Camera lens vs Scope?

[theboyk]

Hello.

I'm looking to get into starting out with some astrophotography and I'm trying to decide between a dedicated scope/camera (something like the Celestron C6-NGT w/ additional Celestron CCD camera system) vs using my current rig and just getting a computerized equatorial mount to attach my camera/lens to.

My first question — how does one compare the "magnification" of a camera lens vs that of a scope? For example, my camera lens if a 500mm lens w/ 1.4x teleconverter. How would this compare to a scope?

Second question — is a 500mm lens + 1.4x teleconverter decent enough to astrophotography (attached to a Canon 30D, mirror lockup, cable release)?

Any advice would be much appreciated!

Thanks,
Kristin.

[Nighthound]

I'm assuming you're not intending on imaging the Moon and planets only from your comment about a computerized mount. First and foremost I would think about a mount that is capable of good results at such focal lengths. Without one it won't matter what you place on top of it.

To address your lens/scope comparison question depends a great deal on the optical quality of each instrument. Is your 500 a Canon "L" f/4? For discussion sake I'll use it to expand on a few things. The 500L would hold its own very well against Achromat refractors and even some of the ED doublet refractors of the same focal length. It would have an advantage in most cases with its faster focal ratio. But for deep-sky long exposure work you would have to be willing to accept some field curvature(distortion that both refractors and lenses have, visible by distorted stars at edges and corners of images) from the 500 lens since correcting the curvature wouldn't be possible. A refractor or Newtonian(called coma in reflectors) can have this corrected by placing a field flattener/reducer between the camera and the scope. I'm not aware of any that will work for a lens. I use the 1.4X TC for daytime use with my 500L with good results but I don't think I would use it for astro work since some quality would be lost and focal ratio would be increased as well.

One other thing about using a lens is the diffraction spikes you'll see on bright stars. Some people like them some don't, just thought I'd mention it. You can see them here:
http://photography-on-the.net/forum/showthread.php?t=666684&highlight=antares

Your 30D will do very well. I use a non-modified 20D and am very satisfied with the results. Once you get a mount that can get you the length of exposures you want you'll need to fine tune alignment and set up skills as well as learn how to best process the exposures you get. There's much to learn so take it slow and continue to do you homework before jumping in too deep. There are plenty of knowledgeable people here that can help so don't be afraid to post any questions you may have.

[chris.bailey]

steves outline of the pros and cons is first class. Only thing I would add is that focussing a camera lens on deep sky objects is no mean feet (not easy peasy with a scope but easier on decent scopes with well behaved two speed focussers).

I started with a 70-200f2.8L on a 10D as my astro rig and very capable it was too. 200mm is a very manageable focal length to begin with and works well with cheaper tracking mounts.

One option is the Astrotrac (google it). Not cheap but works very well with medium lenses and I saw someone recently at a star party get the best pictures of the night with one (he had a Nikon 200mm prime). much lower learning curve than a German Equatorial.

[theboyk]

Thanks for the great responses!

I'm assuming you're not intending on imaging the Moon and planets only from your comment about a computerized mount.

Actually, I'd be quite interesting in imaging the planets, but I assumed I would still need a mount that could track with the rotation for that? I know with the moon I don't need this (the moon is the only thing I've played around with so far — and got me really excited for more!), but I thought I would for planets?

To address your lens/scope comparison question depends a great deal on the optical quality of each instrument. Is your 500 a Canon "L" f/4?

Well, it's not a 500 f/4. It's an older, non-L —*it's actually an f/6.3. It works great for my wildlife work (not ideal, but does the job), but I have a feeling, from your post, it won't really cut it for astro? Considering this, would I be better off purchasing an actual scope (something like the Celestron C6-NGT?) and then attaching my camera to the scope? Is this even possible? Basically, I'm just looking for a way to "get into" astrophotography. I know this is something I'm really going to enjoy, but I don't need to start imaging deep space or anything from the start — I'd much rather start slow and spend the time learning things (that's half the fun, for me anyway).

And I'm still curious in regards to magnification of a lens vs scope. What I mean is, when I buy a 500mm lens, I understand what that means in regards to how "close" I can get to an object (say, vs a 50mm lens), but with the scopes, I'm unsure how this is determined? How do you compare the power of a lens to "get close" to something vs that of a scope? Is there an equivalent?

Again, your advice and information if much appreciated!

Thanks,
Kristin.

[Nighthound]

A mount that can track in sidereal is needed for long exposure and would be recommended for planetary as well. While not mandatory for Lunar it sure makes the process easier not having to recenter the Moon over and over.

The telescope becomes the lens once attached to the camera. The focal length will be that of the telescope, so it's the same principle as a lens in that regard.

Yes, you can attach a DSLR directly to the focuser of the Celestron C6-NGT but you will likely need to replace the stock focuser in order to achieve focus. The stock focuser on most Newtonians are too high profile which places the camera sensor too far back from the focal plane. A low profile focuser like the Moonlight would be my suggestion. If I were going to shoot planetary I would probably go with a Schmidt-Cassegrain(9.25 to 11'' range) on a German Equatorial mount. A German Equatorial would allow you to switch your optical tube to a refractor, Newtonian or whatever for other focal length options. On a side note, the longer focal length will also be nice for planetary nebula and globular star clusters.

A refractor telescope is a great imaging scope for planets as well but a color-corrected(APO) refractor with the needed focal length will come at a hefty price. Refractors have the edge for contrast due to fixed lenses, baffles and no secondary mirror obstruction at the front end of the tube like the reflectors have.

Unfortunately there isn't one telescope that will best fit your desire to image both planetary and deep sky. The focal length that would be best for planetary would make it very necessary to have a high quality mount when switching to photograph deep sky via with long exposure. For planets I would get a scope from 1500 to 3000mm in focal length. This would give you the scale needed and to image I would use a modified web cam, stacking large numbers of exposures. The best I've seen are done this way. The focal ratio doesn't need to be f/4-6 for planetary like I had mentioned for long exposure work.

If you're not sure just yet how far into this you will get then by all means start out slow and don't go spend crazy. But try to think in terms of down the road a bit as far as your mount goes, i.e. a German Equatorial and its flexibility for adding different focal length instruments. The objects out there range in scale greatly and to get that variety it will take different focal lengths to fit those needs.

Personally I have 800mm, 400mm and roughly 300mm focal lengths covered. My 100-400 lens gets me widefield if I want it. At one time I had 1400-2500mm focal length scopes and while the views were great, long exposure imaging at those focal lengths in a huge challenge and requires a mount much more expensive mount than I can swing.

So you'll need to decide where to start and I realize there's a lot to decide upon. I'm trying my best not to confuse you but rather give you some things to consider in your decision making. I remember very well my first scope purchase, it was tough to decide and I wasn't even considering photography at that time. We're here to help if you need it.

[PM01]

Kristin,

The newer 500f/4L EF IS lenses are good enough for astro work. They're not quite as corrected in color or in perfect pinpoint stars as say a Takahashi FSQ106 (probably and arguably THE 500mm range optic to have for astro). Remember that the commercially made lenses are high speed polished, not slow pitch polished like some of the high end refractors out there.

Some have reported distorted stars on the field when using Canon or Nikon lenses. This is very true since they're not as critically aligned or as critically polished as a high end refractor from Astro Physics, Takahashi, Telescope Engineering Company, etc. If Canon polished their lenses the same way as the telescope companies mentioned, they would be much more expensive and there would be considerably less of them out in circulation due to the manufacturing time. Canon can hog out a lens element in a few minutes on a high speed grinder / polisher. It's then ready for the coaters and assembly. Astro Physics refractors (and other high end refractors) can take as much as several hours to DAYS to get the figure critically correct. And this is with multi million dollar machines, reference test plates, compensation and measuring of different glass melt batches, etc.

The main difference between the refractor and reflector is that the refractor (high end) will definitely have more contrast. There is less scatter when light goes through the element and if the element is superbly polished. The reflector on the other hand has a metal layer, whether it be aluminum or silver or even a berol coating. Problem with the metal layer is that the metal itself, when greatly magnified, has a columnar structure to them, which is naturally good at promoting scatter and contrast loss.

You asked about magnification. As a rule of thumb, 50mm = 1x. So a 500mm optic, without adding any 1.3 or 1.6x crop factors, will = 10x. That's more than plenty if you're shooting a fairly wide field.

But if you're starting out, the 500mm canon lens is pretty darn decent for astro. It's not perfect and Canon admits that the lenses that they make are more for the forgiving terrestrial application than rendering point sources critically - astro usage.

While you're at it, and if you feel up to it, here are some articles by Roland Christen, the head honcho of Astro Physics, probably one of THE best names in telescopes.

http://geogdata.csun.edu/~voltaire/roland/

[cspratt]

You could also look at the small scopes put out by Stellarvue. They do a decent job even if their cheaper ones aren't triplets.