Is this normal ?

[Somnium]

Just wanted to check with the collective IIS wisdom about this. whenever i run calibration on my autoguider when it is close to the SCP i always get one axis longer than the other, as seen in the attached. however, when i am imaging closer to Dec = 0 deg, the length of the arms are the same. is this normal or is there something amiss ?

FYI, the X axis in this is aligned to the RA axis and the Y axis is aligned to the Dec axis

[Joshua Bunn]

Yep, Quite normal Aidan. :)

[Somnium]

cool, thanks for that. i would have thought a 40 arc second move (which is what this is supposed to be) would be the same no matter where your scope is pointing ... i am struggling to visualise why this would be the case.

[Joshua Bunn]

Im guessing, the RA rear made a move of 40 arc sec, but because you're pointing at the pole, the arc traced in the sky is less than what the same 40 arc sec of RA gear movement would be if aiming at the celestial equator. I think it just comes down to the fact that the distance around the celestial sphere is less at the pole than it is at the equator. I remember seeing different lengths on my TSX calibration graph.

Josh

[PRejto]

I would question the wisdom of doing a calibration near the pole! With TSX this is unnecessary. I believe the advice from SB is to calibrate near DEC=0 and let the software provide the scaling when imaging in different parts of the sky. I have never seen any benefit to re-calibrating just because I changed target.

Aidan,

How are you going re the issues you posted about in a different thread a while back?

Peter

[Joshua Bunn]

Good advice Peter!

[AlisterNickson]

good advice Joshua!!

[rally]

40 arc secs is less than the full range of atmospheric refraction across the sky in any case.

[Somnium]

i managed to get down to the observatory over the weekend and do some of the tests that you suggested. i pushed the telescope and it bounced back perfectly. i tested the balance and made some adjustments to the RA balance. to get the dec balance perfect in all positions i will need to put a weight at the end of the OTA, something i dont have at the moment but i am sure it is within tolerances. i am going down again this weekend and i am looking at the worm gears. however, when i was down there i wanted to use the opportunity to take some images of the planets. and oh, boy. the seeing conditions were so bad that i could only just see the bands of jupiter, otherwise it looked like a big blob. Similar for mars and saturn. i carefully focused but couldnt improve things, when i noticed that europa was bouncing around like crazy, extremely fast, i realised that was a significant issue and i wonder whether that could be contributing to the issues i am getting. this was on a night where the sky was unbelievably still. i also wonder if this is true atmospheric issues, or boundary layer seeing issues from the mirror. i am going to take a small fan to run some more tests but right now i have not gotten to the bottom of anything really. :(

[PRejto]

Hi Aidan,

Well, you are making some progress!

I trust that you did use a reticle eyepiece to test the bounce back and it perfectly centred in both axis. If so, great. Did you try the small slews test of a few arc-sec in all directions? If you see any backlash this could really mess up guiding especially in declination. (probably less so in RA as the gear is always engaged turning in the same direction. But, any RA backlash would possibly give you a problem right after an east slew.

Don't know what the bouncing up/down might be other than to remark that large scopes are much more prone to atmospheric motion. After looking through refractors of 5-6" most of my life I was quite horrified but what "in focus" looked like trough a 12" SCT. Are you thinking the bouncing might be vibration in RA? If so just turn off tracking for a few seconds and see if there is any change. Good luck!!

Peter