TP: How do I get into astrophotography?

Hi,

I'm considering buying a long reflex lens for astrophotography and see that there's a bunch of Celestron 127EQ (1000mm f/8 newton) available on ebay for cheap, about AUD250 with shipping. Can't say I want to spend much more than this but I know I'll likely need a powered equatorial mount for long exposures.

So, some questions. Do these things have T mounts? Are the equatorial mounts compatible with a motor drive so that I can do longer exposures? What is the image circle size?

Has anyone here much experience with using SLRs on a telescope and can provide any advice? Is there something critical that this device is missing that I'll need? Any alternate recommendations?

(edit) Is it worth the extra dollars for a schmidt-cassegrain or -maksutov? Any recommendations in that department?

Hope this is not too much of an open-ended question... Any and all astrophotog advice welcome!

Edited by brettania - 08 July 2007 at 08:15

You won't be doing much astrophotography with that telescope - perhaps with a point and shoot you might get a shot of the moon. Most Newtonians sold in shops are optimised for visual use, and it may be difficult getting an SLR to reach focus without significant modifications.

That particular telescope is just a good beginner's telescope which, with better eyepieces and a better finder can be very handy checking out the 600 deep sky objects in say Bright Star Atlas by Tirion, and would give decent views of Saturn, Jupiter and the Moon - providing you collimate it correctly. It's probably not the best way to start recreational astronomy though, a pair of binoculars and a good book on binocular astronomy is usually better.

A KM5D or 7D can easily be connected to a Schmidt Cassegrain or good refractor on a good driven equatorial mount with adapters and T mount. The only problem is that they, like most other DSLRs, have an IR filter which stops one from getting the good nebula colours one used to easily get from film - lots of Canon owners take their IR filters off the chip, but their cameras won't autofocus properly afterwards. The KM5D and 7D have another problem in that heat leaks into the CCD.

The other thing one can do is mount the camera piggybacked onto the driven telescope and take exposures of several minutes with standard lenses. The following links are to the only two photos I've taken this way with the 5D and and a 50mm f/1.7 lens, which I foolishly forgot to stop down to f/2.8, hence the distorted stars at the edges. You can see the heat leakage at the top left hand side (N.B. These photos would be considered very amateurish if I posted them at an astrophotography website - they do very much better than this).
Regards,
Renato

Eta Carina Nebula
Eta Carina

Coal Sack in Southern Cross
Coal Sack

Edited by Renato - 05 July 2007 at 15:08

Ronald wrote: It's just as Renato says, with your normal lenses you can do astrophotography too. If you really want to spend money, buy a sturdy motorized equatorial mount. With a lot of luck you might find one for that budget, but you will need a lot of luck, good 2nd hand mounts cost 2x, 3x, 10x more then your budget. The only negative point I can think of in buying a mount without a telescope will be the aligning of the mount.

You are quite right Ronald. Aligning the mount is really tough work, unless you have an alignment scope inside the equatorial head - then it's relatively simple if you live in the northern hemisphere, but still a pain in the backside if you live in the southern hemisphere where I live. Sometimes I get the alignment right in a few minutes, other times it takes 20 minutes and a very sore neck.

But I did forget to mention one thing. If one is in a rural area away from city lights, one can do some basic astrophotography just using the camera and a tripod. Use the 2sec self timer, set a high ISO, with manual focus set the lens to infinity, and use manual exposure with a large aperture and the shutter speed in accord with the following table. One can get some pretty pictures if these shutter speeds aren't exceeded, otherwise the star trailing spoils them.

Effective ..................Declination of Stars
Focal Length(mm)
...............................0s...........50s.........75s

28............................25...........38..........95 max. seconds of exposure

35............................20............31.........76
50............................14...........21..........53
85............................8.1..........13..........31
135...........................5.1..........7.9.........20
200...........................3.4..........5.3.........13
300...........................2.3..........3.6.........8.9
400...........................1.4..........2.1.........5.3

Of course, the above table (from Astrophotography by B.Gordon) was written for film cameras, but because of the crop factor, one can use the figure in the next row (e.g. if you have a 135mm lens attached to a 5D, use the exposure time for a 200mm lens. Declination equates to latitude in the sky. The Pole is at 90s, and the declination directly overhead equals one's latitude.
Renato





Edited by Renato - 05 July 2007 at 17:23

And further.... they quote very low resolution numbers:
1000/10

Unless I'm mistaken, 50 lines/mm is about 1/3 of the A100 pixel pitch. In which case I may as well take a photo at 300mm and just crop the middle 1/9 out.

I have done plenty of astrophotography and I can tell you that it is one of the hardest (and potentially most expensive) forms of photography one can do. And, unlike most other forms of photography, the more money you spend on equipment, the better the results are likely to be.
I wrote a primer here for getting started in astrophotography. It is somewhat dated and was geared for film but the principles still apply. For what it is worth, here are some astrophotos I have taken with my 7D...

Vidgamer wrote: At the risk of hijacking polyglot's thread, I've been thinking about trying astrophotography again, now that I have a new camera. I already have an ETX 60 (which is pretty cheap, by the way, so you might want to add it to the search), so I wanted to attach the A100 and give it a try. However, when I tried astrophotography a few years ago with a small P&S, it was difficult to collect enough light to get really good results, but it's worth noting that I had to mount on top of an eyepiece, so I'm sure that had some loss of quality. If nothing else, it gave me a challenge. :-) (More interesting than astronomy or photography alone -- try to combine the two!) Plus, I often didn't seem to have enough zoom (even though I could add the camera's zoom to the result from the eyepiece). Using an SLR with this 'scope is like attaching a 350mm lens, which I would think would be good enough. But I know that this is just a beginner 'scope, so it's going to have limitations. Is it worth fooling with? So, Renato, how are you getting those great photos with just a 50mm lens riding piggyback? I wouldn't have thought that would be enough zoom (not narrow of enough field of view)? I found the Minolta t-mount adapter at a local store, but I don't think they have the right mount for the ETX-60, so I guess I'll have to mail-order that part.... But the ETX-60 has a built-in mirror to flip between the eyepiece and the camera mounted at the end, so I like to think it'll be easier this time (compared to how I had to do it before!). In theory.

Yes, those photos were taken with a 50mm lens - but with the crop factor it is an effective 75mm lens. Your ETX 60 is a 60mm f/5.8 scope (i.e. 350mm f/5.8 lens), but with the crop factor becomes a 525mm f/8.75 lens which, if I mounted it on my driven equatorial mount, I could get some half decent shots of deep sky objects out of it. From memory, the ETX-60 is an achromat refractor, so I'd be expecting lots of blue rings around the brighter stars though. But you'd really need a good, solid driven equatorial mount to get half decent results (or a really good fork mount, either with a wedge or de-rotating software).

It might be fun using the scope with an adapter for taking daylight shots, especially if you attach it to a high quality Barlow or use a Teleconverter.
The one thing to be wary of that often those adapters are only held in place by the tiny thumbscrew that holds the eyepiece in. So that's a mighty expensive camera being held in place by one thumb screw against bare metal (though many of them are recessed nowadays). I always take the precaution of putting thin adhesive tape around any such adapter so that the screw digs into it. In this way, if the screw does come a bit loose, the camera won't come sliding out.

Basically though, when I used to be enthusiastic about astrophotography and used a film camera, my favourite focal lengths were,
1. 135mm prime lens - as it gave me pictures that corresponded to my Sky Atlas 2000, an example of which is here (shame about the sharpness and the grain - I was using Konica ISO3200)Open Clusters M6 and M7 in Scorpius

2. 500mm f/6.25 (an 80mm telescope) - the bigger deepsky objects seemed better framed in this than in 200mm and 300mm lenses, and little if any guiding was required on my mount.

3.   1280 f/6.3 (a 200mm SCT with focal reducer) - the speed was faster than without the focal reducer, guiding wasn't as tricky as with a 2000mm focal length, and thus there was less chance of the guiding being mucked up by a gust of wind.
Renato

Edited by Renato - 07 July 2007 at 20:51

You can mount a T-adapter onto the end of the ETX-60, so you don't necessarily have the eyepiece issue that you mention. You'd only need an eyepiece adapter if you wanted to stack it onto a barlow, I guess, and get more magnification? But I'm thinking that it might not be worth it, at least on the ETX-60. The light is going to be pretty feint, and it is fidgety getting a camera on the eyepiece, as you say. It's a shame, though, as I do have a barlow lens.

I didn't think about stacking a teleconverter in there, but that's a good idea!

I agree, it'd be fun to use the 'scope in the daylight, except that it's large enough that it's not terribly portable (although, compared to most telescopes, it is).