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Old 06-02-2011, 01:15 PM
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hotspur (Chris)
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Is it possible to use a 70-200 F 2.8 for astronomy photography??

Hello.

I have a 70-200 F 2.8 lens,I bought this for certain aspects of photography,and the images are amazing.I had no thoughts of using for astro work,I have tried using it for a few astronomy photography episodes,But the results had lot of reflections.Is it possible to use this lens for astro work,if so,what settings?

I hear the 70-200 F 4 Non IS,is very good for astro work,along with the 200 mm prime.

No drama if I cannot use this lens for astro work,but would be a bonus.

Thanks for advice.

Chris
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Old 06-02-2011, 01:46 PM
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Cloudyagain (Neale)
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It certainly is Chris.

I have recently bought an adapter to put my 70-200 F2.8 nonIS lens on my SBIG camera. First decent pic the other night attached. Taken at 200mm, external mask to stop it down to about F4.4, 12x5 min subs. I also put a longer light shield on to limit reflections as my first efforts were plagued by them. Need to stop down a bit further as stars not round at the corners. Have made another mask this morning to give about F5.4.

Neale.
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Old 06-02-2011, 02:09 PM
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koputai (Jason)
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Please pardon my ignorance Neale, but why use an external aperture mask and not the ...ah ok, it just occured to me, don't worry....

So, when using on an astro CCD camera you need an external aperture mask. When using on a DSLR, I gather it's fine to use the lens' own internal aperture?

Cheers,
Jason.
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Old 06-02-2011, 02:14 PM
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mill (Martin)
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You should also be able to set the aperture first with the dslr and then connect the lens to the ccd camera.
At least that that is what i think.
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Old 06-02-2011, 02:18 PM
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Cloudyagain (Neale)
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The problem Jason is that you can't easily stop down the Canon lens as it is all done electronically on the camera. H has posted a solution on this forum but it takes a bit of mucking around and you then have the problem of diffraction spikes, depends on which side of the fence you sit.

You have to stop the lens down to get decent looking stars, pity really when you spend all that money.

Neale.
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Old 06-02-2011, 02:19 PM
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Cloudyagain (Neale)
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Sorry Martin, you got in while I was typing.

Neale.
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Old 06-02-2011, 02:48 PM
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hotspur (Chris)
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re 70-200

That's a great image,well done.

I am just looking at the prospect of using the lens on a regular DSLR,I am unfamiliar with 'external apeture mask',where does one buy these,and how to make them work?

All sounds a bit complicated,might just play around with using the 400 prime-seems a bit simpler to use.

Chris
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Old 06-02-2011, 04:10 PM
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mill (Martin)
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Chris it is as simple as making a round hole in a piece of carton and fit it in front of the lens.
Experiment with the size of the hole to get the best result.
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Old 06-02-2011, 05:15 PM
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Cloudyagain (Neale)
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Chris, if you are going to use a DSLR then just stop the lens down with its in built diaphragm.
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Old 06-02-2011, 06:46 PM
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Octane (Humayun)
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The reason for using an external aperture mask is so that you can still shoot at f/2.8 (convenience of speed) without attaining diffraction spikes in the process.

Essentially, you just need to use some step down rings screwed in succession to make a mask.

The process Neale alluded to with stopping a lens down goes like this:

1. With the lens on the DSLR
2. Set your aperture as you normally would
3. Press the Depth-of-Field Preview button (little button near the lens release mechanism button)
4. Whilst depressing the DoFP button, remove the lens

The aperture is then set at whatever value you had it at on the DSLR

H
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Old 06-02-2011, 07:57 PM
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I'm hoping to give this a go with my Nikon 70-200 VRII lens in due course. All of the reviews of this lens say it is pinsharp from corner to corner of a full frame chip. The original VRI version was OK on an APS sensor, but not on a full frame. It will be interesting to see what it is like for more distant objects!

DT
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Old 06-02-2011, 08:34 PM
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hotspur (Chris)
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re lens

Thanks Humayan-how much are these 'Step-down adaptors' and were do you buy the from?

Sounds like there are two methods to use this lens for astro work,Which is good to hear.There is a chap that comes up here to observe,he has just bought one of these lenses,So I will have a chat with him about these suggestions.This is certainly not a high priority photography task-but the lens cost a lot,an d it would be good to use it on some other photography episodes.

The 70-200 range would be quite useful.

Thanks for all the advice by the many people-Chris
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Old 06-02-2011, 09:11 PM
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Chris - a step down adaptor (or "ring") is device to let you put a smaller filter on the lens - male thread on one side and female on the other. They could be 77mm to 67mm. H is just suggesting you use this as an external mask.

An external mask is just a piece of something (can be cardboard) with a hole cut in it - think of it like a Bahtinov mask that has a hole in it, rather than the grid. The aim is to have a circular hole for the light to pass through, rather than the geometrical shape (straightish lines) that the inbuilt aperture blades create. These straightish lines can create diffraction spikes on your final image. You vary the size of the hole to vary the "aperture" of the lens.

I saw an article in S&T about using a hexagonal mask on a telescope to allow you to better see a double star where one star is 10-100x brighter than the other.

DT
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  #14  
Old 07-02-2011, 10:52 AM
space oddity
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step on down

Stepping rings are easily obtainable on the old ebay. Just bought a complete set from China for very little cost. Rings convert to and from the common sizes from 43mm up to 82mm .The common ring sizes are 82,77,72,67,62,58,55,52,49mm. For one or 2 rings, a camera shop will have them for not a lot of money. There are stepping rings up or down which can cover a single large range if you have a specific mask size in mind.
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Old 09-02-2011, 05:54 AM
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ZeroID (Brent)
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Damn!! Another idea to play with !! I've got a beautiful KM full auto 70-300 tele that gets little use and now you've just given me all the ingredients for another workshop session with all my optical bits.
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