Reflecting Telescope as the Main

So now let us consider reflecting telescopes, which are telescopes with a mirror as the main light-collecting element. These come in lots of different flavours such as Newtonian, Schmidt-Cassegrain (S.C.), and Ritchey-Chretien (R.C.) to name just three. Since the main collecting and focusing element is a mirror, the big advantage reflecting systems have over refractors is the much-reduced chromatic aberration (C.A.), and this is very important for imagers. There may well be chromatic aberration "downwind" of the mirror if you use glass elements (e.g. focal reducers or eyepieces), but these are usually very well designed and the C.A. is usually very slight if it is present at all. The Newtonian is a rather bulky instrument and was the most popular reflector, more compact, reflectors are of the S.C. and R.C. design which "fold" the optical path leading to a shorter instrument than a typical Newtonian. I think it can be stated without contradiction that it is the commercial competition between the big two manufacturers, Celestron and Meade, that has led to the possibility of providing amateurs with extremely high quality instruments at what must be considered very reasonable prices; I would go as far as to say "cheap" given the precision engineering involved in creating these instruments both in the optics and the drive mechanics. This commercial competition also leads to innovative and improved design that also greatly benefits the customers. In considering reflectors I will concentrate mainly on the Schmidt-Cassegrain, as this is the amateur scope of choice, but please do consider other designs as they may better suit your needs. For example, a well-designed R.C. will give a better edge-to-edge quality of viewing field image than a typical consumer S.C. and so it should, as it will be a lot more expensive!

Considering S.C. design reflectors we see there are computer-controlled "goto" instruments, those on Altazimuth mounts, those on Equatorial mounts, and they also come in a variety of mirror diameters from six inches to sixteen inches, the latter not falling into the portable category! I would say that if portability is a requirement you really shouldn't consider much beyond an eight-inch diameter mirror. Having said that, for the first two years of owning my eleven-inch S.C. scope I regularly used to carry it in and out of doors. However, as the outcome of this exercise (in both meanings of the word) was a double hernia and two hospital operations, I would rather you didn't go down the same route as me in this instance! I strongly advise that an 11" or greater reflector is mounted permanently at your observing site.

I purchased the Celestron Nexstar GPS 11, which is an S.C. design with an eleven-inch diameter mirror and GPS (Global Positioning System). It has a "goto" capability, a large database of objects on its in-built computer, and it sits on an Altazimuth mount (Equatorial mount options are available, but remember, initially I was only interested in observing and Altazimuth mounts are best for this). For visual observing, the Altazimuth mount gives the most comfortable viewing position, and also the fork arms and flat base lead to a rock-steady structure. When I started imaging with this beautiful scope I used the Altazimuth mounting it came with, but I soon discovered that this was not the ideal type of mounting for CCD imaging. At this point it makes sense to digress into a short discussion of telescope mounts.

The Altazimuth mount http://www.celestron.com/c2/category.php?CatID=9 is a very sturdy mount and ideal for visually observing objects as the eyepiece will be presented at a convenient height and angle for most objects you will observe. It must be a good mount; all the really big professional telescopes use this type of mount! However, when it comes to CCD imaging, this sturdy mount also gives us a major problem. If you look at the geometry of the mount it can of course pinpoint any object in the sky in an x-y kind of way, in that it can rotate on its base (rotational axis straight upwards) and move up and down to lock onto the object you want to view. But the Earth spins on its axis, and thus the stars appear to rotate as well, with the centre of rotation being somewhere near the Pole Star (Polaris) for us viewers in the Northern hemisphere, not straight up overhead. So what? Well our Altazimuth x-y coordinate system scope cannot rotate around the same axis as the stars, and this means that over a period of time, although your star (or other object) is still slap bang in the middle of your field of view and you can see it very well, all the other stars around it have rotated a bit, the rotation distance being greater as you move away from the centre of the field of view. This rotation is called "field rotation" it is the rotation of your field of view over time due to the Altazimuth mounting's inability to follow the rotation about the polar axis. So what again? Well, this field rotation doesn't matter at all when visually observing, that's why the Altazimuth mount is a great mount for observers. But if you are imaging, and if your exposure times are longer than say 30 seconds, then your final image captured by the CCD camera will show the characteristic trailing stars that accompany field rotation. So, one way around the problem is to take short exposures. This is fine provided you have a bright enough object that will allow you this luxury; normally you won't be able to take this way out. The reason I could take this option, for a short time at least, is that I had a "bolt-on" to the Celestron S.C. telescope called a Hyperstar lens. This lens takes the place of the secondary mirror in the S.C. design and turns the S.C. telescope into another design called a Schmidt camera. Technically the Hyperstar lens is a times one corrector lens assembly giving a flatter view in the imaging plane than would occur without the lenses. It also shortens the focal length of the

S.C. telescope and consequently reduces the f# considerably since f# is defined as the focal length of the telescope divided by the mirror diameter. Now, if you are into "ordinary" photography you will know that low f# lenses are called "fast" because they allow you to take pictures with a shorter exposure time than "slow" (large f#) lenses. The same is true to first order (if we want to get technical there is a difference between point objects like stars and extended objects like nebulae) in astrophotography. So my Hyperstar lens allows me to take much shorter exposures than using the telescope in its normal f#10 mode of operation, so short in fact that I can reduce the field rotation effects considerably - but only for relatively bright objects! Also, although the field rotation is reduced, it is not removed. When you start improving your imaging techniques and you become pickier you will find in fact that although small, the field rotation effects are not acceptable, even for exposure times of just a few seconds. You won't get "perfect" round stars - and perfect round stars are one of the main criteria we strive for in good astronomical images. So where does that leave us? Well the sad fact is that if you want to turn out good astronomical images you simply must have an Equatorial mount http://www.celestron.com/c2/category.php?CatID=15. There is no simple way around this problem. But wait a minute; didn't I just say a while back that all the big professional scopes were Altazimuth mounted? And aren't they used almost exclusively for photography? The answer is yes to both questions, and the no-expense spared big professional telescopes have no-expense spared solutions to the problem including very sophisticated software and cameras that can rotate to compensate the field rotation. Although such luxuries are available on the amateur market, the plain fact is that the Equatorial mounting solution is the simplest and most cost-effective solution to the field rotation problem.

If, like me, you have an Altazimuth mounted scope, you can now mount your scope on a wedge and set up the wedge so that the new rotational axis for the base points towards the same rotational axis as the stars. This can work fine, it certainly does for me, but there's no getting away from the fact that the whole system is far less sturdy and vibration-proof than the original Altazimuth mounting. Why should this be? You have cantilevered your scope over at some angle dependent upon your latitude, and it wasn't primarily designed to be as stable in this configuration. Far better that you buy an equatorially mounted scope in the first place if your ultimate intention is imaging and be done with all these annoying little problems!

Returning to the S.C. telescope itself. Focusing is usually afforded by moving the main primary mirror, and the secondary mirror must be accurately aligned (collimated) to the primary to get the best results from your scope. Remember that the optically more robust refractor keeps its collimation rather well - the S.C. doesn't. This once again does not actually matter, as it becomes a matter of routine to occasionally check your scope collimation and make the necessary adjustments to the secondary mirror as required. For the definitive account on collimating your S.C. telescope please visit: http://legault.club.fr/collim.html this says everything you need to know about collimating your S.C. Adjusting three Philips screws fitted to the secondary mirror affords collimation of the secondary mirror. Trying to follow the CCD image on a monitor, whilst adjusting screws using a Philips screwdriver, which of course is located precariously close to the precious corrector plate, simply isn't a fun experience. I very quickly invested in a set of Bob's Knobs http://www.bobsknobs.com/ which replace the Philips screws with a set of knurled knobs that can be finger tightened for collimation. Bob's Knobs are very good value for money, and basically an indispensable item if you want your reflector to be in tiptop collimation condition for crystal clear imaging.

So how far have we got in putting our imaging system together? We have either an equatorially mounted apochromatic refractor with an objective lens diameter of 90mm or more, or we have an S.C. reflector with a mirror diameter greater than six inches, on an Equatorial mount. I shall not describe the different Equatorial mounts available, they all do the job required to varying degrees of accuracy directly proportional to cost, and properly set up they will remove the field rotation problem. What they won't remove, and what will be apparent in the cheaper mounts is periodic error. Periodic error is due to the gears used to move the telescope in R.A. and Dec. not being absolutely precise. Consider the manufacturing difficulties here, precision gearing for very precise tracking and control, coupled with mass-production and a price affordable to the average income earner - we're looking at several mutually exclusive events here. Personally I have not found the periodic error of my Nexstar 11 GPS to be any problem at all in my imaging, something that I find amazing in a mass-produced scope and give full credit to Celestron's engineering department. Do realise that unless your periodic error is VERY bad, the autoguider will be able to take this error out. Periodic error should NOT be this bad in any good quality mount or complete system such as the Nexstar 11 GPS.

To summarise: As a beginner you will benefit from having the biggest aperture, smallest f-number system you can afford (or carry). This will be a "fast" system, and as such you can keep the sub-exposure (individual exposure) times down, which makes the autoguiding process much simpler. A smaller f-number also means a shorter focal length for a given aperture, and this will give you a wider field of view (than a large f-number system), which makes finding and framing your chosen object a lot simpler too. Large aperture, small f-number systems are a win-win in astronomical imaging; this is why I chose the Hyperstar option with the 11" diameter Nexstar 11 GPS reflector. This combination gives you a reasonable aperture with an incredibly fast f#1.85 imaging capability.

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