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7 Essential Accessories for the SCT Owner

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Cloudy Nights SCT Stuff

7 Essential Accessories for SCT Owners
by Allister St. Claire

From the Editor's Desk articles are the opinions and musings of Allister St. Claire. These articles do not represent facts nor are they objective in any way.


SCTs (Schmidt-Cassegrain Telescopes) are the single best selling type of telescope. Far outstripping the sales of refractors, newtonian's and other designs, the SCT remains one of the all time favorite telescopes among amateur astronomers. The reasons for it's popularity are;

  • compact and lightweight design at the 8" size
  • capable of fine views for visual use
  • capable off fine astrophotography
  • inexpensively equipped with GOTO electronics
  • built-in drive tracking even if you don't get the GOTO option
  • a sheer cornucopia of accessories from aftermarket accessory manufacturers

More then any other design, the aftermarket accessory manufacturers have catered to the whims and fancies of the SCT owners. If you can imagine an aftermarket accessory, chances are someone, somewhere sells it. Having owned no less then 14 SCTs in my time, I've put together my top 7 aftermarket accessory list.

#1 Bobs Knobs ($16.00 www.bobsknobs.com)

Collimating or aligning the primary and secondary mirror of an SCT is critical for crisp views. If the secondary mirror is just a hair out of alignment, mid and high power views are washed out. To collimate an SCT the owner points the cooled down SCT at a bright star, defocuses the star so 4-5 diffraction rings are present and then gently adjusts the screws on the secondary mirror. The problem begins and ends here.

The screws on the secondary are either phillips head or allen screws. To collimate the secondary, the owner (in the dark),

  1. stands up and adjusts one screw with a screwdriver
  2. sits back down to see the effect on centering the diffraction rights
  3. stands back up again and adjusts the screw with the screwdriver

and repeats until the diffraction rings are perfectly concentric. Once done at low power the owner repeats at medium power and then at high power. The problems with this procedure are twofold

  1. moving from the eyepiece to the front of the SCT and back makes this a VERY long process
  2. jabbing a metal screwdriver near the corrector plate in the dark is not the smartest thing an owner can do

Bobs knobs are plastic thumb screws which replace the secondary mirrors phillips head or allen wrench screws. Point the SCT corrector end towards the ground, unscrew one secondary screw at a time and replace with the Bobs Knobs thumb screws.

The beauty of Bob's knobs is it alleviates the two problems cited above. With Bob Knobs all the owner has to do is reach around the front of the SCT and adjust the secondary thumbscrew WHILE looking through the eyepiece at the result. No bouncing back and forth between the corrector and eyepiece and no jabbing metal tools near the corrector.

#2 Lymax SCT Cooler ($129.00 www.lymax.com)

Because an SCT is a sealed optical design, it can take quite a while to cool down. Try to observe with an SCT before it's cooled down and all you'll see is wavy, washed out images. Until the optics reach the same temperature as the outside air, it will shed heat inside the tube. When looking through the eyepiece this dissipating heat is magnified and it's like looking at someone's face across a blazing campfire.

The good folks at Lymax sell a simple solution to this problem. The Lymax SCT cooler is a long plastic tube connected to large fan. The tip of the cooler has holes in it. The cooler is inserted into the SCT and the tip with holes will sit between the SCT cone shaped baffle and the corrector plate. Once turned on, air is sucked into the fan, goes through a dust filter and into the body of the SCT. Back pressure causes this internally trapped air to flow back out the baffle and is vented through some holes. See David Knisely's review of the Lymax cooler for full details.

The bottom line is the Lymax Cooler will half the cool down time of all SCTs regardless of their size.

#3 Dew Shield ($69 and up Dew shield selection)

A dew shield performs two important functions for the SCT owner

  1. Slows the formation of dew on the front corrector plate
  2. blocks indirect lights from entering the SCT tube. This enhances contrast

That big glass corrector on the end of an SCT just loves to have dew formed on it. A dew shield is a simple plastic piece that fits around the corrector and protrudes about 6" in front of it. Velcro on the ends hold it in place. If the humidity is heavy, a dew shield will only slow the formation of dew, it can't stop it. If you observe from a high humidity area, you will need to buy an electrical dew heater package. For the rest of us, a dew shield will keep dew at bay for anything but the longest observing session.

Additionally the dew shield blocks indirect lighting from entering the tube. Indirect lights are things like street lights, porch lights or lights coming from the windows of houses. Indirect lighting will lower the contrast of the views making them appear gray and slightly washed out.

Dew shields are sold as simple as the one in the picture above or far fancier and far more expensive. My experience has been they all work the same so buy on price alone.

#4 12 volt battery

Many of the GOTO SCTs can be driven off of batteries or an optional 12 volt power supply. My Nexstar 8i SE can use 8 AA batteries. However, my first night out the AA batteries lasted 1.5 hours. Given as much as I use my scope, you can quickly imagine how expensive it would be to run it off of AAs.

12 volt batteries are sold in the auto department of all the mega stores (Target, Wal-Mart, etc) for right around $35.00. Each of the SCT manufacturers sells a 12 volt cable for their respective models. Unless you like buying batteries by the case, do yourself a favor and get setup with a 12 volt battery and cable.

#5 2" William Optics SCT Diagonal ($139.00 www.astronomics.com)

Every time I get a new SCT (I've owned 70+ telescopes for those wondering), I take the scope out and use the stock diagonal for about 5 minutes. I then gently remove it, go into my garage and pitch it into the garbage can from about 10 feet away. It's a satisfying way to get rid of a piece of junky equipment. I don't even bother to give it away as I know I wouldn't be doing anyone a favor by doing so.

William Optics sells a 2" SCT diagonal with 97% enhanced coatings and a 1.25" adapter for a mere $139.00. The machining on it is beautiful and the optics are great. This diagonal screws onto the visual back of the SCT and can be rotated by loosening the the main thread a tad. A 2" diagonal not only gives one better optics then the stock diagonal, but also allows the owner to purchase 2" eyepieces. 2" eyepieces gives the SCT a far greater field of view at low power then 1.25" eyepieces can.

BTW - for you Nexstar 8i owners, the WO diagonal WILL clear the base when viewing at zenith.

#6 Rigel Quick Finder ($39.00 www.astronomics.com)

I will never understand why Meade and Celestron insist upon shipping their SCTs with those awful straight through finders. As soon as the SCT is pointed above 50-degrees, viewing through it is an exercise in yoga or gymnastics. Even GOTO SCTs need a finder of some kind to perform the initial 2 star alignment. It's also extremely handy when the GOTO misses the object. Personally, I don't use GOTO at all so a finder is critical to finding objects.

The Rigel is what is known as a zero power finder. The Rigel projects LED red circles on the clear plastic view pane at the top of the finder. The user can see the stars through the view pane AND the projected circles on the stars. Put the star in the center of the circle and the object is in the eyepiece. See Tom Trusocks review of 9 zero power finders for details

The best part of the Rigel is it ships with two bases. The Rigel itself clips on and off the base so it can be used on multiple telescopes. Additional bases are $8.00. Buy one Rigel and couple bases and you will never have to buy another finder scope again.

#7 University Optics Konig Eyepieces ($79.00 and up www.universityoptics.com)

University Optics has been selling the Konig eyepiece line for decades. They remain a favorite among owners of longer focal length scopes due to the low price and excellent light throughput. With a field of view wider then a plossel (plossels have a 52-deg fov while konigs have 60-65-deg fov) but with the contrast and light throughput of an ortho, they are great values on the eyepiece market.

The photo above shows two routes an SCT owner can go when buying eyepieces. On the left is my UO Konig collection with a Celestron Ultima barlow. It's the collection I use with my Nexstar 8i. The collection on the right is my collection of Naglers which I use in my short focal length refractor. While the Naglers have a wider field of view, they are much heavier and far more expensive. See the tables below for details

Eyepiece field of view (fov) Price
40mm 2" Konig 60-degrees $119.00
24mm Konig 60-degrees $99.00
16mm Konig 65-degrees $79.00
12mm Konig 60-degrees $79.00
Celestron Ultima Barlow N/A $59.00
TOTAL   $435.00

Eyepiece Field of view (fov) Price
35mm Panoptic 68-degrees $365.00
22mm Nagler 82-degrees $460.00
13mm Nagler 82-degrees $280.00
7mm Nagler 82-degrees $280.00
TOTAL   $1388

If you want to save even more money, skip the 12mm Konig and barlow the 24mm Konig. Eyepieces barlow differently. Some barlow well, others barlow poorly. The 24mm Konig is the sweetest barlowed eyepiece combination I've ever used. Eye relief is perfect and the contrast is wonderful.

One big advantage of the f/10 SCT is it doesn't require expensive eyepieces that are corrected for much faster scopes (f/6 and below). The Naglers and Panoptics are wonderful eyepieces but unless you really, really want the extra field of view, the Konigs will perform just as well as the Naglers in an SCT. Hey, I own boxes of eyepieces but with my Nexstar 8i SE I use the Konigs in the picture above. I do this because they are lighter weight and have some of the best light throughput on the market bar specialty planetary eyepieces.




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