Hi, all. after some help with my AVX mount I recently purchased. It's unguided tracking is appalling. To give an example I get start trails at 1 to 1 1/2 seconds and planets will disappear from a 30mm eye peace in about 5 minutes.
Mount is a an early build unit, (6 pin connectors electronics). Mechanically on purchase it was OK., but hadn't been used for a number of years. After my first night with it, where it didn't track, I overhauled both RA and DEC bearings.. RA now has tapered bearing upgrade and you can spin it like a propeller with an 8SE fitted.(scary to watch) DEC is really good. Back lash in both RA and DEC is fine (both spur and worm drives). I am mech engineer by profession, setting up gears is something I am pretty comfortable with.
Set up. This is my first GEM. So getting a southern polar alignment has been a challenge from Perth given how faint the stars are you have to use. Having said that last night after 3 hours I felt confident i had it aligned. Is there some way once you have done a GOTO alignment for it to tell you how close your polar alignment is?
Yes! From the "Ready" prompt, press ALIGN, scroll to Polar Align and press ENTER.
Scroll to Display Align and press ENTER.
The alignment errors in Az and El are displayed.
GOTO performance is fine using a 2 star alignment plus 4 calibration stars, items appear either within a 13mm eye piece or on my camera view area. (canon 80D live view).
Mode - South EQ
I run the mount calibration before use it
Scope: 8SE. all up weight is 14 pounds with camera, 12.5 for visual viewing.
pic is Jupiter and surrounds showing trails for all objects at 5.4 seconds
Let's see... the 8SE has a focal length of 2032 mm and the Canon 80D has an APS-C size chip (22.5×15 mm). This gives a FOV of 0.63° × 0.42°.
A 13 mm eyepiece produces 156X magnification. If the EP has 70° AFOV, the TFOV is 0.45°.
If your go-tos are accurate enough to place objects close to center of these FOVs, then your alignment is good enough to produce good results using the ASPA ("Any Star Polar Alignment") routine built into the hand controller (scroll to Align Mount instead of Display Align above, press ENTER, and follow the prompts, but read the instructions in the manual first). I recommend you try that. Note that ASPA assumes that the mount base is level; if it's sufficiently off level, the accuracy of ASPA's alignment will be compromised, but "reasonably close" to level is likely more than sufficient.
Balance. I played for hours with RA balance it see if minor changes improves it, and it is very touchy to even 5mm movement from neutral in the position of weight as to how fast it tracks off the target. Best I can get is a planet on screen for 15 minutes. DEC balance, I do pencil balance my gear and mark them on my scope, due to the known stiction in the Dec but I did notice last night, after I finished it was a touch out for the configuration. (Dec now spins pretty freely)..
Funny thing is it seems to track the moon fine when I set it to lunar or even in sidereal, both before and post overhaul.
Is there any way to determine/ measure what rate the RA sidereal is rotating at?
Probably the easiest way to confirm the the tracking rate is to point at a star close to the celestial equator and time how long it takes it to drift a known angle. Rotate your camera so that movement in Dec is parallel to one of the edges and RA is parallel to the other and use the angular size of the FOV as a reference. You can check those numbers above by turning tracking off and timing how long it takes a star to drift from one edge to the other.
Approximate formula (star close to equator):
A ≈ t × Ω
where A is the angle of drift, t is time and Ω (Omega) is sidereal rotation rate. Ω = 7.2921150×10−5 radian/second = 0.004178°/second.
More accurately, it's
Angle = t × Ω × cos(Dec)
but if you use a star within a couple of degrees of the equator, the added term is effectively 1, and you don't have to worry about it (and the process takes less time!)
If you turn sidereal tracking back on, the mount is tracking at some rate ω (omega). Ideally, ω = Ω. If you have the wrong hemisphere set in the tracking mode, ω = −Ω.
The angle of drift after a given time t with tracking on becomes:
A' = t (Ω − ω) cos(Dec)
at Dec = 0
A' = t (Ω − ω)
ω = Ω − A' / t
Using your numbers and assuming the problem is an incorrect tracking rate, with an object at the equator moving from the center of your 30 mm FOV to the edge in 5 minutes...
A 30 mm EP with AFOV of 50º results in 68X with 0.74º TFOV. Center to edge is half that: 0.74º / 2 = 0.37º.
ω = Ω − (±0.37º / 300 sec) [± because we don't know if it's too fast or too slow.]
= 0.004178°/sec − 0.001230°/sec = 0.002948°/sec (about 30% slower than sidereal)
= 0.004178°/sec + 0.001230°/sec = 0.005408°/sec (about 30% faster than sidereal)
Neither of these seem very likely.
Backlash compensation has been set, and is good. Mount moves instantly and smoothly in both direction. Really nice in that area.
Is there any other checks I can do etc to see what the problem is? Sending to Celestron from Australia, is not an option!!
What is a realistic expectation for a AVX to track unguided at that focal length? Having said that it I don't expect it to lose a planet like it does now! My one arm Alt- Az bandit tracks better than this thing does.
My first guess would be that you aren't correctly polar aligned and it's drifting significantly in declination. [But a later reply says otherwise]
That's a long focal length. IMO it's not realistic to expect to track unguided for very long at all using an AVX with a FL > 2m. Most likely less than a minute.
The trailing in your embedded image looks like it could be due to shake, not tracking problems, though.
I have no clue why this would not be apparent for the moon, too. The difference between mean Lunar and Sidereal rates is only about 3%. Something is inconsistent here.
[Edit to correct notation error.]
Edited by SkipW, 06 August 2020 - 09:36 AM.