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01-23-2014, 08:19 PM   #1
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Does the O-GPS1 Astrotracer work with manual mirror lenses?

Hi,

After seeing this picture M31, The Andromeda Galaxy W/500mm f/8 & D800 - FM Forums I am tempted to get one of the cheap 500mm mirror lenses of Bower or Samyang brand to try this myself next time I get to the California desert. I have the O-GPS1 unit but I wonder if the Astrotracer feature works with such a manual lens. I am not 100% sure about the optics but it seems the way the Astrotracer works its function should depend on the focal length of the lens.

Does anybody know more about this?

Thanks!

01-23-2014, 08:41 PM   #2
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It does work on manual lenses, but you need to be aware that the benefit of the O-GPS astrotracer function diminishes quickly as you extend the focal length. for my 21mm the max track is about 4 minutes. For my K 300mm it is about 90 seconds. At 500mm you may likely have max of only 30 second exposures.
01-23-2014, 08:41 PM   #3
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Yes it does work (haven't done it myself, but quite a few examples posted in the various threads about Astro-tracer). You can enter the focal length manually when you switch on the camera with a manual lens attached (also required for SR to work correctly)
01-23-2014, 10:08 PM   #4
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QuoteOriginally posted by jezza323 Quote
Yes it does work (haven't done it myself, but quite a few examples posted in the various threads about Astro-tracer). You can enter the focal length manually when you switch on the camera with a manual lens attached (also required for SR to work correctly)
That's good to know. Thanks!

01-24-2014, 01:25 AM   #5
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QuoteOriginally posted by jezza323 Quote
Yes it does work (haven't done it myself, but quite a few examples posted in the various threads about Astro-tracer). You can enter the focal length manually when you switch on the camera with a manual lens attached (also required for SR to work correctly)
Correct. But one should recall that mirror lenses are fairly slow and thus require longer exposure times to record any nebulosity. And, as said above, Astrotracer accuracy declines with increasing focal length. I consider myself lucky whenever I achieve up to some 40 seconds of tracking with my 350mm f/5.6 mirror lens.

Anyway, here is an example with the Tamron SP 350mm f/5.6 lens plus a 2X teleconverter (= 700mm f/11 system). Stack of 48 exposures ranging from 15 seconds to 1 second; all at ISO 3200:


Astrotracer image with Tamron SP 350mm f/5.6 lens and 2X Teleconverter. Stack of 48 exposures 15 - 1 seconds; ISO 3200

So yes I agree that it does work - within limits. But, with the O-GPS1/Astrotracer, one would often be better off with something shorter and faster:


Astrotracer image with DA*200mm f/2.8 lens: Stack of 8 images at f/3.5 and 8 at f/6.3; both series 20 seconds at ISO 3200.

Last edited by Stone G.; 01-24-2014 at 01:30 AM.
01-24-2014, 09:42 AM   #6
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QuoteOriginally posted by Stone G. Quote
Correct. But one should recall that mirror lenses are fairly slow and thus require longer exposure times to record any nebulosity. And, as said above, Astrotracer accuracy declines with increasing focal length. I consider myself lucky whenever I achieve up to some 40 seconds of tracking with my 350mm f/5.6 mirror lens.

Anyway, here is an example with the Tamron SP 350mm f/5.6 lens plus a 2X teleconverter (= 700mm f/11 system). Stack of 48 exposures ranging from 15 seconds to 1 second; all at ISO 3200:


Astrotracer image with Tamron SP 350mm f/5.6 lens and 2X Teleconverter. Stack of 48 exposures 15 - 1 seconds; ISO 3200

So yes I agree that it does work - within limits. But, with the O-GPS1/Astrotracer, one would often be better off with something shorter and faster:


Astrotracer image with DA*200mm f/2.8 lens: Stack of 8 images at f/3.5 and 8 at f/6.3; both series 20 seconds at ISO 3200.
Wow, these are nice! I guess I'll see how far my DA 55-300 will get me and then decide what to do next. A real telescope may also be an option.

Can I ask you something about stacking? How do the individual pictures look? Can you tell from them that you are pointing at the right object or are they too dark see?
01-24-2014, 10:37 AM   #7
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QuoteOriginally posted by maxxxx Quote
Can I ask you something about stacking? How do the individual pictures look? Can you tell from them that you are pointing at the right object or are they too dark see?
I shall be happy to answer.

Take the 200mm image above:Here is a composite of two single exposures at f/3.5 and f/6.3 respectively. (Click on images to see larger versions):



And a crop of the area that I have been using for the stack above, (you can set a custom-rectangle in Deep Sky Stacker and then do all the stacking withinthat rectangle alone):



Regarding the last part of your question: For stacking, it is advisable to use raw files because you have many more tonal levels to work with. And as such, the ISO setting doesn't matter too much. But as you can see, I shoot at high ISO and therefore, I can usually see already on the LCD display whether I am pointing in the right direction or not. For the very dim objects, it may be tricky and then one will have to use surrounding stars (and a star map) as guides.

01-24-2014, 11:01 AM   #8
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Presumably the focal length is set on the camera in the usual way with manual lenses? If so, for best results you should choose a lens with a focal length that the camera supports - and/or will do if you use a teleconverter - (there may be some mirror lenses with unusual focal lengths).
01-25-2014, 09:42 AM   #9
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QuoteOriginally posted by Stone G. Quote
I shall be happy to answer.

Take the 200mm image above:Here is a composite of two single exposures at f/3.5 and f/6.3 respectively. (Click on images to see larger versions):



And a crop of the area that I have been using for the stack above, (you can set a custom-rectangle in Deep Sky Stacker and then do all the stacking withinthat rectangle alone):



Regarding the last part of your question: For stacking, it is advisable to use raw files because you have many more tonal levels to work with. And as such, the ISO setting doesn't matter too much. But as you can see, I shoot at high ISO and therefore, I can usually see already on the LCD display whether I am pointing in the right direction or not. For the very dim objects, it may be tricky and then one will have to use surrounding stars (and a star map) as guides.
Thank you! This is very helpful. Now I just need some clear sky to try it out. And somebody please tell Los Angeles to turn off the lights at night.
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