Optimizing the Orion 100mm f/6 short tube achromat.

Optimizing the Orion 100mm ST f/6 short tube achromat.

Author: MikeG

BACKGROUND :

The purpose of this article to explain the benefits (and drawbacks) of using a small refractor as one's primary viewing instrument. If one is limited by circumstance or budget to owning a single small scope, then I will outline how one can maximize the performance of a small refractor to provide the most viewing enjoyment possible. With some tinkering and aftermarket parts, an average achromat can be "tuned" or "blue printed" to provide the utmost level of performance possible in a system with limited aperture. First I would like to outline my decision-making process that led to the purchase of my primary observing scope - the Orion 100mm ST (f/6 short tube) achromat.

I will state up front - I have a bias towards refractors. This is not a mindless prejudice against scopes with mirrors, but a personal matter of taste and habit. (So, if you are a mirror man or a CAT person, don't stop reading! You may be able to put some of this to use with your piggybacked RFT.) I cut my teeth on giant binoculars, which are essentially twin rich-field achromats acting in tandem. It's not that I have anything against reflectors or compound designs, but my background experience never involved any exposure to reflectors, so they were a complete unknown to me. So I decided to stick to the familiar - refractors. Once that decision was made, then I quickly narrowed down the field of possibilities : achromat or apochromat. The former was within my budgetary limits, the latter was not. Another easy decision made. My last decision before purchase was the allocation of my gear budget - would I spread it thin and purchase 2 or 3 scopes, or would I put all of my eggs into one basket and purchase a single scope to be my only scope? Rather than have a few under-optimized scope setups that I would be unhappy with in one respect or another, I thought I would attempt to "soup up" a single scope and use the majority of my budget to do it. Now I was ready to shop for a purchase - a single, good, achromat.

ACQUIRING THE OTA :

While eagerly browsing through the online classifieds for refractor deals, I was given a dose of reality. Refractors, even achromats, get expensive quickly as aperture increases. Although I had decided to put my entire budget into one scope, realistically I had to leave myself some change from the purchase so I could also acquire the obligatory accessories - mount, diagonal, finder, and eyepieces. Taking this into account, I had about $200 I could spend on an OTA and still leave myself enough money for the rest. This severely limited my options, and the prospect of a $200 refractor was not exciting to say the least. Two things happened that put my project back on track - a good mount fell into my lap and a hot deal came my way on an OTA. The mount was a surprise eBay find. For less than $50.00, I scored a 1980's Towa EQ-2 mount and wooden tripod. The mount was originally rated for an 80mm long-focus refractor, and it was built like a tank compared to some of the more modern EQ-2's I have seen. I figured it was stable enough to support a 100mm scope and it was in minty condition. Within days, I was offered a nice 100mm Orion ST f/6 achro with rings, carry case, finder and eyepiece for $200 shipped to my doorstep.

Well, now I was in the game. The 600mm focal length OTA balanced nicely on the slightly-undersized Towa EQ. I had a basic mirror diagonal laying around (leftover from an old department store scope), and coupled with the finder and eyepiece which came with the package deal, I now had a serviceable short tube refractor on a decent mount. The key words here being "serviceable" and "decent". I could have stopped here, and used the rest of the money in my war chest for a used 6" dob or fast 6" newt that would also fit on my mount. To me, this little 4" tube was a blank canvas, or that rusted old heap of a car that a restorer can breathe new life into. Having done my homework on fast achromats, I knew false color would be an issue on bright objects - so I purchased a Baader FringeKiller filter to help tame the CA. Speaking of false color, and aberrations in general, I liked the f/6 Orion 100mm better than the other options on the market, which included the similar Celestron 102mm f/5 short tube achromat. Everything I had learned, while doing my homework, states that f/4 and f/5 are truly "fast" scopes, which require very well corrected eyepieces to extract good views. f/6 resides in that borderline area between really fast f/4 type scopes and slower f/8+ planetary scopes. I liked f/6 because it offers a little more magnification than faster scopes and a wider field than slower scopes - while being relatively forgiving on eyepieces. Where a fast f/4 Newt will make mincemeat of a bargain-class widefield eyepiece, a slower f/6 can yield acceptable results without breaking the bankbook on premium eyepieces which cost more than the scope itself.

Around the time I bought the violet-reduction filter, I also bought an Orion enhanced 97% diagonal (1.25") to replace the dubious department store diagonal I was using. Within days, the first batch of accessories arrived - including a GSO 8x50mm RACI finderscope and mounting bracket. A night of low power viewing was rewarding, because it revealed to me my next area of improvement - the focuser. The stock rack and pinion focuser was adequate, but I did not want to settle for "adequate". So I ordered a GSO 2" single speed crayford focuser.....and a 6-point Stellarvue 50mm finderscope bracket to replace the Synta-style bracket that came with the GSO finder. (I just don't like Orion/Synta bracket, some folks like them, some don't).

PARADE OF CRAYFORDS :

The GSO crayford was an easy install. Using a phillips screw driver, I removed the screws holding the stock focuser to the tube. The Orion focuser slid out easily and sold just as easily to another online gearhead days later. (More money back in the warchest.) An adapter ring, also bought with the GSO focuser, bolts into the OTA, and the the GSO crayford focuser bolts into the adapter/OTA. (One note about the GSO for Orion focuser swap - the necessary adapter ring adds several millimeters to the focal length of the tube, thus eating up some valuable focuser in-travel. Some eyepieces, including the 2" GSO 32mm Superview and 2" UO 32mm Konig will not come to focus using a diagonal once the stock focuser has been replaced.) Even considering the loss of focuser in-travel, the crayford was a big improvement over the rack and pinion unit. Focusing was much smoother with no image shift or slop. Finding a crisp focus point was also easier when compared to the stock R&P. I was quite pleased. as happy as I was with the 2" GSO single-speed crayford, it could not handle a heavy load. When using a 2" diagonal, 2" barlow, and 2" eyepiece, there was some slippage when the scope was pointed near the zenith. I had to crank up the focuser tension screw to compensate and this intefered with the smooth action of the focus. A deal came my way on a JMI DX2 crayford which came originally from a Stellarvue scope. So the GSO 1-speed was pulled out and sold to help finance the JMI. To make a long part of this story short, the JMI was a quality unit, but it possessed an even longer profile than the GSO 1-speed. It could handle the increased load of heavy accessories with no problem, but I lost even more focuser in-travel. Many of my eyepieces would not come to focus with a barlow while using the JMI. So, as much as I liked the JMI, I was forced to yank it out and sell it.

I had heard that the 2" GSO 2-speed crayford could handle a slightly-heavier load and I was enticed by the prospect of a 10:1 microfocus, so the scope underwent another focuser swapout - the one speed for the two speed model. I later heard that both models of the GSO crayford have the same load capacity. Whether this is true or not, I am not sure. But I can say that the slippage problem vanished with the new 2-speed focuser, and the action of the micro-focus was very smooth, allowing me to more easily achieve that critical focus point. It was around this time that red warning flags began to go up about my scope's optics. The views through the eyepiece were great, until one pushed the magnification to around 55x. At that point, the image fell apart - bright colored halos around everthing in the field and mushy images that would not crisply focus.

WHAT A DIFFERENCE COLLIMATION MAKES :

Something was dreadfully wrong. Either I had thrown off my scope's collimation by changing out the focuser, or it had lost collimation at some point in the past before I got my hands on it. Either way, it was out of whack. I am no expert on star testing, but star tests didn't look right either. Thankfully, this particular Orion 100mm ST has an adjustable lens cell, and it also came with a collimating eyepiece to adjust the alignment of optical path. However, I was in over my head and anxious to stop tinkering with gear and start doing some observing. So I packed up the scope and shipped it to a friend who is very experienced with refractors and was an expert at collimation of all kinds. I also shipped him the 2" William Optics 97% diagonal that I had purchased to upgrade the Orion 1.25" unit. Upon receiving the package, my optical man immediately confirmed my suspicions - the scope was badly miscollimated, and the diagonal was also out of alignment. Over a course of two clear nights, he collimated the scope and put it through a series of trials and star tests. When he was finished with it and shipped it back to me, the little achro had some new life in it. The improvement in performance was obvious. Low power views were now fantastic and very sharp. Stars were nice little round beads and high power views were surprisingly good. Saturn's Cassini division was easy to see, and high power lunar views were contrasty. Star tests were textbook and the scope would now easily attain the 50x per inch magnification threshold (and more on nights of good seeing). Sure, there was still some color around bright stars at higher powers and on the lunar limb, but it was manageable using a good violet-reduction filter. (I had then switched from the Baader FringeKiller to the William Optics VR-1 which has a more aggressive filtering action and rendering a better view to my eyes.) Using a 3x barlow to squeeze some magnification out the short tube, I could push 300x on the Moon and still get a nice usuable image. Saturn and Jupiter look great up until about 250-275x, depending on the seeing and conditions. Cassini is obvious on most nights, the GRS can be detected on Jupiter, and yet I can still get a 4-degree field at 20x using a long focal length wide-field eyepiece. In fact, thanks to the medium f/6 optics, I could put in a 7mm Nagler and see the entire lunar disc at 87x and have enough room left over in the field to observe occultations. Watching a crisp lunar disc at 87x occult the Pleaides in a field that is barely shy of one degree is a nice affirmation that I made the right choice of scope. The little 4" achro was now beginning to pay dividends. But my quest to push this scope to the limits of it's design and optics was not over. More changes were in order.

THE THIRD MOUNT IS A CHARM :

Around the same time I got the scope back from it's tune-up, I also "upgraded" my mount from the venerable Towa EQ-2 to a modern Orion AstroView EQ-3. This added a measure of stability to my set-up, but the Orion mount had some backlash in the RA gear, and I was not happy with the setting circles on the AstroView. So before it had a chance to start feeling at home, I sold the Orion mount and purchased an old Vixen Super Polaris with illuminated polar finder. Compared to the Orion and Towa mounts, the Vixen was a work of art. Superbly made, smooth as butter, and nice looking as well. And while the AstroView and Super Polaris look similar in size and capacity, the Vixen is certainly more robust. In fact, the 600mm 4" Orion achro is now firmly mounted on that SP mount - it is truly "overmounted" in every sense of the word. I can sit outside in a strong wind, and rack the focuser at 300x and the view is rock solid without any distracting vibration. Of course, some of this is due to the custom hardwood tripod legs I ordered from an old friend. The new legs are 2" thick of Maple and Walnut, non-adjustable with a fixed length and spreader, and beautifully finished. I went with non-adjustable legs to increase stability and remove any flexure points. With the eventual addition of a motor for the RA axis, my mount and tripod issues are completely resolved. Now the little Orion scope was overmounted on an attractive customized rig, it was time to focus on the scope itself again.

FINISHING TOUCHES :

After four focusers and three mounts, the Orion 100mm ST was finally approaching respectability. According to my optical man, (who has forgotten more about scopes than I can ever hope to know) this particular Orion scope was a very good sample, with a well-corrected lens group, textbook star tests, and as much optical performance as one could reasonably hope for in a $200 budget-class achromat. With the addition of the 2-speed crayford focuser, the scope was a pleasure to use - focusing required just a very soft touch without the need to rack back and forth over the critical focus point to nail it down. The Vixen Super Polaris mount and custom hardwood legs provided a stout platform for the scope - able to withstand strong wind gusts without noticeable oscillations in the view. After being opened up and shimmed, the 2" WO diagonal yields very good views and that completed a revonated optical train from objective to visual back. There only remained a couple of minor niggling issues to improve. First, I am not a fan of the Synta-style finder bracket and I am not happy with the mounting point for the finder base that sites on the body of the GSO crayford focuser. It places the finder too close to my head while observing and it frequently got bumped and knocked out of alignment during the course of an evening's observing. To remedy this, I bought a third 100mm scope ring. I attached the finder and finder mount to the third tube ring and attached this about half-way down the tube, out of my way and closer to the objective. With the finder mounted on it's own independent tube ring, I am free to change the configuration of the finder in respect to the scope. Next on the list was a roll of adhesive backed felt liner from the department store. I used the black felt to flock the inside of my dewshield which I thought was too reflective despite being flat black from the factory. The felt is much less reflective and eats up off-axis reflections. Once the finder was located in a more logical position, I went out and found an illuminator for the finder crosshairs. A pet peeve of mine is not being able to discern the crosshairs of my finder against a dark background sky - it makes lining up targets a major pain. I didn't want to purchase an entirely new finder, and the available aftermarket illuminators were quite expensive. So I contracted out the job of building an illuminator to a certain managing editor of a certain astronomy gear magazine. He built me a professional-looking illuminator unit with a rheostat to dim the LED which sat at the end of a pigtail cord running from the illuminator housing. I attached the housing to the DEC-motor bracket on the Vixen mount (I don't forsee using a DEC motor any time soon) and drilled a small hole in the eyepiece barrel of the finderscope, inserted the LED and voila - illuminated reticle finderscope. No more frustration trying to center targets on the crosshairs.

OBSERVING WITH THE FINISHED SCOPE :

After much tweaking, I finally had myself a scope that could perform to my tastes. With a violet-reduction filter, the 4" achromat could easily attain 50x per inch on any target I cared to observe. Images are pleasing, stars are crisp little beads, and with a low power eyepiece, the TFOV of the scope using a budget widefield 42mm SuperView eyepiece was over 4 degrees. Views of star fields in Cygnus are stunning. M24, the Sagittarius star cloud, is a thing of wonder to behold in it's full context with the surrounding sky. Even planets, long considered the bane of fast achromats, are fair game when using a filter to tame the color. Numerous clouds bands are visibile on Jupiter. I have seen the GRS (or as I call it, the GTD - the great tan dot) and I have watched shadow transits of the Jovian moons - albeit on a small image scale. Using a 7mm T1 Nagler and Meade 3x Telenegative barlow for 257x, yields a surprisingly sharp image, despite the fact that such magnifications are generally regarded as "stupid" in a fast achromat. On our bright and forgiving neighbor, the Moon, I have successfully pushed 300x on nights of excellent seeing - granted, the image was going dim and soft at that point, but not to the point of being considered "useless magnification" - I was seeing more detail due to the increased image scale. I have seen Venus and Saturn in the same field of view during a recent close pass - something not easily achieved in slower scopes. But I think the best attribute about this scope, besides it's surprisingly good performance, is it's light weight and ease of use. At 6 pounds bare, and just under 9 pounds fully-loaded (with finder, diagonal, eyepiece, etc), this scope can be fielded with a variety of mounts that are well within the budget of the most frugal astronomer. Cool-down time is almost negligible, and one can start viewing immediately after setting up without waiting an hour or more to achieve thermal equilibrium with the outside temperature. This experience has further cemented me into the small refractor camp - so much so that I have turned down some spectacular deals on much larger scopes of different designs - including a 10" SCT that was offered to me for next to nothing. The old saying of "the best scope is the one you use" has proven to be true in my case, and I have gotten a lot of use out of this humble 4" achromat. Now, pardon me while I grab-n-go!

Clear dark skies to all!

Article and photos by Michael Gilmer (Galactic Stone)