CN FORUMS ARCHIVE Telescope Reviews: A discussion on Astronomical CCD cameras

I'm going to start off assuming a blank sheet of paper - almost any combo of mount/telescope can be made to image, but I'll discuss a system that will enable somebody to quickly come up the CCD imaging curve with the lowest (relatively) investment both in terms of money and effort.

IMHO the most crucial factor equipment factor to getting successfully started is a solid stable GEM mount. Note "stable" from an astrophotography point of view which places more demands on the mount than visual work. I specifically recommend a GEM, not because of anything against fork mounted Celestrons and Meades, but because the mount becomes the basis of a complete imaging *system*, not just a specific *telescope*. If you really get into imaging, you may find you'll start to accumulate different telescopes and cameras for different imaging purposes (e.g. widefield nebulae, deepsky galaxies, etc.). Just as an SLR body is the basis for a system of lenses and accessories, a good mount will be the basis of numerous telescopes/cameras that you can build into an overall astrophotography system.

In addition to the need for mechanical robustness, the drive system for the mount needs to have smooth periodic error. Smooth but not necessarily low, because you'll be guiding or autoguiding. An autoguider can cope with a large (relatively, in astro-photo terms, say 15-20 arc-sec) amount of periodic error if the error accumulates slowly and smoothly (no jumps!). However, if the mount moves erratically, the autoguider, or you if guiding manually, will not be able to correct quickly enough.

A perk, but not a prerequisite, is GOTO. There are only so many imaging hours in a session, and not having to sky-hop to find your target is a big productivity enhancement. But, this is NOT a necessity.

Assuming you're starting with nothing the next item is, of course, the telescope. You want a mechanically solid OTA to reduce the likelihood of flexure between the camera and the OTA, and flexure in the focuser itself. You want a relatively short (500mm and up but below 1000mm) telescope because the difficulty of imaging (polar alignment, tracking, flexure) goes up dramatically with focal length. For that reason alone, despite Celestron and Meade's advertising, I don't recommend an SCT (sorry SCT fans!) as a first platform for learning CCD imaging. Newts place demands on the mount because of the long polar moment of the OTA, and since most Newts are set up for visual imaging modifications often have to be made to bring the focal plane out far enough to reach focus with a CCD camera (either a low-profile focuser or modifying the OTA to bring the mirror forward in the tube). This kind of inexorably leads to a small APO refractor as a first CCD telescope. A quality APO will be heavily built (in the case of TMBs almost over-built) with a tight focuser and rock solid OTA. Because it's an APO, there will be relatively few aberrations to deal with (field curvature being the most common one, and most APO manufacturer offer field flatteners to get rid of that problem).

Lastly, you need some way to guide or autoguide. The easiest way to autoguide is to buy a self-guiding camera, either a Starlight Express or SBIG. I think SBIG's are superior solutions because they're part of an overall system including color filter wheels, adaptive optics, and software.

If you're going to manually guide, then you need either an off-axis guider, or a guidescope. At short focal lengths, a guidescope is by far the easiest way to go because it's easy to find a guide-star. Also, you can guide directly on the object you're imaging, like a comet, which you cannot do with an OAG. The usual rule of thumb is that your guidescope needs to be at least half the length of your imaging scope. The principle here is that you apply relatively high magnification so that any wandering of your guide star can be seen well before it will blur out on your imager. Personally, I can't recommend manual guiding. Half an hour peering through an eyepiece keeping a star centered on a reticle is a LONG time. The last straw for me was manually guiding a half hour film exposure, only to find out I had forgotten to trip the shutter! Pay yer bucks for a self-guiding camera and relax in comfort.

So, some recommendations based on personal experience or the recommendations of other astrophotographers:

- Losmandy GM8: very good price/performance astrophotography capable mount, $1500. Although listed for up to 30 pounds of load (load is for the stuff you put on, NOT including the counterweights), that's for visual work - 20 pounds is about the limit for astrophotography (which is still quite a lot if you have a small APO mounted). Comes with dual drives and is auto-guider ready right out of the box. Polar scope is optional but not necessary anyway (you'll be drift aligning the mount). Can be retrofit with GOTO for another $1600 ($3100 vs $2500 if you buy the GM8 with GOTO)

- Vixen GP Deluxe on HAL110: same class as the Losmandy GM8, $1200. Price seems better, but you need to add dual axis drives and controller for an addition of 2 x $110 + $210 = $430 additional, for a grand total of $1630. NOT autoguider ready, the controller needs to be hacked in order to be able to autoguide. Can be upgraded to GOTO for $1200, which also makes the mount autoguider capable.

The next step up from these two mounts, for not much more money, but a LOT more load capacity:

- Losmandy G11: probably the best price/performance astrophotography capable mount, $2100. Good for 40~50 pounds, depending on how long the OTA is (longer means lower load capacity because of the torque the OTA will put on the mount). People have overloaded the G11 with 14" SCTs and successfully imaged, but that's the exception not the rule. Again, autoguider ready right out of the (large) boxes. This was my starter mount. Can be retrofit with GOTO for another $1600 ($3700 vs $3200 if you buy the G11 with GOTO). A lot more performance than either the GM8 or the Vixen, for not much more money. This mount gives you headroom to grow if you decide you're really hooked on astrophotography. If not, it's a terrific visual mount AND it will handle a C11 or C14 for visual work.

A possibility that has yet to be proved out is the new Vixen Sphinx GOTO mount. Supposedly in the Vixen GPDX load range, autoguider ready, but no real-world astophotography reports yet.

I don't know anything about Tak mounts, but they're highly regarded (Charles?)

On to APO OTAs:

- Televue 85: 85mm/600mm f7, $1700 OTA only. I've played with lots of these and it's a miracle of self-restraint that I don't have one. Quality optics, smooth and solid 2" focuser. Good focal length for introductory imaging, speed's a little slow but CCDs are fast. For small CCDs field curvature isn't a problem, but for larger sensors a reducer/flattener is available for $265. Great match for the GM8, G11 is overkill but will certainly be stable!

- Televue 101: 101mm/540mm (!) f5.4, $3600 package. Tons of examples of outstanding imaging with this OTA on the web. Design has a flattener built in, so the field is dead flat, and it's reasonably fast @ f5.4. Fine with GM8 or G11. Sky and Telescope reviewed on awhile ago and their optical summary came down to: perfect.

- Takahashi FC60: 60mm/355mm f5.9, $899 OTA only. A scope not much bigger than a finder??? CCD cameras are inherently small FOV devices because the sensors are small, so a short focal length will give satisfying photographs of larger DSO objects like the Lagoon nebula. Takahashi will traditionally kill you dead with their nice but expensive add-ons: camera angle adjuster (rotate the camera without losing focus) for $180, reducer for $340.

- Takahashi Sky 90: 90mm/504mm f5.6, $1900 OTA only special at Anacortes right now. Field is curved and some people complain about the amount of color for an APO. Takes the same camera angle adjuster and flattener as the FC60.

- Takahashi FSQ106: 106mm/530mm f5, $3500 OTA only. Petzval design like the TV101, so a dead flat field. Built-in camera angle adjuster, specifically designed for astrophotgraphy and can image up to 6x7cm film (with some vignetting). Due to the proximity of the rear element to the drawtube, needs special ($$$) accessories for visual use. I almost bought one of these but instead opted for the really fast Epsilon 160 astrograph (160mm/530mm f3.3 but not recommended as it's no longer easily available and is ONLY a short-focal length astrograph - but terrific for that specialized need)

- Takahashi TOA130: Charles???

Also recommended: BORG and TMB APOs. The Borg line of telescopes is particularly flexible as the entire system is modular, enabling you to swap finders, tubes, objectives, etc. I'd also like to recommend the Astro-Physics line of refractors, but you can't get one... (I've been on the 155 waiting list for 3 years...)

Self-guiding CCD cameras:

- Starlight Express MX cameras: my first CCD camera was a HX516 (the HX doesn't self-guide). Great starter cameras: very low dark current (a characteristic of the entire line of Starlight Express cameras which use Sony chips for that reason) which made post-processing a breeze. The design of the MX cameras use the interline capabilities of the Sony video chips, meaning that every other row in the chip is used for autoguiding. This means that alternating rows of pixels are used for imaging or autoguiding. Uniquely, that means that MX cameras (exclusively) can autoguide directly on the object you're imaging. So, they can autoguide directly on objects that aren't moving at sidereal rates: comets. The downside is that since half the pixels are being used for autoguiding, you lose half the sensitivity of the camera.

- SBIG cameras: unique (patented) dual CCD self-guiding system. Since there's a separate small CCD for autoguiding, you don't lose sensitivity. SBIG was one of the pioneers of CCD imaging and it shows - their system is well thought out and incorporates the image acquisition/guiding software into the overall solution. Their dual CCD system allows you to add the adaptive optics AO7 unit, which relieves your mount of much of the burden of tracking (meaning that you can "stretch" your existing mount). Charles, I, Florent, all use SBIG cameras.

I personally think the most flexible reasonably priced OTA/mount combo is the GM8 + TV85 for $3200. CCD/35mm capable, can do planetary imaging with PowerMates, great widefield/planetary visual instrument.