Different eyepieces come to focus at different positions of the SCT’s primary mirror, which means that when you switch eyepieces while observing, you need to constantly twirl the SCT’s focus knob to bring each of
them into focus.
With a bit of one-time effort and some relatively inexpensive purchases you can set up each eyepiece so that they all are in focus (or very close) at the same position of the primary mirror. Your eyepieces have then become “parfocal”.
At the same time you can make your eyepieces close to equivalent weights so the scope’s balance is not affected when you switch eyepieces. Finally, assuming you have at least one 2” wide-field eyepiece and thus
a 2” diagonal, you can set up all your 1.25” eyepieces so that they directly slide into the 2” diagonal without switching adapters, and this in fact is part of the process of balancing the weights of your
eyepieces. As I explain below, parfocalizing your eyepieces is easily done after you have added the adapters to bring the 1.25” eyepieces to 2” while balancing them.
Balancing Your Eyepieces / Adapting Them to 2”
If you have a set of eyepieces that includes one or more 2” low-power wide-field EPs, one or more 1.25” medium-power EPs, and one or more 1.25”
high-magnification EPs, you may have noticed that the short focal length (high-magnification) EPs are lighter than the medium-power EPs and quite a bit lighter than the 2” wide-field EPs. You can bring them
all to about the same weight by mounting each 1.25” EP in either one of two types of 1.25”-to-2” adapters.
For each of your small, lightweight EPs, get the Televue "Equalizer". It is a solid
bronze 1.25”-to-2” adapter which weighs a hefty 12 ounces (shown here on the right holding a 7.5mm Takahashi). The extra weight of the “Equalizer” helps keep the
scope in balance; i.e. you probably won’t need to adjust the scope’s balance weights when you switch between heavier EPs and light 1.25” EPs. The Equalizer also allows
you to place those 1.25” EPs in a 2” diagonal; it sells for about $55.
Mount each of your medium-weight 1.25” eyepieces in a
normal 1.25”-to-2” adapter (shown here on the left holding a 14mm Pentax). These adapters are sold by most astronomy equipment vendors such as Hands-On Optics. You can get
one with a compression-ring fitting (that won’t scratch the eyepiece barrel) for about $30 or one without the compression ring for about $16.
By combining each of my 1.25” eyepieces with one or the other of these two types of adapters, I have all my eyepiece weights within a half an ounce of my Televue 27mm
Panoptic. (You probably can’t get all your eyepieces to an equivalent weight of the big hand-grenade Naglers, though.)
Parfocalizing Your Eyepieces
If you have taken the above steps, the only thing you need to add to parfocalize your eyepieces is a parfocal ring for each of your 2” eyepieces. These are not as common as 1.25” parfocal rings, but are available from Scopetronix, DAR Astro Machining, or Gary Wolanski’s Astro Fabricating. These vendors’ parfocal rings
are held in place with small allen-socket set screws that require an allen wrench to tighten. This is good,
because once you’ve located and tightened the parfocal ring in the right place as described below, you don’t
want to accidentally loosen the set screw at night when you’re tired. There are a few other vendors that sell
parfocal rings where the rings are fastened in place with a large knurled-cap screw. This might seem like a good idea but it’s not - it’s easy to loosen that screw when you think you’re loosening the screw in the
diagonal that holds the eyepiece in place, and you’ve then lost the parfocal position. A parfocal ring is one of the few places on a scope where you want a screw to be impossible to turn without a tool.
To parfocalize your eyepieces, aim your scope at a medium-brightness star, and by switching eyepieces find
the one that requires itself to be closest to the diagonal when the star is in focus. That is, after you have the scope in focus for that eyepiece, each of your other eyepieces can be brought into focus by gently raising
them up out of the diagonal (without touching the scope’s focus knob) . Start with your shortest focal-length eyepieces - one of them is likely to be the correct eyepiece to parfocalize all the others to.
You may find that to make one or more of your eyepieces parfocal, they must be pulled out of the diagonal so
far that they can’t be held securely. If this occurs you can get a “Filter Thread Eyepiece Barrel Extension” from ScopeStuff - this barrel extension makes the barrel of an eyepiece longer so that the eyepiece can be
Once you have found the eyepiece that requires itself to be closest to the diagonal when brought into focus,
the rest of the process is easy. You guessed it - without moving the scope’s focus knob, place each of your
other eyepieces into the diagonal, raise up the eyepiece out of its adapter, and when it is in focus, tighten the
adapter so the eyepiece stays there. (If you look carefully at the photo of the Pentax eyepiece above you will
see that it is mounted a little above the adapter to be parfocal.) For your 2” eyepieces, tighten a 2” parfocal
ring where the base of the eyepiece enters the diagonal, so it will drop into the correct position in the future.
One more note: in the above photo of the Pentax, you can see the screw that holds the EP in the adapter,
sticking out of the adapter to the right. When you’re tired at the end of a long observing session, you may accidentally loosen that screw to remove the EP from your diagonal, rather than loosening the screw actually
in your diagonal that holds the whole EP assembly. This is bad, because then you need to re-parfocalize the
EP. (Don’t ask me how I know this <grin>.) So I recommend that you replace the screw in the 2”-to-1.25”
adapter, that holds the EP, with a set screw - i.e. a headless allen-socket screw, that you can get at a good
hardware store. This replacement screw will likely be metric (probably 4mm), depending on where your 2”-to-1
.25” adapter was manufactured. You will need an allen wrench to tighten it once, when you set the parfocal distance, but after that it will just stay in place without the possibility of being accidentally loosened.
If you don’t want to get a 1.25”-to-2” adapter for each of your 1.25” eyepieces, just do the same process as
above but tighten a 1.25” parfocal ring in the correct position. However, I’ve found that when you are working on a particular deep-sky object it is nice to be able to change the magnification by simply switching
eyepieces without breaking your concentration even more by futzing around switching adapters as well, so having each of your 1.25” eyepieces in its own 1.25”-to-2” adapter is very convenient. Plus, 1.25”-to-2”
adapters aren’t really much more expensive than parfocal rings. However, if all of your eyepieces are 1.25”
and you’re using a 1.25” diagonal, you can parfocalize all your eyepieces by getting a 1.25” parfocal ring for all but one of your eyepieces.
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