Affiliation
American Association of Variable Star Observers (AAVSO)
Sat, 08/17/2024 - 14:09
Hello everyone.
I need to know the path length of the Meade LX-80 8-in SCT primary mirror.
In other words, using the manual focuser, the distance between the position of the primary mirror from the most retracted to the most extended focus.
I need this measure to calibrate my electronic focuser.
To obtain this distance I would have to remove the corrector plate and then use a ruler to measure it, which I would not risk doing if I can avoid it.
Perhaps this distance is already factory-specified or has been measured by someone.
Yours,
Ari Siqueira
(SAMA)
I would ask on Cloudy Nights forum for SCTs.
Peter
What model of electronic focuser are you using?
Phil
Hi, Phil,
It's a homemade, DIY, electronic focuser. It has a step motor with a reducer, it grabs the standard Meade SCT focuser knob and rotates it, via a hand controller, or its control software (from a PC), or an ASCOM-integrated software. It integrates with MaximDL, or similar ASCOM-compatible software packages.
The path length value is needed to calibrate the essential focuser parameters: step size, max step, max increment, actual position, park position, etc. As I said, I could disassemble the OTA to figure out the distance traveled by the primary mirror, or I could guess it from outside the tube, but if the dimensions were known beforehand I wouldn't need to remove the corrector plate or the guesswork.
I would post a picture of it here but I don't see the "attach files" button in this forum thread today.
Ari
Isn't travel versus knob rotation specified in the Meade manual?
If you remove the corrector plate, put it back on with the same orientation. Mark it before you remove it.
Alignment after the re-install is easy if you do it out of focus , making the donut evenly illuminated.
Meade optics show oblong stars that are orthogonal on either side of focus.
Ray
Hi, Ray,
No, the manual doesn't say anything about it.
So, I'll take the corrector plate out. I've done it once, so it shouldn't be a problem unless I drop it. :-(
Thanks.
Ari
Hi, Ari:
Before you pull the OTA apart, maybe it's worth revisiting your ultimate goal. I bring this up because knowing how much your mirror moves per step and the total mirror distance traveled doesn't match the motion of the focal plane in an SCT. The secondary mirror "magnifies" the motion of the primary mirror into a larger change in the location of the focal plane.
I, too, have a homemade box that turns the mirror focus knob on my SCT (what I call my "coarse" focuser). I also have an ESATTO focuser in front of the camera (my "fine" focuser). The optics "math" for the two focusers is completely different. When you move the mirror, in addition to moving the focal plane, you also change the f/number of the telescope. A one micron change in mirror position results in a much larger change in focal plane position. But, a one micron change in the ESATTO focuser moves the focal plane one micron (relative to the camera).
You should be able to count the steps between "against the stops" at one end of mirror motion to "against the stops" at the other end without taking the OTA apart. You can estimate where the focal plane is with an eyepiece at the two extremes, and just divide by the total number of steps to come up with a reasonable estimate of the distance the focal plane moves with each step of your focuser.
- Mark M
Hi, Mark,
Your instructions are sound and wiser than risking an accident with my corrector plate or compromising the collimation.
I'll follow them exactly as you described.
Thank you.
Cheers,
Ari
(SAMA)