Autofocus with mirror lock on SCT

[jimipage77]

I am setting up a Meade LX200R 8'' SCT with mirror lock and a orion guidescope on top of it and want to do automated imaging.

I have always imaged with refractors and now I am trying an SCT, I have a question. I understand the mirror has to be locked after focusing to avoid focusing shift during the night. If mirror is locked, no further focusing can be made right? How does people do focusing during the night with an electronic focuser?

[Octane]

Maurice,

My understanding would be that an electronic focuser is a device which hangs off the back of the SCT. Much like how people retrofit Crayford focusers on the backs of SCTs. Once they've locked the main mirror, then, all focusing is done via the Crayford.

Regards,
Humayun

[GrampianStars]

G'day Maurice
The Meade 1209 electric focuser is not for remote use
http://www.mapug-astronomy.net/AstroDesigns/MAPUG/MtrFocus.htm#meade1209

IMO there's only 1 recommended automated focus
via robofocus controlled by software
http://www.rogergroom.com/astronomy/review_jmi_robofocus

[marki]

You will need to fit a crayford focuser to the back of the SCT (I use a moonlite with stepper motor control and a moonlite controller). Set the crayford about halfway then rough focus with the main mirror. Once you are reasonably close lock the mirror and use the crayford by eye or with autofocus routines e.g. Maxim DL if your setup allows.

Mark

[citivolus]

There 1209 can be remotely controlled. While it doesn't allow you to predict a given focus setting as it does not know its absolute position, nor does it offer automatic temperature compensation, it does have the ability to reach focus using software auto-focus routines. It still has significant advantages over focusing by hand.

Additionally, the 1209 is a Crayford style focuser, which virtually eliminates backlash. It has a relatively short travel distance (I recall around 15mm), meaning you can set the primary mirror focus position, lock it, and have enough play to maintain focus throughout an evening; however if you want to switch to a terrestrial target, you will need to refocus by first unlocking the primary and then do the rest by hand.

Microfocusers with absolute value readings and temperature compensation will be of little use to someone outside of an observatory setting, as they need the primary to be locked down and then calibration run. If you move your primary, the calibration values will be useless.

Regards,
Eric

[Merlin66]

I use the #1209 Zero Shift focuser remotely on my 12" and 10" Lx200's.
The ShoeString USB adaptor works very well!

[AndrewJ]

As others have commented, the 1209 can be remotely controlled
however re

The focus lock wont prevent focus "shift" as it cant stop thermal effects
The main thing the mirror lock is supposed to do is address mirror "flop"
as the tube passes through vertical.

Andrew

[Merlin66]

There may be some minor thermal effects, but if the mirror is locked the zero shift focusers seem to do a good job!