16 replies to this topic

[NevadaTom]

I'm trying to justify buying a 2-3 thousand dollar telescope for astro photography when I already have a camera and lenses costing 2-3 times that much. I'm looking for opinions on which way you would go in my situation.

 

So far I have a Canon 5D3 and a variety of "L" lenses to choose from for astro photography. (I also have a CGEM) So let's say I decide to use my 400mm f/5.6 prime lens attached to a 2X tele-converter giving me effectively an 800mm lens for AP. All things being equal, I assume that the image size at the camera sensor would be identical if I used a hypothetical 800mm telescope and mounted the camera at its prime focus. Let's ignore aperture for the moment because I understand that the larger aperture available on this hypothetical 800mm scope would give me a brighter image and therefore shorter exposure potentially. Am I thinking right so far?

 

The Canon 5D3 has a 22.3 megapixels on a full frame 24X36 mm sensor. That gives generous size pixels for collecting more photons without having to introduce too much electronic noise by multiplying the contents of smaller pixels in cropped frame cameras. What would I gain by adding a high end telescope to my present setup? Hanging that heavy camera at the prime focus of this high end scope would render it useless for simultaneous observing. As an alternative, eyepiece projection would give me the option of greater magnification of course. As far as I know dedicated astro cameras are pretty much all prime focus devices, so there would be no greater magnification available there - although they would be lighter in weight and potentially allow switching color filters.  Of course I could also used a cropped frame camera instead of the 5D3 and get simulated magnification that way. Alternatively I could use a lesser quality scope for observing while letting the Canon take the high resolution images. Then too, there is the option of using a modified camera instead of the standard 5D3 to boost infrared response. So many avenues to choose!

 

Tom Clark

[Pinbout]

 

So far I have a Canon 5D3 and a variety of "L" lenses to choose from for astro photography.

 

they're not optimized for full aperture so you'd have to shoot 3x f-stops slower than the fully open, ie f11 instead f5.6

 

Telescopes are better for focusing on infinity.

 

you can do it and see how you like them, we all do but in the end a tv101 will give you a better image.

 

Canon did make some of the optics for the subaru telescope. they display some backups lenses at tradeshow often. can't find my pics.

 

 

[Pinbout]

also if I remember correctly, the circle of confusion on a camera lens isn't a tight of spec as the airy disc of a telescope optic.

Edited by Pinbout, 21 December 2015 - 02:42 PM.

[S.Boerner]

Planning on putting the camera and lens on a tripod?  The Earth is moving and the stars will quickly go out of round.  An old suggestion is that  slowest shutter speed you should use before streaks is 600/focal length so using a 20mm focal length lens would be 600/20 or 30 seconds.  At 800mm the exposure would be less that a second.  That would work for the Moon and brighter planets but not well for DSOs.

 

Fo DSOs you'll need a driven mount (possibly with autoguiding) to have the camera and lens exactly follow the sky.  With moderate focal lengths you can use something like a SkyTracker and accurate polar alignment.  For the 800mm you quote the mount will have to be something much better and designed to carry the load.   If you get a good mount you can piggyback your camera and lenses on a scope.

 

You should spend some time reading past threads in the Beginning and Intermediate Imaging Forum

[Michael Covington]

What Danny says is true of lenses of the 1970s.  Canon "L" lenses are a different thing entirely.  Even non-L Canon lenses of recent design are often quite good.

 

I do the bulk of my astrophotography with a Canon 300/4 lens, wide open.  I get nice crisp star images all the way to the corners of an APS-C-size sensor.  Whether the corners would be perfect with a full-frame sensor, I don't know, but you can always crop the picture.  Most telescopes don't fill a full-frame sensor, after all.

What you want to do is look at the MTF curve of the lens.  You want it high as far as possible away from the center, and you want the meridional and sagittal curves to stay close together.  Many newer Canon lenses (and other major brands such as Sigma and Zeiss) meet this requirement.

[Michael Covington]

The CGEM mount will drive the camera and lens nicely.  You can pair it with a small (50mm or even smaller) guidescope and an autoguider.   

 

[Tony Flanders]

I'm trying to justify buying a 2-3 thousand dollar telescope for astro photography when I already have a camera and lenses costing 2-3 times that much. I'm looking for opinions on which way you would go in my situation.

 

So far I have a Canon 5D3 and a variety of "L" lenses to choose from for astro photography. (I also have a CGEM) So let's say I decide to use my 400mm f/5.6 prime lens attached to a 2X tele-converter giving me effectively an 800mm lens for AP. All things being equal, I assume that the image size at the camera sensor would be identical if I used a hypothetical 800mm telescope and mounted the camera at its prime focus. Let's ignore aperture for the moment because I understand that the larger aperture available on this hypothetical 800mm scope would give me a brighter image and therefore shorter exposure potentially. Am I thinking right so far?

Yes, this is all precisely correct. But f/11 is indeed pretty slow for an astro lens. And 800 mm isn't a terribly long focal length. On a full-frame sensor it gives you a field of view of 24/800 x 36/800 radians = 1.7 x 2.6 degrees. That's a bit small to frame the Andromeda Galaxy, but a good deal too big (not optimally high magnification) for most other galaxies.

 

In general, telescopes tend to give you more for your money than camera lenses, because they don't have all the paraphernalia necessary for terrestrial photography, such as a built-in diaphragm to stop the lens down. But no telescope is as cheap as a lens that you already own and use for other purposes.

 

So I'd say it all depends what you're trying to photograph. By all means start out with the lenses you already own; what ever do you have to lose? Then, after a year or two, if you find yourself limited by your lens selection, you can consider buying a telescope to supplement them.

[NevadaTom]

Replying to all - yes I do have a Celestron A-VX CGEM. The Canon 5D3 and 400L lens with 2X tele mounts nicely on it. I glued together a stack of stainless steel fender washers to match up the camera bottom with the foot on the 400mm lens. The mount is nice and rigid on a longish Vixen rail. So far I have had only one suitable night for imaging since I put this setup together and shot a series of 38 frames of M31. Came out pretty nice actually, no color fringing on the stars, but nowhere near the detail and color in M31 I have seen others produce. Stars were round with no hint of trailing at all. Exposure was ISO 1600 at 30 seconds f/11.0. The skies around here aren't all that black so I hesitated to increase the exposure. I suspect a longer shutter will improve detail when I get the chance to head out into less light polluted skies.

 

The 2X teleconverter automatically loses 2 stops and reduces sharpness a bit. Couple of challenges with the Canon 5D3: The LCD live view screen does not articulate at all so it is difficult to aim precisely. I suppose I could use a mirror when the camera is aimed at an awkward angle - or get a right angle eyepiece. I had to hope the GOTO mount put me in the ballpark. I had manually pre-focused the lens prior to mounting the setup on the Celestron A-VX. That worked out well, and M31 wasn't too far off center. I do have a very nice Baader Planetarium finder scope that would help with aiming the camera, but I need a way to mount both of them side by side since I can't piggyback the finder on top of the Canon. Of course if I was using a real telescope, mounting the finder would not be a problem. I polar aligned the mount as best I could; but calibrating the GOTO was hard to do looking through the camera's eyepiece since there are no crosshairs to nail the exact center of the frame. Autoguiding will help when using much higher magnification.

[Kendahl]

Since you already have the lens, why not mount it and the camera on your CGEM and give it a try. Your biggest problem will be that you don't have the equipment for autoguiding. That will limit exposure time to a minute or two at best. Use the photographs you collect to teach yourself how to astronomical image processing.

 

I don't know how well the teleconverter will work with the lens. When I proposed doing the same with my L series 100-300 mm f/5.6 zoom, I was advised that the resulting images would be disappointing. I do know that I can tell the difference at 300 mm between my L zoom and a cheaper one.

 

The other problem with the teleconverter is that your focal ratio will become f/11.2 which will require four times as long an exposure or four times higher ISO. Either one leaves your with more noise to subtract.

[Jon Isaacs]

As others have said, you have the lenses, that's the way to start.

 

My only recommendation:  Image at F/5.6 rather than F/11.2.  You probably don't really gain image size at F/11.2 because the image is not sufficiently sharp nor the tracking sufficiently precise to take advantage of the larger image scale.  You'll get much faster exposures, you can probably drop the ASA to reduce the noise..  When you get everything sorted out, maybe the longer focal length might be useful...

 

Danny's comment about full aperture sharpness, telescopes have their greatest resolution wide open, most camera lenses are sharper when stopped down.  But camera lenses typically are designed to provide flat fields without much loss of resolution across the field.  Telescopes operate very close to the theoretical limit but there is a significant drop off as one moves away from the center unless a field flattener is used or the design is specifically meant for astrophotography.

 

Jon 

[dmilone]

Tom,

You mentioned that you had trouble looking at the camera's LCD screen. Well the camera's optical view finder is even worse. It makes a lousy eyepiece.

I used to use the Canon EOS Utility to control the camera remotely with my laptop. That way you can pipe the camera's live view video feed to the laptop screen and you don't have to lie down on the ground or get into awkward positions to look through the camera's LCD screen. You have full control of most of the camera's settings, you can zoom in on a bright star for focusing, and you can set it to take as many exposures as you need in sequence. Set it and forget it. Also, the images are saved directly to your hard drive, not to the CF card.

[NevadaTom]

Yes Sputnik, I do have backyardeos to remote the camera functions to my laptop. I've tried it indoors with my Canon 5D3 and it works well - even has crosshairs in the center for finding. Definitely the way to go as soon as the weather moderates enough to sit outside and not freeze my fingers while using the computer. ;-)  For Nikon users they also make backyardnikon.

[Vondragonnoggin]

If you already have the lenses, definitely make use of them. AVX + Nice lenses seems like a great way to go if you already have the lenses.

 

Certain lenses good for viewing through too with the right eyepiece.

 

My comet hunter - PVS-7 gen 3 night vision goggle into a Tamron 300mm F/2.8 w/modified 2" telescope back to use 2" filters. Great widefield views and great live Ha nebula hunter too.

 

Either can also be used with an intensifier and relay lens to my Nikon DSLR for much shorter exposures if I want.

 

I got a Tokina this size too w/ F mount for my Nikon. I know these are not like the Canon quality, but they do have ED elements and provide good views both visually and photographically.

 

 

 

[Ed Wiley]

I use 50 year-old Zuiko prime lenses on my old Canon 350 (modified). I stop them down 2 stops. I am not a critical imager, but I do note that the stars are a bit bloated as the lenses are not apo lenses. Doesn't bother me,but I am not aiming for a spot in S&T. Try your lens on a tripod and 30 seconds, you will be surprised at what you can get.

 

Ed

[chummee]

Sorry for replying to an old post, but I had the same question and thought my reply might be helpful to other beginners struggling between buying a telescope versus a camera lens. 

 

Everything being equal, a telescope has an advantage over a camera lens for astrophotography because 1) a telescope allows you to place a guidescope on top of it, which is essential for exposures over 1 minute long and 2) a telescope allows you to put a filter or a filter wheel in the optical path much more easily. 

 

But if you have a fast prime lens, the advantages of a telescope are diminished because you can do away with a guidescope since the exposure time can be greatly limited. Even if the prime lens is slower, a side-by-side mounting system can be used to add a guidescope to the set up to allow for longer exposure. Also, a field flattener is not necessary for camera lenses since it's already built in as part of the lens. 

 

The drawback of camera lenses is that their lens collar often only has one hole for the screw, making it easy for the lens to wiggle while mounted on the dovetail. But I suppose a simple 3D printed part can easily fix the issue. 

 

In sum, if you have a decent prime telephoto lens, I'd recommend using it to try out astrophotography first. If there is a strong need to use filters, such as light pollution filter or narrow band filter, then buying a telescope is a good option. Obviously, if you also would like to observe through an eyepiece in addition to taking photos, buying a telescope is a must. 

 

The above is coming from a person who has tried out astrophotography for only a few months. I do not claim to be an expert, but I have just gone through this thought process not too long ago. Hope this is helpful to other beginners. 

[aeajr]

This is really an imaging discussion and, as it says in the title of the forum:

 

Beginners Forum (No astrophotography here - please read the forum description)

 

I am moving this to imaging.

[chummee]

In case of interest, I tested my Sigma 150-600mm on the Swan Nebula. The image is generally soft and the stars have a weird cone shape. I wouldn't recommend this lens for serious astrophotography. 

 

 

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