Scopes and Aperture

[Radar]

I want to pick some people's brains here.

Recently when I saw through some other people's scopes (a 12 inch LX 200 and a C14), I was very impressed with the views. In fact, seeing through these scopes has made me want to spend more money (which I don't have) in the coming year and try and buy one.

These scopes had three metres focal length (from memory). Even with that kind of focal length (magnification), the objects we were looking at were extremely bright. Compared to other scopes I have used, imaging through these should not be so difficult though, because of the light gathering abilities. So I'm thinking not so long exposures are needed.

If I was looking for a long focal length scope to do visual and film imaging through, what scope would you recommend and why?

And if I was looking to buy a long focal length telescope for imaging only what should I buy?

Thanks

Ray

[AstroTasmania]

Hi Radar

I too will be very interested to see what other people suggest. I have never been a SCT fan, at f/10, just too slow with a narrow field of view, certainly not a scope for wide field shots. Using film, as you do, is so much slower than digital, your exposures would not be short. Not many SCT's have good PE, essential for long exposures, also the SCT's have a very large central obstruction which reduces contrast and fine image detail.

There are better optical systems for photography, I would do a Google/Wikepedia search and find a comparison between each optical system. Also remember that a standard SCT on a twin fork mount has limited space through the forks.

Do a LOT of research and read reviews, not just manufacturers claims, here is a start:http://en.wikipedia.org/wiki/Catadioptric

Just my cents worth,

Clear skies...

[rogerg]

With long focal length comes many imaging problems directly caused by the long focal length. I'd suggest what you'd be better off with is large aperture with shorter focal length (F/4 - F/6), but that's where optics start getting really pricey.

The longer the focal length:
1) the more susceptable your setup is to atmospherics.
2) any guidescope must be longer also
3) the more expensive the CCD to be able to near 2 arc seconds/pixel imaging size
4) the more susceptable yoru setup is to mechanical irregularities such as PE and backlash
5) the more accurate your pointing accuracy must be
6) the mount must perform that much better overall

Optimal image scale for digital imaging tends to be around 2 arc seconds per pixel. In a 3m scope this means you need a CCD with large pixels ($$$) or a focal reducer to shorten the focal length. If you image at 1 arc second per pixel you're going to suffer from atmospherics deteriorating your image most nights. If you image at 3 arc seconds per pixel life will be a breeze.

With my LX (fl=3048 @ F/10) I always image with a F/6.3 reducer which in my setup takes the focal length down to about 2160mm (not F/6.3, more about F/7.5). That equates to about 1.043 arc seconds per pixel on my CCD, if I remember right. This means I only accasionally have perfectly sharp images, most of the time atmospherics blur things to varying degrees.

If I had a bigger chip I'd using binning to take that up to 2.086 by binning to 2x2, but my chip is so small that just makes the image unsatisfactoraly tiny (something like 450 x 300 pixels).

Using a large chip (or film) on a scope like mine reduced with a focal reducer usually results in unsatisfactory distortions and other bad effects such as vinyetting and comma, making for less than good images. This is something the people who buy STL11000's have been finding. Where as if you spend the $$ on a good F/4 or F/6 10" scope you'd have the shorter focal length straight off without a reducer and the field will be flat with no vinyetting. Comes down to quality of optics and what the optics are designed to work at optimally, how flat & big their field is etc. This is also one reason refractors are so popular for larger surface area CCD's - a F/6 110mm APO is probably a more workable solution than a F/10 or even F/6 12" SCT.

The advantage of long focal length is seeing fine detail when:
1) you have perfect atmospherics
2) you use adaptive optics of some variation
3) your mount and optics are premium

I did once do some film imaging through my LX @ F/10. Worked OK, I was reasonably happy with the results considering all.

If you are after narrow field high "magnification" imaging then a long focal length scope (SCT) might suit, but the job likely won't be easy. On the other hand if you don't specifically want that tiny detail you can only see when imaging at 1 arc sec/pixel under perfect viewing condtions, I'd stick with a shorter focal lenght and perhaps smaller aperture as a result (refractor).

My 2 c worth. And I'm a little scatter brained today, so sorry if it doesn't make sense - just ask for clarification.

Roger.

Edit: Btw, I asume you realise F/10 is F/10 no matter what the focal length. So if your current scope is F/6 and you buy a new scope that's twice the aperture but still F/6, your exposure times will be exactly the same but the FOV will be narrower. Getting that new bigger scope to F/4 to have faster exposures normally results in optical problems/distortions as mentioned above. So the gain is resolution/FOV changes, not exposure time.

[AstroTasmania]

Hi Radar,

Good points raised by Roger. What I would also suggest is to sit down and ask yourself this question. "What do I want to do most with this new scope?"

Photography with film or with CCD.
Even the fastest film is slow compared to CCD
Wide Field or Narrow field.
Fine planetary detail.
Planetary solar system, bright objects.
Deep sky-faint objects.
How good is the mount?
Average nights of good seeing per year.

All of the above!

Remember, there is no single scope that will do all of the above - at your pocket depth.

A Cassegrain/Newtonian offers long & short focal ratio's which combines fast wide field, slow & narrower field - but start at $5K upwards.

A real Ritchy-Cretian scope - at F/6-F/8, mid focal ratio, again very expensive.

I think that if you answer the questions above, it will start to determine where you will start looking. Also, some makes of SCT are better than others!

Clear skies...

[Radar]

Thanks for those great answers Shev an Roger.

I want to image smaller DSO's such as the crab, ring etc. At the moment, if I were to image these objects, they would appear quite small on the film. I want to be able to enlarge these kinds of objects so that when I shoot on film they are of a decent size, and detail in them can be resolved. This is basically my reason for tinkering with the idea of getting a longer focal length scope.

Hmm, so many angles to think of.

:hmm:

[AstroTasmania]

Hi Radar,

I think this might fit the bill, worth a look anyway.

http://www.pbase.com/snowlep/kuhn

Clear skies and the above...

[rogerg]

Ray,

What's the focal length of your Takahashi Epsilon E160 ?

I suspect that using your existing equipment would be ideal for such images if you had a CCD with an average pixel size.

I'd suggest your decision might come down to:
1) new camera
or
2) new telescope

You could achieve your goal by either of the above 2. I'd err towards option 1 seeing as you'd go that way eventually probably, and your existing setup (G-11, 8" SCT, Takahashi E160) sounds very good optically & mechanically.

Match the right CCD to your setup and you should be able to achieve quite nice objects of things like M1 and smaller.

Roger.

[Radar]

Nice link Shev.

Rog, the focal length of my scope is 530mm. I never really thought of using a ccd camera to shoot these smaller type objects. What kind of size do you think I could get?

Does the smaller imaging surface (the ccd chip) mean I need better guiding and polar alignment? I imagine it would.

Regards

Ray

[rogerg]

What you want to do is work out what pixel size suits your telescope(s) for imaging. I suspect something normal, around 9 x 9 microns will be suitable, but you should do the calculations and test it. This will tell you something about the camera that would best suit. Then you find cameras in the suitable price range and specifications from people like SBIG, Apogee, Finger Lakes, Meade, Orion, etc.

Ron Wodaski's calculator is useful for the task:
http://www.newastro.com/wodaski/pick_a_camera.htm

There are pages that tell you the actual calculations to do it yourself also - google will find them.

You are right, that your guiding, PE and other mount characteristics will have more demanded of them when you increase the resolution at which your imaging with a CCD.

You should be able to work out from the above if you're going to be able to achieve what you want with simply a new camera, or if you do want to change telescopes. 530mm for the Tak is still fairly short. But then, my Megrez 80 (480mm) is great for things like NGC 2070, M17, M8, etc. I haven't tried M1 but it's in that ballpark area of size I think isn't it? Besides, I guess you might be guiding through the Tak and imaging through the 8" which would provide a much longer focal length (probably too long).

[AstroTasmania]

Hi Radar

Well it looks like Roger has talked you out of using film, which was your original plan!

Get with the force.

Clear skies...

[Radar]

Hi Radar

Well it looks like Roger has talked you out of using film, which was your original plan!

lol, he does make a good point. :hmm:

Thanks for the replies guys. :thumbsupmate:

[AstroTasmania]

Hi Radar

Check this link to see what can be done with a DSLR and a good 130mm refractor. Is this possible with 35mm film?

http://www.californiastars.net/gallery/m042_20d_toa130.html

Clear skies...

[Radar]

Wow! Great image. A good shot of Orion is next on my hit list. I'm hoping for a trophy shot.

[Duncan]

Hi all,
Great shot.M42 will be the first i look at to when i get up and running.In the meantime i'll enjoy the wind and rain, not:ahh!:
Cheers,
Duncan

[Tenacious Del]

I can't really add anything to this thread, except that this thread was a good read. I learnt quite a bit. So thank you.

Sorry to hear about the weather Duncan.