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PHD signel to noise, and star not moving enough


DaveS

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I *think* this belongs here, but could equally be under "cameras" or "'scopes", or even "mounts"so if the mods want to move it, that's fine by me.

When trying to frame a guide star in PHD using the kit in my sig I rarely get a snr above 6, usually it's 5-5.5 or so with the most I've seen of 9. I'm thinking this is very low, so what do others do who are imaging from light polluted sites?

I've also been getting errors of the type "calibration failed, star did not move enough". Could this be due to low snr, or should I be looking elsewhere? The mount perhaps?

I said it was messy, and not fitting into one single category, sorry.

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Hi Dave,

Not sure about the SNR, but so long as you have a star in the green box it should work!

The star not moving enough is easily solved by changing the 'Calibration Step' value found by clicking the brain icon. I use a value of 1400 with my ED80 and 400 with my C8 & reducer. So, have a play with that figure and you will quickly see what a difference it makes. I think you should aim for between ten and twenty steps for good calibration.

Good luck!

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Can try defocussing the guide star a bit, also make sure you're calibrating on a star not a bright pixel, slew the mount with the hand control, if it moves it's a star if it doesn't it's a hot pixel.

Alter the calibration steps to make the guide square move more, not by a little bit, crank it right up and experiment,  it won't do any harm, try to get it to calibrate in around 15 steps, depends on the scope / camera combo and also how high in the sky you're imaging, best to practice around the " middle" of the sky.

Dave

Another thought, do you take a dark frame ?

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Thanks. I hadn't thought of changing the Calibration Step, dur... and it's always useful to use your brain! I'll give it a go, next clear night. I've been seeing calibration steps going up to 60 before PHD gives up.

I'll also check that I've got the chip aligned to RA and Dec, I think it is, but may have moved.

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Thanks. I hadn't thought of changing the Calibration Step, dur... and it's always useful to use your brain! I'll give it a go, next clear night. I've been seeing calibration steps going up to 60 before PHD gives up.

If you're using the ST80 as your guide scope, try increasing the calibration step to 2000 and see how many steps it takes to calibrate.  I've read in various places that you should aim for calibration to be achieved in 10-15 steps.

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If you're using the ST80 as your guide scope, try increasing the calibration step to 2000 and see how many steps it takes to calibrate.  I've read in various places that you should aim for calibration to be achieved in 10-15 steps.

Yes, as advised in the paper written in the US astronomy magazine. 10-15 steps. Works for me. Calibration step makes a big difference through my Lodestar so worth experimenting.

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OK, now waiting for a clear night to put theory into practice.

What sort of snr are people getting? Is 5-6 too low?

I'm currently guiding live as we speak.  I was using a 1s exposure time and getting SNR of around 6 with no problems.  I've since increased the exposure to 2s and getting an SNR of around 10 - I've done this to smooth the guiding a bit due to the odd wind gust. 

Again I've read that 2-3s is probably a better exposure rate as with anything lower you tend to 'chase the seeing conditions'.

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Re:

....

What sort of snr are people getting? Is 5-6 too low?

  It depends on how they calculate the centroid of the star image. Also it would be useful to publish how they define SNR. Normally SNR numbers are quoted in decibels (db). When the signal and noise are of equal power one would say the SNR is 0db.

Is an SNR of 5 or 6 too low?  I would say that an SNR.with a power ratio of 5 to 6 is "huge" ( 7 to 8 db) and there should be no difficulty tracking with excellent accuracy. I would expect to get quite good tracking when the noise power is twice the signal power ... tracking through clouds!!

Jeremy

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So, things to try:

Increase exposure time from 1 sec to 2-3 sec, though at that point my lousy LP starts to make itself felt.

Check the step size PHD is using, and crank it up to 2000 if it's too low.

Try calibration at a good distance from the pole.

OK, now for a break in the clouds. Yeah, right!

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Amazingly it's been clear, so I've been able put my plan into action.

Increasing the step size to 2000 has certainly helped, as has tweaking the focus a little, I'm now getting SNR of 6.5 or thereabouts.

The only thing to sort now is why PHD has a tendency to go deaf, and not "hear" the selected port for the server.

Oh, and PHD2 is a total disaster for me, not only does it hang, it doesn't even close nicely, have to use task manager and give two or three kills.

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Do you take a dark frame at the same length as your exposures ? this can help find a bright enough guide star when there doesn't appear to be one.

Would also increase exposures to 3 - 4 sec's though this doesn't appear to make the guide stars any brighter but helps stop the guiding chasing the "seeing"

Dave

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I'm not sure about increasing the exposure length, as the background starts to intrude too much, but I'll give it a go. Currently using 2 sec.

When PHD loses communication, it's usually after I've had to shut it down. I have to do a restart of the computer in order for it to "hear" the port again. It's not showing up in device manager as it's own port.

I also had another crash last night when windows reported that a USB device had stopped functioning. However I think this may be a computer problem as my USB mouse on its own port also failed. Plugging in a new hub also had no effect. Oddly the mount was still connected through the EQDIR cable, but that was showing up as a COM port, rather than a USB device.

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I agree with all that's been said above with respect to calibration etc.  In fact to save time I tend to calibrate at around 2 seconds, and then increase the "exposure time" to 5 seconds or more to avoid atmospheric disturbance.  I use inverted commas when I say exposure time because you can adjust the exposure time using the cam dialogue button.  So although I have set an "exposure time" of 5 seconds on PHD, the actual exposure time may be much less than this, according to what I set in the cam dialogue box.

Regarding the USB problem, I also cannot stop PHD2, and sometimes have problems with the connection.  But I have found (using my old laptop running XP), that I have two USB plugs.  One of them seems to be much faster than the other, so having established which is which certainly helps the problem. But I still have to stop PHD2 using the task manager.

Chris

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