Drake Ward said:
I am in the process of purchasing my first telescope. I have been told that a reflector is the way to go. Biggest bang for the buck! I have a lot of beginner questions I would like answered and thought I would try asking here. I have been involved in photography for over 25 years and would like to try astro-photography. Why are some newtonians "short tubes" which seem to also be a larger aperature. What is it that actually determines the aperature size? I have been looking at a Meade 10 inch f4 Newtonian on a Meade mount. In Canada this is called a LXD75 SN-10AT. Is this a good way to go or will I be disappointed and quickly wish I had purchased something else. I appreciate any help I can get.
Thank you
Hi:
I am not much of a astrophotographer but I do know the basics and have a lot of experience with telescopes.
- Short tube Newtonians: This term is most often used for smaller Newtonians that use a fast spherical mirror and a barlow-like corrector to lengthen the focal ratio. Not a good design.
- For astrophotography, the most important equipment is not the camera, it's not the telescope, it's the mount. If the mount is not stable, if it vibrates and/or does not track accurately, it really doesn't matter how good the scope is or the camera. In a world where exposures last minutes and the target is moving at the rate of 1/4 degree per minute but resolutions are measured in terms of a few arc-seconds, a solid mount that precisely tracks is most important.
- Astronomers think in terms of focal length and objective diameter which we call aperture. Focal ratios are fixed so an F/4 is an F/4.
- Regarding the LXD-75... The problem here is that the scope is too big for the mount. People do some nice photography with mounts like the LXD-75 and the Celestron CG-5 ASGT but these are really "entry level" mounts they use much smaller scopes, typically an 80mm apochromatic refractor.
Such a combination is much more forgiving of tracking errors because of the shorter focal length of the scope as well as the size and weight, a good quality 80mm F/7 refractor might weigh 6 lbs, the SN-10 probably weighs more than 30 lbs.
A couple of more issues to consider. Reflectors typically need to have their optics aligned, collimated. This is not so easy if you are not familiar with the process. They also typically are sensitive to temperature, to get the good sharp images, they need to cool down, that takes a while.
A good site, is Jerry Lodriguss's astropix.com He has a lot of information, some nice photos as well as what amounts to a long tutorial:
http://astropix.com/HTML/I_ASTROP/TOC_AP.HTM
It may sound like I am not a big fan of reflectors but the truth is that I love Newtonians and I love refractors. I am aware each has a place, a role. In the Astrophotography world, apo refractors make imaging a lot easier.
Another site worth looking at if you want to get some ideas of what is available and what can be done is the www.astro-physics.com Astro-physics is Astromarter Roland Christen's company that has been at the forefront of astrophotography for 30 years. If you want a mount that can handle a 100 lb telescope and do it in the colds of antarctic, a scope that is about as perfect as it can be, A-P is the one.
Jon Isaacs
