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Crayford focuser for SCT: 1 vs 2 speeds

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#1 Steve Daniel

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Posted 14 February 2013 - 03:43 PM

I'm looking at Crayford focusers for my CPC800. Do I really need two speeds, especially if I don't plan on doing any astrophotography? How fine is the focus control you get with a one-speed focuser? I'm specifically looking at the Crawford Machine extra low profile focuser for SCTs.

#2 ZRX-Steve

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Posted 14 February 2013 - 06:14 PM

Your CPC has a single speed focuser and you are upgrading it to another single speed focuser. Can you share your reasons for upgrading? Personally, for the $40 or $50 delta, I'd get the two speed. You'll make it back in resale if you ever decide to sell.

#3 ZRX-Steve

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Posted 14 February 2013 - 06:18 PM

P.S. I have a C8 that i use with Mallincam. I upgraded to a GSO 2 speed specifically for the fine control. But that's for APhotography / Video Astronomy. If I wasn't doing Mallincam, I wouldn't have spent the upgrade $$'s just for visual.

/s

#4 Steve Daniel

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Posted 15 February 2013 - 12:25 AM

The two reasons for upgrading are: image shift when focusing with the stock focuser, and wanting finer control during focusing. How much finer than the stock focusing would a single-speed Crayford focuser be? I suppose it may vary per manufacturer...

#5 hottr6

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Posted 15 February 2013 - 08:16 AM

I would suggest staying with the single-speed. Two-speeds are necessary only for AP or at f/ratios faster than f/5 for visual work. But if doing AP, you'll be better served with a motorized focuser. I also find 2-speeds need regular maintenance to keep them operating at 100%. You'll save money and frustration as well. YMMV.
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I use a JMI on my MCT. I did the research and both Crawmach and JMI units add the least length to the optical train of all the Crayfords commonly available.

Shane in grey-zone NM

#6 cliff mygatt

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Posted 15 February 2013 - 09:34 AM

I agree with a single speed focuser for an SCT, I have a JMI as well and I am quite able to get great focus with the single speed. Good Luck!

#7 fmhill

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Posted 15 February 2013 - 10:23 AM

I'm looking at Crayford focusers for my CPC800. Do I really need two speeds, especially if I don't plan on doing any astrophotography? How fine is the focus control you get with a one-speed focuser? I'm specifically looking at the Crawford Machine extra low profile focuser for SCTs.


I have two telescopes with dual speed crayford type focusers and two scopes with single speed focusers and trust me, if you are going to spend the money and go through the effort to upgrade, definitely upgrade to 2 speed, they are vastly superior to the single speed focuser. You will never regret going to the two speed type whereas if you go with another single speed, you will always wonder what you are missing...

#8 hottr6

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Posted 15 February 2013 - 01:34 PM

I'm looking at Crayford focusers for my CPC800. Do I really need two speeds, especially if I don't plan on doing any astrophotography? How fine is the focus control you get with a one-speed focuser? I'm specifically looking at the Crawford Machine extra low profile focuser for SCTs.


I have two telescopes with dual speed crayford type focusers and two scopes with single speed focusers and trust me, if you are going to spend the money and go through the effort to upgrade, definitely upgrade to 2 speed, they are vastly superior to the single speed focuser. You will never regret going to the two speed type whereas if you go with another single speed, you will always wonder what you are missing...

It really depends on the focal ratio of the 'scope you are using. I have 2-speeds on my 105 f/13 and 90 f/11 'fracs, and the second-speed is a useless appendage that never gets used. On a f/5 RFT and faster it would be invaluable.

The OP has a slow SCT... he won't miss a thing in visual use.

Shane in gray-zone NM

#9 richardlowney

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Posted 15 February 2013 - 04:09 PM

A lot depends on how good your optics are. If you have a snap-to focus, you won't be disapointed with the two speed. My first views through mt 8" Edge convinced me two speeds would be very beneficial. I purchased the Crawmach. Its an excellent focuser and is low profile (2" total).

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#10 Steve Daniel

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Posted 15 February 2013 - 04:36 PM

Thanks all, these help. Richardlowney, what did you mean by "snap-to" focus?

#11 richardlowney

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Posted 15 February 2013 - 05:00 PM

Snap-to focus means to me very little focuser knob travel to achieve perfect focus. Mostly seen by very well made or short focal length refractors such as the NP101. I was pleasantly surprised by the difficulty of achieving focus with the 8" Edge using only the standard focuser. Also, almost undetectable mirror flop. I will say I had a newer C8 that was equally as good.

#12 richardlowney

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Posted 15 February 2013 - 05:33 PM

Let me put this a little better. Snap-to focus means from an out of focus image to a very well focused image with very little focuser knob travel which is why the two speed is, in my opinion, the better choice. As a previous poster said, if you spend the money on a single speed crayford, you now have two single speed focusers.

#13 Steve Daniel

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Posted 15 February 2013 - 11:56 PM

Ah - thanks for clarifying! I'm mostly leaning toward two-speed now.

#14 Namlak

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Posted 16 February 2013 - 02:57 PM

Another vote for 2 speed. I have the JMI on my C8 and find that the low-speed knob requires far less torque (as you'd expect) and thus much less wiggling of the scope when focusing, allowing much better finger-tip control when observing. I find this is also very helpful when focusing for AP at 10x live view on my DSLR screen so my Bahtinov pattern isn't hopping around like crazy.

#15 Starman1

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Posted 16 February 2013 - 06:25 PM

The two reasons for upgrading are: image shift when focusing with the stock focuser, and wanting finer control during focusing. How much finer than the stock focusing would a single-speed Crayford focuser be? I suppose it may vary per manufacturer...


In your scope, every millimeter of mirror travel results in 5mm of focus travel, so the focus knob is a "coarse focus".
With a rear-mount focuser, every millimeter of focuser travel results in 1mm of focus travel, so even a 1 speed focuser is 1/5 as fast in focusing--a "fine focus" knob.
A 10:1 2-speed arrangement is hardly worth it for a scope with such a large depth of field, though most rear-end focusers are 2-speed now.
If slowing the focusing down is important, a 2-speed replacement focuser for the standard focuser knob is all you need. This replaces the standard knob, and does not add anything to the back end of the scope to increase the f/ratio.
You see, adding the rear length of the focuser increases the f/ratio of the scope and narrows the field of view. For visual use, you want the back end to be as short as possible, not longer. Plus, these focusers are oriented to imaging, so they have very little travel--you STILL end up using the standard knob for coarse focusing.

If you aren't photographing with the scope, why add a rear focuser? Slight image shift when focusing doesn't prevent very tight focusing, and the replacement focuser knob system somewhat reduces it anyway. You can also reduce image shift by running the focuser from one end to the other a couple times about once a week to keep the grease in between the two primary baffles evenly distributed.
Image shift is an issue for the astrophotographer (putting the image on the chip just so), but it's really not an issue for the visual observer.
I slowed down my focusing with a custom-made 2.5" focus knob, but the various 2-speed focus knobs available now work very well (JMI, Starlight Instruments) and enable extremely fine focusing, even if there is a little image shift. I owned an SCT for 11 years, and the image shift never bothered me because it didn't interfere with focusing. But the larger focus knob did make finer focusing possible.






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