Now that I have fabricated an acceptable environmental housing to shield my permanently set up mount and scope from the heat and wild weather of Laos while not observing, I am moving on to the next stage of set up.
Now trying to set up a Tak EM200 mount on my pier, and match it to my TOA130. The OTA is attached with the standard Tak clamshell.
This is the first time I’ve considered the cone angle with this arrangement.
What I have noted however, I have found confusing.
In my alignment process: (with the luxury of being in the northern hemisphere)
1. I have adjusted the EM200 so that the polar scope is centred on Polaris.
1a. I have confirmed that as I rotate the mount, Polaris remains in the centre.
2. With the mount at top dead centre, I have inserted shims as necessary in the clamshell to bring the scope to point to Polaris. ( with dec trim as required to centre )
3. Now for the problem! (I’m using a 12mm illuminated reticle eyepiece.)
When I rotate the mount 90° to the right, Polaris appears to be too high, ie, about half way from centre, to the edge of view on the RHS.
When I rotate back to TDC, Polaris is centred as before.
When I rotate the mount 90° to the left, Polaris again appears to be too high, ie about half way from centre to the edge of view on the LHS.
At TDC, Polaris is centred!
That cannot be, to my reckoning.
I have repeated this process several times with the same result. I cannot understand the mechanics of how this can be.
Hopefully someone smarter than me may be able to explain?
It suggests there may be a bow in the axis of rotation.
Maybe if I crank the gears so that I make a 90° shift in the mechanical shaft position of my current TDC, it would change?
Brian,
When you rotate the mount left or right the position of the pole star up and down is controlled by the Dec. not influenced by the core angle etc.
What happens when you use the Dec axis left/ right to bring the pole star back to the centre? Is it still OK when the scope is sitting directly above the mount?
Check that first before we look are other solutions.
Ken
Hi Ken,
No, I understand that the Dec doesn’t affect the cone angle. The deck is only used to bring the alignment star back onto the (effective ra) line, as there is a little slop in the dec plane.
So, no, with the scope at TDC, I get the star in centre of view, but as I rotate the mount either side, the star appears to move higher in the screen. - suggesting the OTA is physically pointing toward the centre of rotation, since I am viewing without a diagonal in the optic train.
What I am seeing is like a an elliptical trace, flattened toward TDC, and elongated at either side. I see the same through the finder scope also.
Brian,
I understand.
Just as a double check what happens when you align the mount to the pole star. With the telescope say to the west?
Is the pole star still central with the scope above?
In the east is the pole star back-in the centre?
Hi Ken,
I can give that a try tonight if the cloud gods are willing.
At the end of last night’s viewing, I did not home the mount. Rather, I powered down and slipped the clutches. Hence the relationship between the RA ring gear and the mount body has moved by about 90°.
Theoretically it shouldn’t make any difference, but!
Cheers, Brian
Brian,
Just to make sure I fully understand. When the mount is rotated 90deg either to the east or west of the pier the pole star appears to have moved in Dec (north or south?)
The 90deg rotation is in RA……
What is the extent of the movement? Seconds? Minutes? Degrees!!!!??
If it’s not an issue with the cone error, then I think it could be that the RA and Dec axis of the mount are not at right angles to each other……
(I’m trying to model the issue with my mount.)
Hi Ken,
No. It is not a Dec issue. When adjusting for cone angle, moving the dec serves no purpose except to bring the target star to the middle of the RA target line for viewing.
With the scope at TDC, I try to bring the target star to the centre of view. I do this with two adjustments. The critical one is by physically shimming the OTA to point up, or down as required to bring the target to the middle height of view. (Trying to make the line of the OTA exactly parallel with the axis of rotation of the mount. Dec adjustment aligns the LR direction, shimming aligns the up down)
The dec is only adjusted to bring the star left or right into the field of view.
When rotating the mount 90°, the effect of shimming now affects the apparent L-R position of the target star.
What I expect to see when I rotate the mount to -say the right, is the target star appearing a little to the right of centre, and when I then rotate the mount to the left, I see the target appearing also to the right. The difference between the two positions needs to be adjusted with shimming so the L and R become the same.
Then, ideally, with the target star aligned through the polar scope to be in the centre of rotation, through the OTA, the target star should also appear to be be in the centre (with appropriate dec adjustments) regardless of the angular position of the mount. TDC, left 90 or right 90, and everywhere between.
Hi Ken,
Yes, terminology seems a barrier. But, when I’m referring to left or right, I’m referring to rotation of the RA axis around the centre of the mount. My reference to TDC is with the OTA at the 12 o’clock position, the left and right are with the OTA at 3 and 9 o’clock around the mount axis.
None of my attempts at resolving this issue have been fruitful with the assumption that the mount is internally aligned as one would expect. Given the Conan-Doyle approach, I’m becoming convinced there is something amiss within the mount. I am not the first owner, but assumed all was good when I acquired it. In retrospect I have had a number of possible issues attributable to the mount, but could not positively identify them. I moved on from the Tak to a PMX and problem solved. But now, given the poor seeing in Laos, the PMX is wasted, so I’m looking to step back down to the EM200.
I have used this OTA with the same clamshell on a EQ6 mount and resolved cone error misalignment without issue.
My plan is to substitute the EM200 with my EQ6 , using same clamshell and OTA to isolate the source of problem.
Thank you Ken for your efforts so far.
Cheers Brian
