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Author Topic: Zeiss Lenses for Astrophotography [Long]  (Read 2809 times)

noisejammer

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Zeiss Lenses for Astrophotography [Long]
« on: May 16, 2013, 11:59:41 PM »
A few days back, there was a discussion regarding whether the ZE 2/25 was a reasonable lens for astrophotography. I've been clouded out since then but caught a break under a dark sky. I decided to try put this to rest - at least for my lenses. The usual caveat applies - I tested one sample of each. Take this for what it's worth.

From inspection of their published performance, I believe the 2/25 Distagon, 2/50 MP, 2/100 MP and 2/135 Sonnar should be the sharpest lenses Zeiss offers. I own copies of the first three, so this these will have to do. I used a 5D Mk II with about 11k clicks. it had just come back from a sensor cleaning at Canon.

Very few flat-field astrographs are faster than f/4. Further, these lenses all exhibit 2 stops vignetting at large apertures which would make them unsuitable for astrophotography at f/2. To keep things reasonable then, I decided to stop the lenses down to f/2.8 and f/4.

I used a 3x loupe and 10x magnified live view to achieve critical focus on Spica, a bright, blue-white star, about halfway to the edge of the image. After focusing, Spica was relocated to the centre of the image. I did not use stop-down focusing - I think the camera stops itself down when it's in live view mode anyway. Note that critical focus was not achieved when the lenses were turned to the hard stop corresponding to focus at infinity. It is reasonably close but the spot size from stars was bloated by about 50%.

I used mirror lock up and the camera is piggy-backed on my heavy duty observatory mount. White balance was set to sunlight. I released the shutter using a timer and a cable. If I missed something, too bad...

The exposure was 30 seconds with long exposure noise cancellation enabled and ISO was adjusted to correctly expose the dark sky. This records lots of stars (some of which will be saturated.) This was to allow faint aberrations to be observed from a point source.

Here are my results -

ZE 2/25 @ f/3.2
http://i1141.photobucket.com/albums/n581/noisejammer/ZE_25_f32.jpg?t=1371272295
Centre - faint stars form images of 1 pixel. Bright stars are round but exhibit spherochromatism of about 1 pixel.

Midfield - Faint stars form images of 1 pixel. Bright stars remain round with slight radial CA (perhaps 1 pixel.)

Extreme corner - Faint stars - < 2 pixels. Bright stars form images about 2x2 pixels with some sagittal astigmatism. This extends over about 6 pixels. There is some radial CA - perhaps 1 pixel wide.

ZE 2/25 @ f/4
http://i1141.photobucket.com/albums/n581/noisejammer/ZE_25_f40.jpg?t=1371272328
Centre - faint stars illuminate a single pixel. Bright stars form perfectly round discs with barely visible spherochromatism.

Midfield - faint stars illuminate a single pixel. Bright stars form perfectly round discs with barely visible radial CA.

Extreme corner - faint stars are close to single pixels. Bright stars exhibit slight sagittal astigmatism (4 pixels). Some radial CA (perhaps 2 pixels, green to blue.)

ZE 2/50 MP @ f/2.8
http://i1141.photobucket.com/albums/n581/noisejammer/ZE_50_f28.jpg?t=1371272351
Centre - faint stars are near single pixels. Bright stars are perfectly round with some violet fringing on Spica.

Midfield - faint stars are near single pixels. Bright stars are nearly perfectly round. Radially reversed coma-like pattern visible on extremely bright stars - probable indicator of field curvature.

Extreme corner - faint stars appear round. Bright stars exhibit sagittal astigmatism with a small fraction of their light spread out over 6 pixels.

ZE 2/50 MP @ f/4
http://i1141.photobucket.com/albums/n581/noisejammer/ZE_50_f40.jpg?t=1371272362
Centre - all stars appear round. CA visible on Spica (but it is a special case.)

Midfield - all stars appear round. CA only visible on the brightest stars.

Extreme corners - faint stars are single pixels (these were not visible in f/2.8 image - perhaps because of vignetting.) Bright stars are slightly distorted from round but have minimal astigmatism.

ZE 2/100 MP @ f/2.8
http://i1141.photobucket.com/albums/n581/noisejammer/ZE_100_f28.jpg?t=1371272377
Centre - faint stars are single pixels. Spica is a perfectly round disc with clear diffraction rings around it. Some CA is visible - but Spica is a special case of a very hot blue star.

Midfield - all stars appear to be round. Satellite tracks are easily visible as single pixel lines.

Extreme corner - all stars are round. There is no evidence of lateral CA.

ZE 2/100 MP @ f/4
http://i1141.photobucket.com/albums/n581/noisejammer/ZE_100_f40.jpg?t=1371272389
Centre - all stars are perfectly round with their apparent diameter being dictated by their brightness. Spica exhibits some CA but less than was visible at f/2.8

Midfield -  all stars are perfectly round with their apparent diameter being dictated by their brightness.

Extreme corner -  all stars except the brightest recorded are perfectly round with their apparent diameter being dictated by their brightness. Bright stars have slight sagittal stretch but it's barely visible.

Conclusions
1. When focused accurately at f/4, each lens can produce superb results.

2. The ZE 2/100 MP and ZE 2/50 MP are quite usable at f/2.8 however CA and slight astigmatism may hinder things.

3. The ZE 2/25 should not really be used for astrophotography with a faster aperture than f/4.

4. The user should be aware that the each lens has about 1-stop illumination fall off at f/2.8 which is probably undesirable for astrophotography.

5. The user should be aware that critical focus cannot be expected if the lens is simply focused against the hard stop.

6. It would be reasonable to expect similar performance from the ZE 2.8/21 and 2.8/15 if they are stopped down to f/5.6. I can't think of many good reasons to use such wide lenses for astrophotography (except making star trails or images from a static mount.)

If ppl really want the raw data, I'll make that available on the Canon DSLR Digital Astro group

Clearest skies
73 DE TO.
« Last Edit: June 15, 2013, 01:08:04 AM by noisejammer »

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Zeiss Lenses for Astrophotography [Long]
« on: May 16, 2013, 11:59:41 PM »

Click

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #1 on: May 17, 2013, 07:40:27 AM »
Great post. Thanks for sharing.

Mr Bean

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #2 on: May 17, 2013, 06:36:49 PM »
Thanks for the info NoiseJammer.

Note that critical focus was not achieved when the lenses were turned to the hard stop corresponding to focus at infinity. It is reasonably close but the spot size from stars was bloated by about 50%.

Interesting point. I've noticed on my Zeiss 15mm that the hard stop doesn't seem to match with what the focus indicator says in the view finder. I'll try the focus on the stars, when the weather clears (live view + loupe).
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Mr Bean

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #3 on: May 19, 2013, 08:36:57 AM »
I must admit, the 15mm is a little tricky to test for accurate focus with live view ;)

But after a little testing last night, under a reasonable starry sky, the hard infinity stop on the Zeiss seems to correlate with actual infinity :)
5D mk3 with grip, 300 f4 L, 100 IS Macro L, 50 f1.4, 50 f1.8, 40 f2.8 pancake, 35 f2, 1.4x TC III, Zeiss 15mm f2.8, 24 f1.4 L
580EX II, MT-24EX Macro Flash
EF 12mm and 25mm II Extension tubes

lilmsmaggie

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #4 on: May 19, 2013, 09:35:32 AM »
Thanks for posting noisejammer.  I've been considering the Zeiss 21/2.8 for night sky (milky way, night sky in general -- but not astrophotography). 

I have a wide-field f7 APO triplet for that   ;D  I'd be interested in seeing the images you obtained during your tests.
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noisejammer

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #5 on: May 19, 2013, 12:35:42 PM »
You're welcome - I've been meaning to test these lenses for a while. The evening was clear and it seemed like an appropriate thing to try.
I've amended the original post to add links to the images.

On preparing the images for upload, I also discovered that I'd set the 2/25 to f/3.2 rather than f/2.8. I have corrected this too. By implication, f/2.8 would be even worse...

@ lilmsmaggie
I tried my ZE 2.8/21 piggybacked at Starfest last year. It's a great lens but (imo) is unsuited to this application. The image quality is fine (esp at f/5.6) but it is next to impossible to keep the earth out of the image. This means you throw away substantial parts of the frame. In my case, the scope's dew shield also protruded in the middle of the frame. I expect the 2.8/15 would be considerably worse.

BTW - I have a triplet apo too (TOA 150). There's nothing like having the right tool for the job!
« Last Edit: June 15, 2013, 01:09:25 AM by noisejammer »

lilmsmaggie

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #6 on: May 20, 2013, 01:00:35 AM »

@ lilmsmaggie
I tried my ZE 2.8/21 piggybacked at Starfest last year. It's a great lens but (imo) is unsuited to this application. The image quality is fine (esp at f/5.6) but it is next to impossible to keep the earth out of the image. This means you throw away substantial parts of the frame. In my case, the scope's dew shield also protruded in the middle of the frame. I expect the 2.8/15 would be considerably worse.

BTW - I have a triplet apo too (TOA 150). There's nothing like having the right tool for the job!

I'd love to have a Tak anything  but my SV90T will have to do.   Wasn't planning on piggybacking just use my regular tripod mounted w/ 5D2 and the 21  -- 30 sec or less exposures.  I have considered an AstroTrac but I've been looking at the iOptron iEQ45 as well.   If I'm not mistaken, the full AstroTrac travel package actually costs more than the iOptron iEQ45.
5DMKII, T3i, 24-105L, 100 2.8L Macro, 70-300 4-5.6L

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #6 on: May 20, 2013, 01:00:35 AM »

noisejammer

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #7 on: May 20, 2013, 09:59:57 PM »
... I have considered an AstroTrac but I've been looking at the iOptron iEQ45 as well.   If I'm not mistaken, the full AstroTrac travel package actually costs more than the iOptron iEQ45.

I built my first barn door tracker while on an eclipse chasing trip in Zimbabwe - so for your amusement here we go.... stripped from an astronomy site I contribute to

I made a barn door tracker the night after the 2001 TSE. I was staying at my BIL's house in a "leafy suburb of Harare." (In case you miss the reference, that's ZANU-PF speak for people who have running water and occasional electric.)

After visits to all the hardware "stores" we could find, The only piece of wood we had was about an inch thick and three feet long, none of the hinge screws matched each other and since I could only find three screws, the fourth was actually a nail. The domed nut didn't have the same thread as the rod I turned and the T-nut was effected by using Vaseline, slotting part of the rod and threading the wood....

The handle was removed from a piece of junk we found lying in the street - and since it had a 1/4" Whitworth thread, was probably the only part that fitted. I prefer a straight handle because it clearly shows orientation and doesn't require you to grasp the mount. Attachment to my uber-wobbly tripod was easy, attaching camera to the upper plank was less obvious. I can't remember this bit but I recall it involved rubber bands and beer.

Anyway, I persevered... I fitted a 17 / 3.5 lens to my OM-2 and ran a 40 minute exposure on 400 ASA Fuji..... from a house boat on Lake Kariba while surrounded by hippopotomae.

Apart from the green cast, all was fine. Mars was overhead and clearly showed off its colour against Antares. It's said that dark skies and enthusiasm cures many ills... It worked for me!


There are some excellent designs - this is probably the best I've come across - http://www.garyseronik.com/?q=node/52

lilmsmaggie

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #8 on: May 21, 2013, 02:12:12 PM »
I recall seeing Gary's BD tracker design.  I'm not much of a DIY'fer, although I'm sure I could save quite a bit of money.  My nephew is the woodworker of the family  ;D 

He's got all the tools and equipment, so push comes to shove he could help me put it together.  Lately, I've been checking this out:

http://www.ioptron.com/index.cfm?select=productdetails&phid=6f80ac1c-1ec6-4a43-8a7c-1e135839da81

Very portable.  Vixen makes the Polarie but it doesn't come with a PAS.  All-in-all, for serious AP, I would like to have a GEM.  Ironically, I had a Mach1GTO but sold it.   :-[  The iEQ45 makes for a more lightweight and portable package but tracking accuracy may not be its forte.


... I have considered an AstroTrac but I've been looking at the iOptron iEQ45 as well.   If I'm not mistaken, the full AstroTrac travel package actually costs more than the iOptron iEQ45.

I built my first barn door tracker while on an eclipse chasing trip in Zimbabwe - so for your amusement here we go.... stripped from an astronomy site I contribute to

I made a barn door tracker the night after the 2001 TSE. I was staying at my BIL's house in a "leafy suburb of Harare." (In case you miss the reference, that's ZANU-PF speak for people who have running water and occasional electric.)

After visits to all the hardware "stores" we could find, The only piece of wood we had was about an inch thick and three feet long, none of the hinge screws matched each other and since I could only find three screws, the fourth was actually a nail. The domed nut didn't have the same thread as the rod I turned and the T-nut was effected by using Vaseline, slotting part of the rod and threading the wood....

The handle was removed from a piece of junk we found lying in the street - and since it had a 1/4" Whitworth thread, was probably the only part that fitted. I prefer a straight handle because it clearly shows orientation and doesn't require you to grasp the mount. Attachment to my uber-wobbly tripod was easy, attaching camera to the upper plank was less obvious. I can't remember this bit but I recall it involved rubber bands and beer.

Anyway, I persevered... I fitted a 17 / 3.5 lens to my OM-2 and ran a 40 minute exposure on 400 ASA Fuji..... from a house boat on Lake Kariba while surrounded by hippopotomae.

Apart from the green cast, all was fine. Mars was overhead and clearly showed off its colour against Antares. It's said that dark skies and enthusiasm cures many ills... It worked for me!


There are some excellent designs - this is probably the best I've come across - http://www.garyseronik.com/?q=node/52
5DMKII, T3i, 24-105L, 100 2.8L Macro, 70-300 4-5.6L

noisejammer

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #9 on: May 22, 2013, 01:48:09 PM »
I sympathise about parting with your AP mount - we have one at our club (carries a 5" AP) and it's pretty good indeed.

There are plenty of finger powered barn door designs that require little or no effort. They offer more than satisfactory performance over a 30 minute session. (It seems there's an 80-20 rule in everything.) Here's one that a friend published.... http://psychohistorian.org/display_article.php?id=201303261529_barn-door

At $400 for the tracker, the iOptron is getting quite steep and although it's compact, it is something of a one-trick-pony. You might consider the Losmandy StarLapse which is half of a GM-8. It's possible to use this for tracking or add the other half and it works as a GEM. It even works as a jig for making time lapse movies.


niteclicks

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #10 on: May 22, 2013, 03:27:40 PM »
That Losmandy looks very interesting, I've been looking for something a bit more potable than my Atlas EQ-G.

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Re: Zeiss Lenses for Astrophotography [Long]
« Reply #10 on: May 22, 2013, 03:27:40 PM »