20 replies to this topic

[Scott Beith]

Does adding a Crayford focuser onto an SCT eliminate the movement of the mirror in the OTA when focusing? Essentially does this lock the mirror in one spot preventing image shift?

[Don W]

Yup

[Scott Beith]

Thanks. Need to save pennies for that too I guess.

[Sky Captain]

Hey Scott,
On the Meade any way you have to use the mirror lock knob after it has cooled for about an hour then set the focuser (elect. crayf) to mid position, focus main mirror, use mirror lock and away you go.

Kerry

[Dean]

I would think so. My Meade came with an electric microfocuser - you get the focus close, lock the mirror and use it to fine focus, so I don't know if you would need another focuser.

[Sky Captain]

Just adding the crayford by it self won't lock the mirror, but unsure about celestrons.

If that is what you meant in your post.

[Scott Beith]

Sounds like the Meade has a good focusing system, thanks for the help.

[jrcrilly]

Sounds like the Meade has a good focusing system, thanks for the help.

Well....

Meade's focusing system needed a lot of help. The current model deals with it nicely. Even better is the Moonlite motorized Crayford in Meade blue (my Classic didn't come with the Meade microfocuser).

[Scott Beith]

Moonlite gets my vote for a replacement focuser anyday. I am still thrilled with the one on my CR150.

[Rat8bug]

Does adding a Crayford focuser onto an SCT eliminate the movement of the mirror in the OTA when focusing? Essentially does this lock the mirror in one spot preventing image shift?

Yes and No! The mirror can shift when the weight of the mirror causes the focus mechanism to move, within the tolerance on the focus rod, et.al. Using an auxially focuser at the eyepiece end, allows the focal length to remain constant during focusing. This is an absolute necessity for imaging and stacking photos. One can also use a Borg Helical focuser; which works as well or better than a crayford focuser....

http://www.barrie-ta...stro_photo.html


Ciao...Barry

[Eddy]

Hi Scott,
I agree with Barry. I do have the JMI (NGF-C) and it helps a lot for focusing although it does not lock the mirror. You will eliminate mirror shift because you are able to focus with your Crayford, but gravity might still be able to move the mirror while your scope is tracking.
By the way, since we are talking about Crayford focusers, consider a dual speed (Feathertouch, Moonlite and soon WO). Although I like my JMI very much, I regret sometimes I did not save longer to get a dual speed Crayford focuser. Especially at higher magnifications or when you consider imaging.
Hope this helps a bit.
Cheers,

[Dean]

but gravity might still be able to move the mirror while your scope is tracking

Meades have a mirror lock to prevent this. I don't know if it completely solves the mirror shift problem, but I've done a fair amount of imaging with an LX200 and I've never had the focus shift with the mirror locked.

[melvy]

My Meade 8" had much more mirror shift than my C8 does. The Meade however had the mirror lock so for AP is was a non-issue. The C8 seems to have a much lighter mirror and very little shift, but for AP it is still a little more of an issue (really not bad though.)

[celestial_search]

Scott:

Rod Mollise did an excellent review of the W.O. SCT Crayford focuser. It was entitled "Who Needs a Crayford Focuser for an SCT." See his website.

[Scott Beith]

Thanks everyone. You guys are full of useful info.

[Tim2723]

One extra point of interest: The W.O. Crayford is a beautiful piece of machinery, but it won't fit on anything smaller than an 8" scope (cost me $16 in shipping to figure that one out).

Also, why the heck is Beith asking about SCTs anyway?

[celestial_search]

Thanks everyone. You guys are full of useful info.

Sometimes we are full of other things than info!

[Starman1]

Scott,
Image shift on an SCT comes from 2 sources:
One (the most obvious) is from the space between the outer primary baffle (attached to the mirror) and the inner primary baffle (attached to the tube/cell). Since the screw that drives the mirror forward is offset to the side, this causes the mirror to first tilt one way, then the other, as it moves back and forth through focus.
Two is the play between the screw attached to the focus knob and the pin that is attached to the mirror backing. This screw has a hole in it that fits over the pin, but the hole is often larger than the pin, resulting in "dead space" when the focuser is reversed near focus.

I installed a new focuser on my Meade that eliminated the dead space on the focus screw movement (it actually pinched the mirror pin), but it didn't eliminate image shift from the baffle tolerances.

A slight image shift of a couple minutes either way is really not annoying if you know what's happening and just live with it. But it is a disaster for CCD photography (the image can be moved right off the CCD chip). This is why, I believe, Meade added the microfocuser to the back of their SCTs. Visually, though, the microfocuser is not essential.

If the tolerances on the central baffles are too great, a lot of image shift can occur. Either a new baffle can be machined to close up the tolerances (reducing image shift), or a much heavier grease can be used to reduce the play. You know people who can provide either solution.

Another solution is the addition of a rear-mounted Crayford focuser. I don't recommend this solution for a few reasons:
1) Every SCT's maximum performance, optically, is with just one distance setting between the primary mirror and the corrector plate. Aberrations grow exponentially when that distance grows or shrinks appreciably. This optimum distance is accomplished with a rear optical extension group that doesn't exceed a certain set length--usually with a diagonal only. The addition of the SCT focuser (which doesn't eliminate the need for a diagonal), extends this back distance, requiring a non-optimum setting of the distance between the primary mirror and the corrector plate.
2) These focusers weigh a lot, adding to the inbalance caused by the use of large diagonals, eyepieces, large finders, etc. You can add a sliding weight under the tube, but that weight will have to get fairly heavy to balance a lot of heavy rear accessories, and this will cause more wear on the bearings over the years. If you do photography, I have seen as much as 15 pounds of extra weights to balance the tube with a lot of accessories.
3) The back distance becomes so large the scope will not clear the base. You can set the dec/altitude drive to not allow the scope to approach the place where the focuser will hit the base, but this creates a big hole near the zenith in alt-az mode, and precludes easy polar alignment when mounted on a wedge (though has no effect on a GEM mount).
4) The more things thread together on the rear of the scope, the more play there is in everything. A friend with a 12" Meade would collimate his scope perfectly in a "straight-through" method, then attach focal reducer, Crayford focuser, diagonal and eyepiece to the back to discover the "sag" in all the elements took the image out of collimation. By trying a variety of combinations, he discovered that the most sag was in the Crayford focuser, so eliminating it essentially solved his miscollimation.
5) Last, but not least, unless your eyepieces are parfocal, the SCT Crayford focuser focuses too slowly.
When I would go from a 22 Nagler, to a 14mm UWA, to an 8.8mm UWA, the focus had to change appreciably (using them on my dob, the drawtube moves over half its total throw). It was easy to accomplish with the mirror-moving focuser, but took forever on my friend's SCT Crayford.
To make a long story short, he gave up on it, and I never bought one because of his experiences.
That they work is not deniable.
That they are necessary or even desirable is.
IMHO, of course.
Just though you should hear the other side.

[Scott Beith]

Tim,
I am on a 3 year quest for aperture.

Frank,
I agree. :o

Don,
I will try whichever SCT I decide upon (Celestron or Meade) in stock configuration, then decide which accessories to purchase. I may find the $$$ is better spent on something else, or I may eventually decide to go the Crayford route. I won't know until I have quite a few nights under the stars with the new scope. Your advice is sound and I appreciate it. I never accessorize a scope before I know what really needs to be improved. My Yard Cannon was a work in progress for well over a year - as little things popped up, improvements were made. Any other way of deciding purchases could result in wasted $$$.


Thanks again for everyone's help.

[Rusty]

There are two kinds of mirror movement (and since I've not discovered comprensive definitions of either, here are mine):

1. Mirror "shift" is the movement of the image when changing the direction of focus. Celestron, for example, sets their scopes up to be aligned when "backing off" focus, i.e., turning the focus knob counter-clockwise.

2. Mirror "flop" is caused by gravity, where the mirror moves slightly due to the change in altitude of the OTA.

The former is most noticeable in visual, the latter, in imaging (where nobody's fussing with the focus).

Focus-enhancers using the SCT's stock focusing system won't cure either problem. An outboard focuser such as a Crayford, will minimize the effect, since the mirror can be "snugged up" to minimize "flop", and of course, "shift" will no longer be an issue...

[Sky Captain]

I have always thought that a crayford style (may be 8 minature bearings) mirror movement system would be best to remove any mirror flop.