A scope’s manual focus knob changes the scope’s focus at a rate determined by the gear ratio chosen by the scope’s manufacturer. The question is, what rate of focus change should the manufacturer have
chosen? There are two rates-of-change of focus needed by scope users:
- When you switch eyepieces you want the focus knob to change the focus rapidly, since eyepieces from different manufacturers can have radically different focus positions (if you haven’t made them parfocal).
- Once you have achieved a “close” focus for a given eyepiece, you want the focus knob to change the focus very slowly so that you can zero in on a very precise focus.
This is a mutually exclusive pair of criteria for scope manufacturers to achieve - no matter which one they choose, many users will complain that they chose the wrong rate-of-change of focus position.
But there are a number of ways for you to meet both of these requirements with an SCT, as described below.
A Larger Focus Ring
You can achieve a finer focus by adding a larger-diameter ring to the stock
focus knob on your SCT - the larger ring allows you to make finer movements by turning it rather than the scope’s focus knob itself. The classic way to do
this has been to purchase a grommet that fits on your focus knob, and then drill a hole in the top of the lid of a peanut butter jar, into which the grommet
fits. You can then slide this assembly onto your SCT’s focus knob. But many of us feel that when you’ve paid two or three large for your scope, a
peanut butter lid is rather crude. A nicer alternative is sold by ScopeStuff, which offers a 2.5” diameter
fine-focus ring (shown here on the right, mounted on my U2K) for a variety of Celestron and Meade SCTs, for $28. And Jim Henson offers to include a pack of peanut butter scent for those who miss the full
experience of the old-fashioned do-it-yourself fine-focus ring - you can’t beat that offer <grin>.
A Dual-Rate Manual Focus Knob
It might seem obvious that SCT manufacturers should provide a focus knob that can be turned at both a fast and slow rate, but due to intense price
competition they haven’t done this. However, Starlight Instruments makes a product that does accomplish this - their “Microfocuser”. This focus knob
replaces the original focus knob in Celestron or Meade SCTs, and provides two focus knob “speeds”. The outer focus knob moves the SCT mirror at the
same rate as the original focus knob, but an inner focus knob moves the mirror at a tenth the normal rate, allowing you to more precisely adjust your
SCT’s focus. The cost is $190; this is a bit pricey compared to the electric focus motor options described below, but it does have the advantage of not requiring you to futz around
with more connecting cables and control keypads than you already have with your SCT. I have one and it is very convenient when I just want to do visual observing (but a motorized focuser is essential when doing
astrophotography). Starlight Instruments doesn’t have a Web site that shows this product but it is described and sold on the Starizona Web site - on that site go to Telescopes - Telescope Accessories -
Focusers. (Note that Starlight Instruments also makes a rear-cell-mounted non-electric focuser called the “Feather Touch Focuser” which is a different product than the “Microfocuser”. You will be able to
distinguish these products very easily by looking at their respective prices - see the next paragraph...)
Separating an SCT’s Mirror-Movement Focus from an Eyepiece-Movement Focus
Another option is to use the SCT’s manual focus knob to move the primary mirror to change focus rapidly, but then to move the eyepiece
itself slowly to achieve fine focus. Basically this requires that you leave the scope’s manual focus knob alone but add an additional focuser between the scope’s rear cell and your eyepiece. JMI makes two such units - one that is motorized (the Next Generation Focuser - “NGF-S”) and one that is manual (the “NGF-SE”). Both are
designed to thread onto the rear cell of an SCT. The NGF-S unit (shown on the right here) which includes an electric focus motor, sells for about $280 and the manual NGF-SE sells for about $170.
(Starlight Instruments sells a similar non-electric focuser with street prices starting at $265.) With these units you can use the SCT’s focus knob to get close to a good focus quickly, and then use the
NGF-S or -SE to fine-tune the focus.
Update: JMI has recently changed the model designation of the NGF-S to NGF-CM (for “crayford motorized”) and the designation of the manual NGF-SE to NGF-C. According to JMI, the major design
change is in the NGF-CM focuser. The large external motor has been replaced by a smaller integral motor and the addition of manual adjusting knobs. This makes the NGF-CM focuser lighter and more compact
than the NGF-S and allows both manual and motorized operation.
Note that the NGF-S comes with a small keypad (shown on the right) to control
the direction and speed of the focus motor. You may replace this with a controller that provides a digital read-out of focus position. In this case JMI adds a position
encoder to the focus motor unit (similar to the encoders within the two drive axes of GoTo scopes, that tell the computer which way the scope is pointing). There is then a different
hand control unit (shown on the left) which controls focus direction and speed but also provides a digital readout of focus position. This is very slick, but also rather pricey. You
need to order the NGF-S, the “DRO” encoder, and the “HUDRO” hand unit, for a total cost of about $500. (Note that you need to order the NGF-S and the DRO encoder
together since they are factory-assembled.) Note that astrophotographers have major heartburn with this digital readout system because the hand unit
forgets its settings when it is turned off. This makes it difficult to return to specific focus positions from night to night, which basically was the original purpose of the digital readout.
Another complaint with the HUDRO unit is that it uses a standard 6-conductor phone cable rather than a coiled cable like the one used
by the U2K’s hand control unit or the other Motofocus units. The problem with the 6-conductor cable is that it is rather stiff and tends to get in the way when you are using your scope. A
coiled cable would be a lot more convenient (although, I’m not aware that anyone actually manufactures a coiled 6-conductor cable, but it would sure be nice...)
I personally use the NGF-S unit without the DRO option and have found it really helpful for achieving fine focus. I do think the DRO option is way too expensive for the value it provides.
Note however that JMI also has introduced a “Smart-Focus” option - if you have the NGF-S unit with the DRO encoder, instead of adding the $140 HUDRO hand unit you can add a $340 software system that allows you
to automatically achieve focus with a CCD camera using a computer. Don’t underestimate the value of automating the focus process - the vast majority of CCD camera users will tell you that the most
time-consuming aspect of taking photos with a CCD camera is the excruciating process of getting good focus on the camera’s CCD chip, and they would tell you that $340 is a small price to pay to automate
this process. On the other hand, at the total price for this setup you should also consider the Robo-Focus system described at the bottom of this page.
Note 1: If you have a 10” or larger Celestron or Meade SCT, you should consider also purchasing JMI’s
“ADPT3SCT” adapter, which replaces the standard Visual Back adapter provided with those SCTs. The problem with the standard Celestron or Meade Visual Back adapter is that it provides a 1.5” exit hole
which matches the size of the exit hole on the rear cell of an 8” SCT. But the 10” or larger Celestron or Meade SCTs have a significantly larger baffle tube and thus a 2” exit hole in the rear cell. When they sell
you a scope with the standard 1.5” Visual Back adapter, that adapter actually cuts out much of the light the scope can deliver to your eyepiece. The JMI “ADPT3SCT” adapter provides a 2” exit hole and thus
eliminates unnecessary vignetting of the light path, and it only costs $45.
Note 2: If you contemplate using a Celestron or Meade skylight filter mounted to your rear cell to keep
dust out of your scope, on an 8” SCT the NGF-S and a 2” diagonal can add enough length to prevent the diagonal from swinging past the fork base with the skylight filter in place. There is, however, an
inexpensive replacement solution to this problem. A Hoya 46mm UV filter (available from any good camera shop or camera mail-order vendor) fits perfectly inside the JMI “ADPT2SCT” mounting ring that
connects the NGF-S to an 8” SCT’s rear cell. It will keep dust out of the SCT and only costs $10, a lot less than the rear-cell-mounted skylight filter. (Presumably, other reputable filter brands besides Hoya
would also fit well, but I bought the Hoya and since it worked I had no reason to test any others.)
Note 3: If you have a U2K, there is a small jack on the far left of the fork base, that is labeled “FOCUS”.
You can plug the coiled 2-conductor cable from a JMI motorized focuser (either the above NGF-S or the MFC8 discussed below) directly into this jack and use the U2K’s hand controller to control the JMI
instead of using the JMI controller. (This won’t work if you have the optical encoder and HUDRO option, which uses a 6-conductor cable.) To control a JMI focuser from the U2K’s hand controller you hold down
the MENU key and press the UP or DOWN keys to change the focus position. This moves the JMI motor at its highest speed only, but that’s fine for visual work and having only one hand controller is very convenient.
Celestron’s Fastar system mounts the CCD camera on the front cell of Celestron SCTs. With this
system you can only achieve focus by moving the SCT’s primary mirror, and so the NGF-S or -SE options described above cannot be used to focus the scope. You can only achieve fine focus with a CCD camera
in the Fastar system by fine-tuning the SCT’s primary mirror position. You can do this with the Starlight
Instruments manual “Microfocuser” knob described above, or by substituting an electric focus motor for the Celestron’s manual focus knob.
The most basic electric focus motor you can add to the focus knob, is the
JMI Motofocus Model MFC8 which is made to fit the U2K and Celestron C8s. (JMI has a similar model to fit Meade LX scopes but since they can’t use a
Fastar system the NGF-S or -SE would be a more flexible and convenient, albeit more expensive, option for Meade LX scopes, since the scope’s focus
knob could be used to get close to a good focus quickly, and then the NGF-S or -SE would be used to fine-tune the focus.) The MFC8 includes the focus motor shown above, and a
replacement focus knob for your scope, that accepts the focus motor. This Motofocus also includes the small keypad described in the NGF-S discussion above, to control the direction and speed of the focus
motor. The MFC8 sells for $120.
An upgrade to the Motofocus is to add a mechanical read-out of the focus position to
the basic MFC8. This is useful when you are immersed in Fastar astro- photography and you want to get back to the focus positions that worked for you on previous
nights. The JMI Digital Focus Counter (Model DFC) is shown on the right - click on the thumbnail to view a larger picture.
It is a small mechanical dial showing focus position, similar to an automobile odometer (their term “Digital Focus Counter” is a bit misleading in today’s times where “digital” tends not to mean a mechanical
system like this one, but rather an electronic system). The DFC needs to be added to the Motofocus when you order it (it is added in the manufacturing process), and adds $90 to the price of the Motofocus.
A third electric focus option is to add the electronic read-out of the focus position also described in the
NGF-S discussion above, rather than the mechanical one. As noted above, in this case JMI adds the “DRO” position encoder to the focus motor unit and you add the “HUDRO” hand control unit. To do this
you need to order the MFC8 Motofocus motor, the “DRO” encoder, and the “HUDRO” hand unit, for a total cost of about $320. As is the case for the Digital Focus Counter, you need to order the MFC8 and the
DRO encoder together since they are factory-assembled.
Many astrophotographers using the Celestron Fastar system have found that the most precise focus is achieved with the Robo-Focus system made by Home-Dome. As shown on the right here, this
unit adds a focus motor that moves a substitute focus knob through a belt drive. This unit can be very precisely controlled through a computer. It starts at about $400 and has a number of options that
help in astrophotography - it’s worth checking out. And unlike the JMI HUDRO, it does remember focus positions after it has been turned off.
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