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Help - stars and coma, can anyone tell me if this is normal?

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    Aenima
    Aenima  1 England
    14 Jul 2013 - 2:17 AM

    Hi,

    Recently got a nikon for imaging without a telescope, widefield shots of stars etc. but even choosing a nikon lens that focused infinity ive still noticed problems - especially coma, where the stars look bullet shaped.

    As im new to both lenses and widefield imaging i'm hoping someone can tell me how normal this coma is and what it might mean for the lens.

    -dsc0020crop.jpg

    -dsc0022crop.jpg
    -dsc0018.jpg

    These are crops of several seconds exposure @ f100mm lens is: nikon 28-100mm AF nikkor

    -dsc0017crop.jpg

    Any help greatly appreciated!

    Regards,

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    14 Jul 2013 - 2:17 AM

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    monstersnowman
    14 Jul 2013 - 5:33 AM

    Is it in the viewfinder or image? Obviously slow exposure will show some movement or movement when pressing the shutter button in the image if not using some remote release. Also mirror shake will cause movement on distant subjects. Maybe the sturdiness of the tripod. Cheaper tripods are useless if not using remote or ir/radio triggers.

    bigalguitarpicker

    I suggest using a much shorter exposure. Shoot lots of frames, then combine them in a programme such as Registax.

    Gundog
    Gundog  1624 forum posts Scotland
    14 Jul 2013 - 11:25 AM

    At any exposure of more than about 10 seconds, stars will start to "trail". Plus everything Monstersnowman says.

    Dave_Canon
    14 Jul 2013 - 11:46 AM

    How did you focus? I would suggest that it is important to focus manually in this case. Many lenses now (particularly longer lenses) focus beyond infinity so the infinity point is not at the end but very close. I believe this is done to allow for expansion etc.

    Dave

    simmybear
    simmybear  732 forum posts
    14 Jul 2013 - 3:23 PM

    Dave I'm sure Stephen Hawking would love to know what is "beyond infinity" lol

    Aenima
    Aenima  1 England
    14 Jul 2013 - 6:48 PM

    Sorry, huge gap in information on my part.

    Ok. : camera was mounted on a tracking equatorial mount/telescope piggyback. The 28-100mm lens was manually focussed to the hard stop that is as 'infinite' as it gets with the focus ring.

    Apologies for the missing info, I was in a hurry as the lens has a limit of a few days before I can decide whether to send it back.

    For 40 bucks I cant ask too much. But as astrophotography is my main interest the way the lens handles stars is important.

    I bought nikkor because a few m42 lenses Id tried didnt reach infinity, hence a nikon brand for a nikon camera seemed a good way to go.

    Id be very interested to know if the lens is normal for the modl - nikon silver AF- 28-100mm F3.5 5.6 g (40 gbp)?

    Also if there is a reasonably priced alternative or even any tips for this one that deals with coma better.

    Many thanks for all the replies, very much appreciated and helpful,

    Regards,

    Gundog
    Gundog  1624 forum posts Scotland
    14 Jul 2013 - 7:00 PM


    Quote: How did you focus? I would suggest that it is important to focus manually in this case. Many lenses now (particularly longer lenses) focus beyond infinity so the infinity point is not at the end but very close. I believe this is done to allow for expansion etc.

    Dave

    Correct fact - but wrong reason I believe. I was told by the Nikon guy at Focus back in March that the reason modern AF lenses appear to focus "beyond infinity" is to prevent them being damaged when auto-focus takes them to infinity. The hard stop is beyond that point to allow a degree of latitude.

    But yes - it means that, if you want to manually focus at infinity, you have to set the lens at the infinity mark rather than at the end of its focussing rotation.

    Aenima
    Aenima  1 England
    14 Jul 2013 - 7:04 PM

    Its definitely coma - and in the image, the stars look like seagulls. Tongue

    Aenima
    Aenima  1 England
    14 Jul 2013 - 7:06 PM

    When manually focussed just 'before' infinity the stars looked like donuts so the hard stop is likely the right point to focus on this particular lens. Even so the coma is pretty severe enough to be the bigger issue for me.

    Last Modified By Aenima at 14 Jul 2013 - 7:08 PM
    Aenima
    Aenima  1 England
    14 Jul 2013 - 7:16 PM

    1--dsc0020crop.jpg

    Cropped.

    Aenima
    Aenima  1 England
    31 Aug 2013 - 3:26 PM

    Well, I returned the lens to the extremely helpful seller, and swapped it for the 55-200mm AF-S nikkor which is the same design as my kit lens and does the 'past infinity' thing so I can be sure of it reaching the point where stars focus properly. However, the issue of coma is still quite bad as these lenses I'm using are very budget range, but at least its par for the course and with a few exposures 'stacked' the stars begin to merge to the point where they look better in a picture once stacked and processed.

    cygnus.jpg

    This is JPEG'd to death but the stars look like stars at least Smile

    Regards,

    Aenima

    Sooty_1
    Sooty_1 Critique Team 31125 forum posts United Kingdom192 Constructive Critique Points
    1 Sep 2013 - 12:04 AM

    I would suggest it's more to do with the relative cheapness of the lens, and the fact that zooms are not usually as well corrected for aberrations as they are much more of a compromise (after all, how many people that use budget lenses are likely to be shooting for critical sharpness in the outer field at night?).

    I would suggest finding out which focal length you are most using for wide field and buy a prime lens of that length. 24, 35 and 50 mm are popular for astrophotography. Nikon primes are excellent, and if you can use a manual lens on your camera, a relatively cheap way of doing it (some camera/lens combos lose some functionality, but most are useable).

    Also, as above, stacking exposures is much better than single exposures now, allowing you to cut out background light and effectively increase your signal to noise ratio, giving better contrast and colour rendition.

    Nick

    Aenima
    Aenima  1 England
    2 Sep 2013 - 2:26 PM

    Thats very helpful advice Nick, many thanks!

    Yes, figuring out which i use most is a good plan, I kinda realised prime lenses were more what I needed, although I intend to use the camera in other ways so my zooms are still appreciated, I think a very high f/l telephoto would be the way to go as most of my targets are small even through a 8inch telescope OO!

    thank you for the suggestions, Smile

    Regards,

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