Last updated on
4/11/
2006

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A Rich-Field Scope

As I mention on the Nice-To-Have Accessories page, there are some spectacular deep-sky objects (several star clusters and nebulae) that are too large to be really appreciated in a normal scope regardless of how wide-angle an eyepiece you use.  Also, when you are observing objects in constellations that are within the Milky Way, you are missing some incredible views of the Milky Way because of the narrow field of view of your telescope. You could mount an f/6.3 Focal Reducer on your rear cell, which converts an f/10 scope to an f/6.3 scope and provides you with a wider field of view. But it is very inconvenient to remove the items you have on your rear cell, add the focal reducer, and re-mount the rest of the items.  The majority of the objects you want to observe at any given time, require a narrow field of view so switching the focal reducer in and out becomes a real pain, and opens the possibility of damage to your diagonal and eyepieces since you’d be doing this at night in the dark.

Therefore many amateur observers mount a small rich-field (wide field-of-view) refractor on top of their main scope.  Such refractors are specifically designed to supplement your primary scope’s capabilities.  They have very short focal lengths to achieve a wide field of view, and tend to have 80mm-diameter (3”) objective lenses (to keep costs low).  They are therefore commonly called “Short-Tube 80s” or “ST-80s”.  With one of these scopes mounted on your SCT a wide field of view is available when you need it but your SCT retains the narrow field of view necessary for most deep-sky objects. Also, a rich-field scope makes a great finder scope when you need to star-hop to find an object (which, even with a GoTo scope, eventually you will do because many interesting objects aren’t in any GoTo scope’s database). 

These scopes include a ¼”-20 threaded plate at the bottom which is the standard thread for camera tripods, and are intended by their manufacturers to be mounted on a tripod for use.  However, SCT owners mount them on a camera adapter attached to a Dovetail Plate on the SCT itself.  The rich-field scope then tracks the stars because the SCT’s mount does, and of course it becomes a GoTo scope if the SCT you have is a GoTo scope.  It is important to note that you will need to add a 5 pound or so weight below your OTA to achieve dynamic (vertical) balance with any of the rich-field scopes discussed here - see below and the Balancing an SCT page. So keep in mind that the dovetail plate plus camera adapter, and 2D balance weight system are additional expenses if you don’t already have them.

The least expensive and thus very commonly used Short-Tube 80 is manufactured by Synta, the Chinese company which makes most of the refractors sold by Orion and Celestron, and whose scopes are re-badged by many vendors around the world.  It is a 400mm focal length f/5 scope that sells for about $200, a very reasonable price (and therefore it is commonly purchased by astrophotographers for use as a guide scope as well). In North America it is available in three versions which basically only differ in the color of the scope. 

You can get it in black as Celestron’s “80mm Wide View Spotting Scope” to match Celestron SCTs, from Oceanside Photo & Telescope; see below for a photo of the Celestron version.

The same scope is available in dark blue from ScopeStuff in the U.S., and also as the “Skywatcher 804” from Focus Scientific in Ottawa, Canada.  The Skywatcher 804 (shown in the photo on the left here) and the Celestron have a dual-clamp mount that has a ¼”-20 threaded plate at the bottom.  Meade SCT owners will appreciate that dark blue color <grin>.

Finally, this scope is sold by Orion (part #9386, shown here on the right) in white. One potential advantage of the Orion version is that occasionally Orion puts this scope on sale for less than $200. The Orion does differ from the Celestron and Skywatcher versions in that its mount is a single plate fastened to the OTA rather than a dual clamp-ring mount. (This isn’t very important because ultimately you will probably want to use a different mount than any of these versions provide, as I explain below.)

Another 80mm rich-field scope is the 80mm Right Angle SuperFinder sold by Lumicon (shown here on the left). Specifications are not given on their web site but Lumicon tells me it is a multicoated f/3.8 scope with an aluminum OTA that weighs 2.6 pounds. It appears to sell for $220 (the Lumicon web site is a bit fuzzy as to what is provided with which version of this scope). Note that since this scope was designed to be used as a finder scope it lacks the rack-and-pinion focus mechanism of the other scopes described above (which is presumably a major reason why its weight is lower as well).  It is focused by a helical drawtube which is common for finderscopes; this is not very convenient but acceptable if you only plan to use one specific eyepiece in this scope since in that case you only focus it one time.  In any event if I were considering this scope I’d purchase the “Straight Thru” version for $200 and add the Williams Optics 1.25” 45° erect-image prism diagonal discussed below.

At a price of $200 or so, these are achromatic (the primary lens is a two-lens, or doublet system) rather than apochromatic (the primary lens system uses a three lens [triplet] system or more) scopes. To focus all visible wavelengths at one focal plane you need at least three lenses, so these doublet achromats exhibit some chromatic aberration - generally manifesting as violet fringing around bright objects (e.g. bright stars, the moon, or Jupiter). Users of these scopes don’t seem to find the amount objectionable, perhaps because a good apochromatic scope like this usually costs over five times as much. But there are two other Short-Tube 80 scopes that have somewhat better optics and/or focusers, than the above Synta-made scopes at a correspondingly higher price (and, unfortunately, a greater weight). They are particularly viewed as more appropriate if you want to use the scope for astrophotography as well as visual observing.  Note also that the violet fringing can be reduced with a Minus-Violet filter, discussed at the bottom of this page. Also, note the very reasonably priced 80mm apochromat recently introduced by Celestron, also discussed below.

Anacortes Telescope offers a nice version of the Stellarvue AT1010, the AT1010A (shown here on the left ) for $500.  It is an attractive 80mm f/6 achromatic scope that includes a tripod mounting plate, weighs 4.75 lb. and has a good reputation.


 

William Optics offers the Megrez 80, an f/6 “semi- apochromatic” scope (shown here on the right) priced between $550 and $670 depending on the options you choose. (Some options include the Williams 1.25” 45° image-erecting prism which is probably the best of that type of diagonal available.) It also includes a tripod mount and weighs 4.85 lb.  It’s not clear what Williams means by “semi-apochromatic” but it presumably has less chromatic aberration than the achromatic scopes discussed above.  Williams has a very good reputation but I note that they do package this scope with a Minus-Violet filter so the Megrez 80 can be assumed to have some residual chromatic aberration. Nevertheless it is still substantially less expensive than most true apochromats.

However, Celestron has introduced an apochromatic 80mm scope - the 80ED refractor (#52280).  The “ED” signifies low-dispersion glass and Celestron is marketing this scope for a price of a little over $400, which is a remarkable price for an 80mm apo. It is an f/7.5 scope (which is still capable of wide-field views) that weighs 4.5 pounds, and could be used as a very portable “grab-and-go” scope as well as a rich-field scope atop an SCT. There are no reviews of this scope available yet, but it’s likely to be worth considering since the other 80mm apochromats available, are a lot more expensive.  (If I had to place a bet, I’d bet that this scope is the Japanese Vixen 80mm ED scope, and Vixen scopes have a very good reputation.)

There is a substantial difference of opinion among SCT users, as to whether it is important to spend the extra money to purchase an apochromatic refractor as a rich-field scope.  One group believes that an achromatic refractor is sufficient considering the cost of most apos.  A second group believes that an apo is always worth the extra cost.  A third group asserts that an apo is worth the extra cost because it’s a better scope if you want to use it as a portable travel scope separately from your SCT. Personally I’ve been a member of the first group, although the price of the new Celestron 80ED may make the cost consideration much less of an issue, depending on whether its quality proves to be good.
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I mentioned above that all of the Short-Tube 80 scopes are provided with a ¼”-20 threaded plate at the bottom because they are intended by their manufacturers to be mounted on a tripod for use. To mount one on an SCT, as a minimum you will need to mount it on a camera adapter attached to a Dovetail Plate on the SCT itself.  But there are a number of considerations you need to be aware of. On the right here is a photo of the Celestron version of the Synta ST-80 (mounted on my U2K) which can help you better understand the following:

1. ST-80s tend to weigh from 4 to 5 pounds, even after you have removed the small finder scope that comes with them (which isn’t needed for this application).  To achieve dynamic balance you will need to add a 2-dimensional counterweight to the bottom of your scope - on my scope (as shown on the right) I have two 2.5-pound weights below the scope plus an additional 2 lb. weight (the silver weight in the photo) that I made and added to get everything in dynamic balance.

2. Note that the ST-80 in the photo isn’t mounted with a ¼”-20 camera adapter, but with a pair of scope mounting rings.  You can mount the ST-80 on a camera adapter on your dovetail plate, but that makes it difficult to aim the ST-80 so that it is parallel to the SCT’s optical axis. Even though the ST-80 has a much wider view than the SCT, eventually (especially when you start to use the ST-80 to locate objects that aren’t in your GoTo scope’s database) you will want both scopes to point to exactly the same location in the sky. Most dovetail plate systems do offer camera adapters that provide 3-dimensional aiming capability but such an adapter isn’t likely to retain the ST-80’s alignment with your SCT as you remove and replace the ST-80 over many nights of observing - the ST-80’s length produces a much longer lever-arm than the single camera screw was really designed to hold firmly.  (Plus, such 3-dimensional camera adapters tend to place the ST-80 even farther away from the SCT’s optical axis which makes dynamic balance even more difficult to achieve.) And 3-dimensional camera adapters typically cost as much as a pair of scope mounting rings anyway. So I use the Losmandy dovetail plate and mounting ring system; it is robust but pricey and I discuss other systems on the Dovetail Plate page on this site.

3. Notice that the ST-80 is positioned fairly far forward on the SCT. This is so that the ST-80’s eyepiece is far enough forward that (using technical astronomy terms) it doesn’t hit your head or poke you in the eye, when you bend down to look through the SCT’s eyepiece <grin>.  In fact it is positioned so that you can easily look through either eyepiece and thus readily take in a wide-field view while using the SCT. This means that the counterweight needs to be located back almost at the rear cell, at least on an 8” SCT.

4. If you look carefully at the photo above you will see a white stripe just below the place where the ST-80 sits on the dovetail bar, as well as white stripes above where each of the two counterweights are attached to the counterweight bar below the SCT. These are strips of typewriter correcting tape (raise your hand if you remember using typewriter correcting tape) that make it easy to get everything in the right place when the scope is set up for an observing session.

5. If you use the Losmandy dovetail-plate/ring system, you can make a pair of aluminum plates that securely fasten the bases of the two rings together, as shown in the photo on the right.  (Aluminum bar stock is available in 3’ lengths from a good Ace Hardware store.) This creates a solid unit that will retain the ST-80’s alignment as you remove and re-install the ST-80 on the dovetail plate from night to night. 

There is another modification also shown in the photo. The Losmandy 4.25” rings fit the ST-80 closely, so the very long mounting screws Losmandy provides for the rings are way too long for the ST-80. Worse, because of the 3-point mount one of the screws is directly at the top and that screw projects so high that I was concerned it would snag clothing, etc., in the dark. I purchased shorter (1”) screws with nylon tips, and drilled and tapped new holes in the rings to make a 4-point mount for the ST-80. The reason new holes were needed, is that Losmandy provides 3/8”-24 (fine thread) nylon-tipped screws with their rings. So you’d figure all you need to do is purchase shorter screws of this thread, right? Well it turns out that Losmandy makes their own and no one else sells 3/8”-24 nylon-tipped screws in quantities less than 1000 (you do want nylon tipped screws, to protect the ST-80 from becoming scratched). Because I also use these rings to hold a smaller-diameter guide scope (for which the Losmandy screws are the right length) I decided to add a set of new 4-point mount holes with new, shorter (1” long) 3/8”-16 nylon tipped screws for the ST-80 (which McMaster-Carr does sell), and keep the 3-point mount holes with the original Losmandy 3/8”-24 nylon-tipped screws for the guide scope.

6. I replaced the mediocre (to put it kindly) 45° prism provided with Synta ST-80s, with a Williams Optics 1.25” 45° erect-image prism diagonal.  The Williams, albeit expensive at $100, has an 89% transmissivity which is higher than the Synta-provided 45° prism and thus provides a brighter view.  Although there are 90° reflecting-type diagonals that provide an even higher reflectivity than 89%, a 45° correct-image prism diagonal is really convenient when a rich-field scope is mounted on an SCT.  As noted in #3 above you want to position the rich-field scope so that its eyepiece is located close enough to the SCT’s eyepiece, that you can easily look through either eyepiece without repositioning your observing chair. This is very difficult to do with a 90° diagonal.

7. I’m using a 12.3mm Orion “Epic ED-2” eyepiece on the ST-80.  The Orion Epic ED-2 eyepieces provide a nice 20mm of eye relief which is good in this type of setup, and have an Apparent FOV of 55°. So the 12.3mm on the ST-80 provides an Actual FOV of 1.69° which is wide enough for a good view while providing good magnification (33X).  At $68 I have found it to be a really good eyepiece for this application.
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On the subject of chromatic aberration in achromatic reflectors, a set of new products has entered the market which offers an inexpensive way to improve the chromatic performance of these doublet scopes. They fall under the generic term “minus-violet” filters, that are intended to reduce the false color violet fringe around very bright objects in medium-priced achromatic refractors.  Sirius Optics produced one of the first such filters - the “MV1” - that received good reviews by users - for example, Gary Hand of Hands-On Optics noted that it provides sharper images and this is a typical observation by folks who have used it.

In the April, 2004 issue, Sky and Telescope magazine tested a number of these filters.  They concluded that a combination of a Baader Fringe-Killer and a Baader Neodymium filter, or alternatively the Sirius Optics MV20 filter, worked best on larger refractors.  But the Fringe-Killer/Neodymiuma combination tends to absorb a lot of light so it isn’t such a good idea for small refractors like an ST-80.  They liked the Baader Fringe-Killer alone, for small scopes.  But either way, they noted that any of the available minus-violet filters noticeably improve the performance of an achromat; even though they won’t turn an 80mm achromatic refractor into a $1300 Televue apochromat, they are worth the small investment.  I bought the Sirius Optics MV1 when it first came out, and I’ve been happy with it on my ST-80.


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