Help with phd time to calibrate?

[SimonfromSussex]

The old saying goes if it aint broke don't try and fix it, however the time it takes phd to calibrate for me is possibly too long?

Once it has calibrated everything runs absolutely fine and I can guide ten minute subs no prob. but it normally takes 20-30mins to initially calibrate, is this too long?

The other night I calibrated in the eastern plane, took 4 subs and then hit the horizon. Had to then re-calibrate in the western plane and consequently lost nearly an hour of imaging time calibrating twice!

I'm using a modded Phillips web cam but in normal mode, through a finder scope (straight through).

I'd be really interested to know how long other people take to calibrate using phd, especially if you have the same set up as me?

I'm wondering if a better guide camera would help?

Cheers

Simon.

[Freddie]

I calibrate in about 12 steps per diection so it's all done in about 5 mins. How many steps does it take to calibrate on each axis for you. If it is too many, you need to adjust the step duration by clicking on the brain and adjusting in there.

[SimonfromSussex]

Thanks Freddie, usually 40-50 steps for me!!!

I'll have a play with the step duration, cheers.

[chrisshillito]

Hi Simon,

In order to calibrate PHD has to see a certain amount of pixel movement. You can increase the amount of movement observed per calibration pulse by either guiding with a higher magnification or by increasing the guide rate applied during each pulse. That said I wouldn't advise changing guide rate just to satisfy calibration - calibration is only intended for PHD to learn about your setup, not to define it. What guide rate do you currently use and how long a calibration step have you set?

You might try using a longer focal lenth for the guide system - if you can, slip in a barlow.

Chris.

[jgs001]

There's an option in the PHD brain (Calibration Step), what is that set to ? For my ST80, I had to increase it significantly.

[bamus]

for finder scope value for Calibration Step should be around 2500 to 3000 (higher when you calibrate on star with declination higher than 50 deg and lower when you calibrate close to dec 0)

[SimonfromSussex]

Thanks everyone, currently my calibration step is set at 1500.

I will bump it up to 2500/3000 and give it a go.

5 min calibration would be brilliant!!

Many thanks again.

[chrisshillito]

Simon,

Increasing the calibration step isn't going to speed up the calibration. It will cause the calibration to complete using a fewer number of steps (pulses) but this is countered by the increased duration of each pulse (which is what calibration step sets).

Calibration step adjustment is only there for those situations where you cannot get enough pixel movement for calibration to complete (phd will give up after 60 steps or so) .

If you want to speed up the movement as opposed to changing the amount of movement then you need to change the pulse guide rate or use greater magnification.

Chris.

[Shibby]

Also, after a meridian flip you can just hit the "flip calibration data option" and you're away!

[bamus]

Simon,

Increasing the calibration step isn't going to speed up the calibration. It will cause the calibration to complete using a fewer number of steps (pulses) but this is countered by the increased duration of each pulse (which is what calibration step sets).

Calibration step adjustment is only there for those situations where you cannot get enough pixel movement for calibration to complete (phd will give up after 60 steps or so) .

If you want to speed up the movement as opposed to changing the amount of movement then you need to change the pulse guide rate or use greater magnification.

Chris.

hi Chris

is calibration less accurate if you are performing it with 15 steps only and with less amount of time (by the means of increasing calibration step)? I think I don’t fully understand what you are saying, but increasing calibration step always helped me to increase calibration time with short focal length scopes.

[chrisshillito]

hi Chris

is calibration less accurate if you are performing it with 15 steps only and with less amount of time (by the means of increasing calibration step)? I think I don’t fully understand what you are saying, but increasing calibration step always helped me to increase calibration time with short focal length scopes.

Changing calibration step will have no impact on the accuracy of calibration. In order for calibration to complete PHD keeps issuing guide pulses until the star has moved some fixed percentage over the image width/height.

Calibration will fail if this doesnt occur within 60 pulses (or steps in phd speak). The distance the star will move with each pulse is determined by the guide rate applied and how long it is applied for (calibration step). If you set a low calibration step the the pulse is short, the star doesn't move much, and so phd will need to issue more pulse to achieve the required calibration distance. This isn't a problem provided the movement is completed within 60 steps. Conversely, if you lengthen the calibration step each pulse will be longer so fewer will be required to move the star the required calibration distance.

Increasing the calibration step should not speed any thing up, only the distace the star moves with each step (pulse). The speed (in terms of pixels/sec) at which the star appears to move is dependent on the guide rate and the stars declination. With increasing declination the star will appear to move smaller distances and you may need to increase the calibration step to ensure the required calibration movement is achieved within the 60 step limit.

Chris

[SimonfromSussex]

Thanks again everyone for the help.

Finally got a clear night last night to try the advice out.

Upped the calibration steps to 3000 and it calibrated in about 22 steps, much quicker and the guiding was good, excellent!!