Manual ‘Rotator’

[jif001]

I have a Sharpstar 61EDPHII that has a built-in manual rotation mechanism, which is great for altering the orientation of different targets within the frame. Unfortunately there is no scale or reference points that allow the same orientation to be re-set next time I go for a target I’ve imaged before, and keeping track of different orientations used for different targets is difficult. So, I fixed that …

Using the NINA framing assistant I platesolved the current field of view and asked for NINA to tell me the camera orientation. By altering the rotation of the scope and repeating the platesolve / orientation query I eventually got the camera to zero degrees orientation (perfect precision is not necessary). I marked this zero position on the OTA. 

By measuring the diameter of the OTA I could calculate the circumference in millimetres (you could just measure it directly with a tape measure - again, perfect precision is not necessary). Since the full circumference covers 360 degrees I could then calculate the number of millimetres per degree and established that 10mm of circumference is near-enough 15 degrees. So, starting from the zero position described above, I marked the tube at 15 degree (10mm) intervals up to 180 degrees. By aligning the appropriate number with the datum mark I can now ‘dial-in’ any orientation I need. Using the NINA framing assistant I can determine the orientation I want using the orientation slider and then set that on the scope. 

The markers were made using a label printer, each one individually printed and then overlapped slightly to get the correct 10mm spacing. You could easily do the same with masking tape and a pen. 

One thing to be wary of is the need to avoid physical clashes as you rotate the scope. I’ve had to position the dovetail in the clamp so that I can get the full 180 degree rotation without the EAF clashing with anything on the mount. This has resulted in the arrangement being back-heavy and unbalanced in dec but with the limited use I’ve had (about 2 hours) due to our customarily poor weather, it seems to guide and track ok. With this mount (EQM35) some dec imbalance can be helpful in managing backlash, but If it proves to be a problem I’ll investigate adding some weight to the front end of the dovetail.

It’s important that if the setup is ever dismantled it can be put back together again in the same way (I occasionally switch the camera and filter wheel onto a Samyang 135) and so additional alignment marks have been added to achieve this. These allow me to align the filter wheel with the focus tube markings, and align the camera with the filter wheel so that ultimately everything is properly aligned.

 

Edited May 15, 2023 by jif001
text colour corrected

[pipnina]

I've attempted something similar. My focuser has a built in rotation feature but sadly it slops when not done up so tight, that it won't rotate any more 😕 (3 brass screws allow rotation, another one thumbscrew locks and unlocks rotation, the brass ones need to be done up every time it's moved for some reason)

Glad yours is working however! That looks like roughly what I wanted to achieve with mine!