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Author Topic: Astrophotography telescope 'lens'?  (Read 3899 times)

Caps18

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Astrophotography telescope 'lens'?
« on: December 02, 2011, 06:53:09 PM »
Over the last two nights I have been using my 5Dm2 with a 300 f/4 with a 2x & 1.4x stacked teleconverters to get it to 840mm, and it took some great pictures of the Moon, although I think another 2x would be good for the Moon (~2000mm to get it full frame).  At 840mm, I could see Jupiter and 4 moons, but Mars is too small.  I would guess you would need ~25000mm to get Jupiter full frame on a 5Dm2.  I might need to get a planetarium to get that size of a telescope...

http://www.telescope.com/Astrophotography/Astrophotography-Telescopes/Orion-Atlas-10-EQ-G-Reflector-Telescope-with-GoTo-Controller/pc/-1/c/4/sc/57/p/24735.uts

Besides the auto pointing tripod, do you have any other suggestions?  Would I need to support the camera hanging off the side?  Could I use the teleconverters to 'zoom in'?  Has anyone had any luck with this type of setup?  Is there a better telescope for this kind of photography in the $2000 range?

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Astrophotography telescope 'lens'?
« on: December 02, 2011, 06:53:09 PM »

Jettatore

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Re: Astrophotography telescope 'lens'?
« Reply #1 on: December 03, 2011, 02:29:29 AM »
I briefly researched astrophotography, and one thing that was made clear to me, was that light gathering ability was more of a concern than actual magnification, which is more so handled by the eye-pieces you attach to the telescope.  Fortunately youtube and the rest of the net is filled with videos and info on this subject.  I want a good telescope, but being so close to New York City and the other largish nearby cities there is little hope of filtering out massive amounts of light pollution.

We had a big power outage a month or so ago, and I went outside and just looked up at the sky, and the amount of stars I could see with bare eyes was crazy vs. a normal night here.

MintMark

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Re: Astrophotography telescope 'lens'?
« Reply #2 on: December 03, 2011, 05:06:03 AM »
You can read all you need to know and a lot more at http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM.

Think of the telescope as a prime lens that is manual focus with a fixed aperture. How you use it depends on what you're photographing. An adapter allows the camera to fit where an eyepiece would normally go. Yes, you zoom in by adding more lenses. For telescopes you can use Barlow lenses (designed to magnify eyepieces). Camera teleconverters work too, but the camera doesn't like the absence of a lens so you have to work around that. The more magnification you add the dimmer the image gets, so it only works for small bright targets (Jupiter and Saturn). For small dimmer targets you need more aperture. A lot of planet images start off as video, then the best frames are selected and averaged together.

For faint targets (galaxies and nebulae) you have to do long exposures. You need a mount that tracks the apparent movement accurately. This only happens when it is properly aligned parallel to the earth's axis. The longer the exposure and the longer the focal length the more accurate the mount and the alignment has to be. This is where the added weight of the camera comes in... it can be a problem if you are operating near the limit of the mount.

Anyway, the link I gave describes the sorts of things you can do with different equipment... you can do quite a lot with just a fixed tripod. There are also man made targets up there too, such as the international space station flying past and Iridium flares.

Hope that helps,

   Mark

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Re: Astrophotography telescope 'lens'?
« Reply #3 on: December 03, 2011, 05:41:51 AM »
Decide if there are particular subjects you're going after. For planets you want a longer focal length. If you go deep sky objects, shorter focal lengths and ratios are often more useful.

Don't forget about diffraction too, particularly for planets or other small objects. You need a big enough physical aperture to resolve details. I have a 104mm aperture scope, which I often used in 2650mm f/26 configuration, and that's way into diffraction softening there so it'll never be sharp. But with that I can see two bands on Jupiter, and the rings of Saturn. I've not had any luck with Mars or Venus though where I'm happy if I can make out a disc.

Specifically for imaging planets, the DSLR might not be the best choice anyway. While it is not something I've successfully tried, the use of webcams to capture a stream of video is a popular method, where the video stream is then fed into software to sort out the best frames from the bad and stack them together to get some really nice results.
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aldvan

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Re: Astrophotography telescope 'lens'?
« Reply #4 on: December 03, 2011, 09:06:48 AM »
I started 'astrophotography' with my 5D MkII, catching Moon and Jupiter pictures. Than I decide to go for something more specific. I bought a Meade L200, a wonderful telescope, just for realizing that my  point of observation in town was spoilt by huge light pollution.
By the way, with a focal lenght of 2000mm (my Meade's) you get a very small Saturn at direct focus, and something bigger through a Baader 13 mm ocular.
The best thing you should get is a place free of light pollution, allowing a very little noise and radical crop...

Edwin Herdman

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Re: Astrophotography telescope 'lens'?
« Reply #5 on: December 03, 2011, 09:48:48 PM »
So here's what you do:

- T1i-ish or newer camera.
- Get an EOS to T-mount adapter.
- Get a telescope that seems to match what you want.  Obviously it should allow a t-mount connection (the "t" seems to stand for Tamron, surprisingly enough it was apparently a legacy mount style of theirs).
- Get a motorized (hopefully computerized) mount for it (if you're handy you can construct one yourself; there are some guides on the net).  This is important because the very long exposures required for a good photograph mean that the Earth will rotate during the time of the photograph, and so you will get star trails if you don't follow the part of the sky you're trying to image.  On the other hand, this can be used to good effect (point at Polaris and take a long exposure).  Some of the computerized mounts should offer an automatic orientation ability, which doesn't really stretch your knowledge of the stars and takes a long while to complete, but it might be accurate if you're having trouble finding something or just don't really know where to look.

Now you can take long exposures.  You will be using "bulb" mode to do it, partly because the telescope will be a "slow" lens (f/8 or smaller) pointed at an already dim sky.  A very sturdy telescope mount will be helpful, and so will a remote release.

If you don't like the way the photos are turning out, try using the Registaxx software to "stack" multiple images to reduce noise.
Specifically for imaging planets, the DSLR might not be the best choice anyway. While it is not something I've successfully tried, the use of webcams to capture a stream of video is a popular method, where the video stream is then fed into software to sort out the best frames from the bad and stack them together to get some really nice results.
DSLRs are used by serious amateur photographers who don't have enough scratch to buy one of the professional imagers ($X000-$X0000+ and not likely to be less than the cost of a professional full frame camera, for what is basically a box with control lines for a computer connection).  DSLRs can capture video, too; usually on par with the webcams (if not better), and the sensor of a DSLR will be larger as well.  There is also that not minor detail of webcams being unsuitable for connection to a telescope.  However, I'm not sure that with either video method you will have enough of an exposure to capture enough signal to resolve astronomical objects, especially through a slow (f/8 or smaller) telescope lens.

TexPhoto

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Re: Astrophotography telescope 'lens'?
« Reply #6 on: December 03, 2011, 11:32:36 PM »
Check eBay for a used telescope that does what you want.  Pennies on the Dollar.

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Re: Astrophotography telescope 'lens'?
« Reply #6 on: December 03, 2011, 11:32:36 PM »

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Re: Astrophotography telescope 'lens'?
« Reply #7 on: December 04, 2011, 08:59:30 AM »
- Get a telescope that seems to match what you want.  Obviously it should allow a t-mount connection (the "t" seems to stand for Tamron, surprisingly enough it was apparently a legacy mount style of theirs).
While some scopes have dedicated adapters to go to T-mount, the fail safe option is to get an eyepiece adapter. This fits between the camera T-mount adapter and the scope eyepiece holder.

Quote
- Get a motorized (hopefully computerized) mount for it (if you're handy you can construct one yourself; there are some guides on the net).  This is important because the very long exposures required for a good photograph mean that the Earth will rotate during the time of the photograph, and so you will get star trails if you don't follow the part of the sky you're trying to image.  On the other hand, this can be used to good effect (point at Polaris and take a long exposure).  Some of the computerized mounts should offer an automatic orientation ability, which doesn't really stretch your knowledge of the stars and takes a long while to complete, but it might be accurate if you're having trouble finding something or just don't really know where to look.
One more detail here: if you want to do half serious long exposure work, you need an equatorial mount, or a mount that can be adapted to such. While Alt-Az mounts can track, you will get field rotation over time. Sticking the alt-az mount onto a wedge is one way to convert it to an equatorial mount. This means one of its axis of rotation will match that of the earth.

Quote
Now you can take long exposures.  You will be using "bulb" mode to do it, partly because the telescope will be a "slow" lens (f/8 or smaller) pointed at an already dim sky.  A very sturdy telescope mount will be helpful, and so will a remote release.
A timer remote off ebay has proved invaluable here!

Quote
If you don't like the way the photos are turning out, try using the Registaxx software to "stack" multiple images to reduce noise.
Stacking is practically a requirement for deep sky stuff, especially if using a DSLR. I have to admit I haven't tried registax in a long time, but it seemed more suited for processing the video output of webcam imaging. Deep Sky Stacker is targeted at deep sky objects. IRIS does everything, if you can ever figure out how to use it! Beyond that is a selection of commercial software also. I've started using PixInsight as I got frustrated by DSS' limitations.

Quote
DSLRs are used by serious amateur photographers who don't have enough scratch to buy one of the professional imagers ($X000-$X0000+ and not likely to be less than the cost of a professional full frame camera, for what is basically a box with control lines for a computer connection).  DSLRs can capture video, too; usually on par with the webcams (if not better), and the sensor of a DSLR will be larger as well.  There is also that not minor detail of webcams being unsuitable for connection to a telescope.  However, I'm not sure that with either video method you will have enough of an exposure to capture enough signal to resolve astronomical objects, especially through a slow (f/8 or smaller) telescope lens.
The webcam suggestion is specifically for planets, and typically moon surface detail too. Planets are small and bright, and DSLRs with their relatively big pixels are harder to work with. You get the crop factor advantage with a webcam, not imaging tons of empty space. A DSLR video doesn't work as well here since you're effectively using far too big a pixel size and might even fall between line skipping, although with the latest ones which can do sensor crop it might not be so bad. Actually "webcams" are so popular a method for doing this, the scope manufacturers do their own dedicated versions too: the Meade LPI and the Celestron NexImage.

A DSLR does come in as a value option for deep sky imaging, where you can make use of the size and can be matched with either lenses (which is what I use) or scopes.


For example, this is a total of less than 30 minutes (14x2m) of exposure time from last night, 100% crop. I know it can be a lot better, but the weather isn't something I can control so I ran out of time to do any more. That was with a modified Canon 450D (increases sensitivity in deep reds), H-alpha filter, 135mm f/2 and astrotrac mount. Light pollution included at no extra cost. I wish I didn't sell the 300/2.8 now...
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Edwin Herdman

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Re: Astrophotography telescope 'lens'?
« Reply #8 on: December 04, 2011, 09:03:41 PM »
Thanks for those updates.  I'm surprised to hear about webcams, but you obviously know what you're talking about there!

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Re: Astrophotography telescope 'lens'?
« Reply #8 on: December 04, 2011, 09:03:41 PM »