
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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
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Old 06-09-2014, 07:17 PM
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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.
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Old 06-09-2014, 08:08 PM
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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.
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Old 06-09-2014, 08:18 PM
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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.
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Old 06-09-2014, 08:21 PM
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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.
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Old 07-09-2014, 11:39 AM
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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.
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Old 07-09-2014, 01:33 PM
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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
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Old 07-09-2014, 03:10 PM
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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?
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Old 07-09-2014, 03:25 PM
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Quote:
Originally Posted by Spookyer View Post
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?
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.
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Old 07-09-2014, 03:29 PM
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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.
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Old 07-09-2014, 03:51 PM
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Ok guys, the front element is 77mm across, what sort of size do you suggest for the hole in the aperture mask?

Brett
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Old 07-09-2014, 03:52 PM
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Brett,
You don't mention the distance from the rear camera flange to the CCD....
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Old 07-09-2014, 04:27 PM
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Quote:
Originally Posted by Spookyer View Post
Ok guys, the front element is 77mm across, what sort of size do you suggest for the hole in the aperture mask?

Brett
First of all I would go to the Photozone website which shows tests on the lens.
http://www.photozone.de/nikon_ff/456..._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
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Old 07-09-2014, 07:14 PM
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You can buy a filter step down ring to stop down the aperture. Reasonable cheap out of Asia on eBay.

DT
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Old 07-09-2014, 09:27 PM
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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
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Old 10-09-2014, 08:37 PM
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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.
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