Focusmax V5 and multistar: feedback from users?

[Stephane Basa]

Hello everyone


I finally switched to Focusmax V5 and the transition went smoothly. It was an opportunity for me to redo my models properly.


On this occasion, I tried using the MultiStar mode. It does indeed work, but I found the process terribly time-consuming. I understand the reason (it takes a bit of time to get several stars) and I confess to being a little disappointed by the method, which seemed interesting at first.


I'd be interested to get some feedback on this method. Is it used routinely? If so, do you have any particular tips for dealing with the long integration time?

Thanks for your feedbacks.

[Richard Asarisi]

Hi
I use multistar focus exclusively for my system and achieve 1.x arc sec results every time. Are you saying that it takes to long to find focus? my times run about 120 to 250 sec. But what does it matter? Its all automated and it does its job, then hands control back to ACP. I spent a lot of time testing the multi star focus for Steve and spent many nights collecting data and results for both methods. My results clearly showed that multistar achieved better results consistently over single star.

[Stephane Basa]

Hi,

Great feedback, very useful. Indeed, it takes several minutes and I confess that it destabilizes me a lot. Does everyone take so long? If so, it's normal and not related to my system. In that case, I can adapt myself.

[Dick Berg]

Hi Stephane,

I also tested multi-star for Steve. The focus times will vary depending on what's in the field of view (brighter or dimmer stars), how large the camera FOV is, and how large your telescope aperture is (mine is a 12-inch SCT). A larger scope and larger FOV could result in somewhat shorter total time.

In sparse fields, with mainly 11-12th magnitude stars and my 12", it will take as much as 5-6 minutes to complete focusing. I have set the maximum exposure cutoff quite high - 10 minutes.

What I like best about this for me, though, is that it focuses at the target object location - no slewing away to focus is needed. My mount doesn't have the best pointing accuracy anymore. This avoids multiple pointing exposures.

Another plus for me is FocusMax's temperature compensated focusing. I spent a good deal of time a year ago developing a focusing "equation" that I have used with great success since, covering a range of about 20C degrees ambient temperature. This means I focus once at the beginning of the evening and then forget it. Typically the temperature change during the night is usually not more than 7-8 degrees C, and my equation is quite accurate over than range.

[Stephane Basa]

Hi Dick,

What worried me was how long it would take (I have a 50 cm with a FoV of 23 arcmin, which I think is comfortable). But I deduce that this method is long in any case, and that's what worried me. After all, given the method, it's not too surprising in the end, but I needed to be reassured that I was not missing something.


Ah, I'm interested in your method for managing focus with temperature. I saw a post about that. Is that what you're talking about?

[Dick Berg]

Hi Stephane,

Could you please post a link to that post? I don't think it's my post, but I just don't remember. Thanks.

[Stephane Basa]

The post I have in mind is this one

[Dick Berg]

Stephane,

(Believe it or not, I'd forgotten about that ... )

There are at least three ways to do this (if you have a TC focuser, like the Optec TCF-series or something similar).

1. You can follow the instructions in the TC manual to develop an equation that is implemented directly in the focuser EPROM hardware.
2. You can do what I've done - independently take a few nights worth of single images using multi-star focusing, then plot the focus position vs. temperature and get the equation that way. Then implement this directly in FocusMax per instructions there.
3. Using the equation you get in example 2, write javascript to move the focus at the beginning of an image using the ImageStart function in UserActions.

Note: 2 & 3 depend on having some independent way to measure and access the temperature, whether that's from a weather station (may be too far from the telescope OTA) or a sensor attached to the OTA. (Optec has such an animal.)

Do you have something at hand? and something in mind?

EDIT: Attached is a script that could work in UserActions.ImageStart(). I haven't tested this.

[Stephane Basa]

Excellent, it reassures me when I forget something.


Thanks for the advice. In my case, I have a temperature sensor at M1 and one at M2, which should be fine. But I've never made the effort to take 1 or 2 nights to make regular measurements. This is clearly the right thing to do in order to have clean measurements and I'm going to schedule it.

Then, for the implementation, the option 2 is probably the easier and I will also check how to do that on FocusMax.

As always, your advice is very useful and I truly admire the time you spend improving your system

[Dick Berg]

Thanks, Stephane. Let me know if you run into any roadblocks.

If you're going to implement 2, then you only need to include the top section of my script in the UserActions. This will add the temperature and focus position (as found by FocusMax after each focus operation) to the log, which you can then turn into a linear equation of position vs temperature.