On Sun, Jul 25, 2021 at 02:35 PM, <pcboreland@...> wrote:
Paul,
I'm somewhat mystified why you set your TVC value to zero when imaging. And what do you mean calibration in both directions?
The purposed of TVC as I understand things is to nullify Dec backlash. Just as an autofocuser has a similar backlash correct facility. If your guide rate is say 1s, and your measured backlash greater than 1s and TVC = 0, then you are unable to make a Dec correction before the next pulse comes along. This will surely leave to over correction. If backlash is say 2s, then using TVC it's possible to zero out the backlash entirely I've found. However, PHD2 cannot issue pulses shorter than 20ms, so in practice I think it would be best to shoot for a backlash of 50ms. This value of 50ms though cannot be measured it seems by the guiding assistant, but has to be determined adaptively. I look at the value after about 1hr of guiding. In the guide log I posted the backlash was measure at below 20ms so I'll need to reduce the TVC value. My value of 70 is equivalent to 17.5 at the 0.56 arc-sec step rate. I am interested in seeing where I end up. One thing is clear I'm not touching my Dec worm from her on. It's locked down:)
Peter
Peter, there are some things that are too good to be true :) TVC is useful for visual, but more often than not results in sub-par guiding performance. The reasons are complex, but here are some, from my experience:
1. TVC/backlash is needed only when reversing DEC direction
2. Reversing DEC direction is a pretty complex mechanical operation. There's change in torque, change in speed, change in motor duty cycle to speed up and slow down, change in all between all the gears and slack in-between them. Remember, you're reversing the direction of your entire telescope and counterweights -- this is a hard process with a lot of variables. The heavier the scope, the harder this becomes.
3. With any amount of polar misalignment (even a tiny one), your DEC axis will be mostly lagging behind or getting ahead. If your guide corrections are not excessive, you'll very rarely need to reverse DEC corrections. You just need to keep correcting at the same rate and in the same direction
3a. If you have to reverse DEC direction frequently, you're chasing seeing or some mechanical vibration in the mount. In both cases, I'd rather not have TVC on (see below)
3b. When and if you do need to reverse DEC direction, I'd rather the mount not do so quickly. A little uncorrected backlash is actually a good thing in this case, since the mount will not switch directions Instantaneously. If the direction switch is caused by a passing air cell or tube current, I'd much rather not try to correct for it.
4. As been noted before, the amount of backlash varies with the physical OTA position and weight distribution. You're just as likely to undershoot (which is OK) as overshoot (not OK) with any specific TVC setting at different positions in the sky
If you ever try to guide with one of the DEC directions switched off (the one that's not needed for correcting for polar misalignment), you'll see why TVC is really not necessary. You mostly don't want to, and mostly, don't need to reverse DEC directions, at least not frequently. You also don't want to correct with a full aggressiveness of the guider to avoid overshoots. For all these reasons, I prefer to leave TVC at zero.
And no, I never measure backlash from a guide log. There is too much noise in the data guiding for me to trust the results. Measure backlash directly, using a guider and a watch, or do a guider calibration or special tools that exist for this purpose.?
Regards,
? ? ?-Paul
Paul Kanevsky