SharpCap Forums

If I want the script to refocus every half hour,
is it possible for the script to refocus on the planet,
or should it refocus on a star(s)?

Hi,

refocusing on stars will be much more reliable... Focusing on a planet works, but the signal is often quite small and other factors (variable wind strength, thin cloud/haze, etc) can also lead to fluctuations in the focus measurement that might be enough to hide the signal that the autofocus tool is looking for. By comparison, focusing on stars using multi-star FWHM is far more robust.

You could have a sequence that periodically did this

* Increase exposure to 2s (possibly adjust gain too)
* Focus on stars using multi-star FWHM
* Plate solve + sync to correct any position drift
* Goto the target planet again
* Put the exposure/gain back to the original values

Even given this approach, unattended planetary imaging is going to be a challenge - deep sky may need more expensive equipment, but I think it is easier to automate.

cheers,

Robin

Dear Robin,

Thank you for explaining about re-focusing on stars being
better and for suggesting an outline procedure.

What do you see as the main challenges for unattended
imaging?

I have bought one of these but have yet to be able to
evaluate it properly.
https://www.firstlightoptics.com/other- ... ector.html

It is really kind of you to engage with me in this way.

God bless you
Sean

Hi Sean,

I think the problems are really the normal ones of any astro-imaging

* focus
* camera settings
* pointing

However, they are all made harder by the fact that you are operating at about f/20 to f/30 rather than f/5 to f/7. Smaller field of view, less distinct signal in the focus measurement, etc. Everything has a bigger chance of just not working, either repeatedly or just every now and then. It's probably worth working up to fully unattended via a remote control stage where you try not to go to the telescope and make all the required options using a separate PC via remote desktop, etc. That should help you find the snags and gotchas.

cheers,

Robin

Dear Robin,

Thank you. Your approach sounds good.

I think my first experiment may be to try to replicate the
findings of this chap who reckons that the focus varies
linearly with temperature.
https://www.youtube.com/watch?v=Jyk8i64Gl9A

Then the obvious thing to do is to set up a small computer
that reads off the temperature from the tube
and adjusts the focus accordingly between videos.

If this works, then, as well as simplifying focusing, it
might avoid the need to keep repointing the telescope.

Hi Sean,

yes, focus temperature compensation is a feature of some ASCOM drivers for electronic focusers. It is likely to be roughly linear as you are (largely) compensating for the expansion/contraction of the materials of the telescope as the temperature changes. Ideally you would want a nice observing session where the temperature varies and you can record the focuser position for a range of temperatures to get the calibration worked out.

cheers,

Robin

Dear Robin,

Wow! That's brilliant. I had no idea that this was possible, thank you.
Looking at other posts on the forum, I see that you have said that
one can turn it on (after calibration) in Sharpcap. I also see
that my ZWO EAF does not do this.

Please are you able recommend a focuser compatible with Celestron 8
and/or Celestron 14 tubes?

Hi Sean,

you could investigate the Pegasus Astro FocusCube for SCT (https://pegasusastro.com/products/focus ... -for-scts/) . I have 2 Pegasus motor focusers (but not the SCT version). They are very good, but do require a 12V supply in contrast to the ZWO. There is a temperature compensation option in the ASCOM driver (but again, this is something I have not used).

cheers,

Robin

Dear Robin,

Thank you. This sounds like a good plan.
I am talking to my dealer about this.

Dear Robin,

After discussions with Damian Peach, it seems that to get
the best results, you must recollimate every time you adjust
the Atmospheric Dispersion Corrector to adjust for altitude
changes unless the target is high in the the sky in which
case you don't need the ADC.

Not for the faint hearted!