Temperature compensating focuser

[E_ri_k]

Hey all, I have been absent for a while due to work commitments, but hoping to get some imaging time soon! I am still looking at my options for a focuser upgrade kit.

I guess this question goes out to you guys who use temperature compensation focusers. I have noticed my scope is quite sensitive to temperature differences, and it's annoying having to refocus every 2nd frame.

So before I spend any money, once calibrated, should I expect it to keep focus for the whole session, or is that wishful thinking!

Erik

[acropolite]

The Sharpsky focus controller and motor has excellent compensation capabilities.

[Chris85]

As I understand it, yes, that's the way it should work. I guess you'd refocus with filter changes, but maybe not if you use offsets? I've yet to try temp focusing myself, but I do have one of these on the way http://www.usb-foc.us/ to help me get started.

Cheers,
Chris

[PRejto]

The only way temperature compensated focusing can work is if the change in focus is linear over the range of temperature you hope to image in. And, the only way to find out is to actually graph the focus change vs temperature to confirm. Many scopes are not linear. I know that FocusMax V.4 has a wizard that will plot focus position vs temperature over many hours, and I think the Optec controller FocusLynx will also do something similar. The other critical issue is the location of the temperature probe. Microtouch puts the probe inside the controller and that is utterly useless (stupid! actually). Optec offers a kit with an external probe that will operate a Microtouch motor but only if you also use the Optec hub. Of course, it will operate with Optec and Starlight Instrument hub controllers/bipole motors.

Peter

EDIT: This is rather interesting. http://www.innovationsforesight.com/education/real-time-autofocus/

Scroll down to this section: "Focus versus time analysis using SharpLock"

You can see how non-linear most of the plot is. Of course, every excursion would mean that temp-compensated focusing would fail. Naturally every system is different so you might find your scope linear enough. I think most people using temp. compensated focusing do so as an aid to focusing fewer times but not as a replacement for focusing.

By the way, if you are talking about your TOA150 it is possible to fit an ONAG-XT and a TOA-67 flattener with some cameras. The back focus is just long enough. I've done exactly that with my TEC180 and have been using Sharplock/FocusLock software for incredible autofocusing. With this system temperature simply doesn't matter. You can image all night and never focus.

[E_ri_k]

Thanks, I have had a look at their website, only unsure on how to mount it?



Thanks Chris, I'll have a look at their site :) I can set offsets for my filters. Would be interested to hear how it works out for you.



Thanks for the info. Good point about the change having to be linear, I didn't think of that......So it is still necessary to adjust the focus either way, and this is not a way to eliminate it completely. Thats good to know.

I did have an ONAG, not the XT version though. I sold it because I wasn't confident it would be a good match for the TOA. I never had a chance to try the SharpLock software with my old scope. Thats amazing that you never have to re focus! If I had the spare cash I'd probably invest in one again, but with the low exchange rate, I'd e up for a few $K when I add in a focuser upgrade....

Erik

[E_ri_k]

My method for refocus is slewing to the nearest bright star and putting on my focus mask, the slew back to the target. I'm assuming not, but is there another method without having to continually move the scope? (Unless a there is a nice bright star in the frame)

Erik

[PRejto]

Hi Erik,

Wow, too bad that you had an ONAG and sold it! The automatic focusing with it is brilliant not to mention the super wide FOV for finding guide stars. Oh well, I'm not aware of any other system that can do this though there has been talk for a while that Optec is going to adapt this for use with OAG. That would be game changing!

SGP can focus without slewing away from the object of interest. You might want to check that out.

FocusMax, both new (V.4) and earlier free versions (V.3) can automatically slew to a focus star, auto focus, and slew back to your target but require PinPoint or T-Point to do this.

All of these systems work but all take time. This is where FocusLock/ONAG shines. It just maintains focus all night. You need not focus again even starting up on the next night of imaging as long as the guide camera position is unchanged. FL takes about 1 min, maybe 2 at most to automatically focus the imaging camera after a guide star is selected. After that I can image for many hours and simply not focus. This is tough to beat.

Peter

[E_ri_k]

Hey Peter, yeah I know, I was a bit reluctant to part with it, but I need the money to fund other astro stuff! You're right, it is a tough one to beat, I was thinking the other night why it couldn't be adapted to work with an OAG. I really love the ease of finding a guide star with it. I'm using my ED80 for guiding the TOA and its FOV is pretty good.

I think I'll have to buy SGP :p looks pretty good, just another life of software to learn haha.

Erik

[PRejto]

Hi Erik,

It almost sounds like you might be confusing OAG with using a separate guidescope. I don't think I ever heard anyone quite describe finding a guide star as "easy" using OAG.....

Peter

[E_ri_k]

Ahh, don't know what happened there...? Typed the sentence in the wrong spot, that happens when I type on my phone! I meant I loved the ease of finding a guide star with the ONAG. I know it's harder with an OAG, that's why I sold the one I had to finance the ONAG.

I ended up buying this last night, and a bracket to go on the focuser.
http://www.usb-foc.us/shop/product/usb-focus-heavy-set-2

I'll get SGP tonight and start familiarising myself with it.

Erik

[Chris85]

Hi Erik, I received my USB_focus v3 the other day and finally got it installed. A bit of stuffing around to get it to work and they didn't have a bracket for my focuser, but hopefully now it's sorted. What I have discovered is that I couldn't get it to work through my usb hub (Hitech astro mhp) and that I needed to run a separate usb all the way to my laptop. No biggie as my laptop sits beside it but a consideration to think about.
Good on you for getting SGP as well :thumbsup:
Chris

[E_ri_k]

Hey Chris, looks good:thumbs:! How long did it take for the kit to arrive? What problems did you have getting it to work? Good to know about the hub issue. I have 2 ports on the PMX which are used, but another seperate hub also.

I'm going to have a play with SGP tonight. The interface reminds me a little of MaximDL, which I have been using for quite a while. Im guessing I have to get PHD Guiding to be able to use all the features in SGP?

Erik

[Chris85]

Hi Erik, the kit took just a little under 2 weeks from ordering to get to Perth, but that included time spent at their end getting in more motors as they didn't have enough in stock.
The adapter I ordered was for the TS APO 3" which I figured was the closest to my 4" as they're all made in the same shop, so although not an exact fit I managed to get it work.
Originally it wouldn't lift the weight of my image train. I solved this by removing the motor and adapter, and setting the tension on the focuser just enough to hold it while pointing at zenith. I probably had it too tight to begin with.
With SGP I'd recommend getting the latest Phd2. Works a treat together.
Cheers

[E_ri_k]

Cool, thanks, received an email the other day, it was sent out on Monday, so shouldn't be too long I guess. I'll get PHD 2.

Did you get the heavy set or the standard? I got the heavy only because it recommended it for 4" focusers...

Erik

[Chris85]

Hi Erik, yes like you I got the heavy set as I too have a 4" focuser, plus my reducer and image train is slowly bulking up. You'll have to power it with 12v. I tried usb and it just doesn't have enough juice
Cheers

[bugeater]

Wouldn't the change in focus just need to be reproducible? Even if non-linear you could still compensate based on its previous non-linear behavior. In practice I don't know if this is how the various software packages work though, plus it may be easier to just have a certain change in temperature trigger an autofocus cycle.

[PRejto]

As far as I know nobody has designed software that can do that. I know from keeping records of my focus points/temp from night to night that even when the outside temperature is the same as a previous night the focus point is often not the same and is different enough to exceed the CFZ (using FocusMax). I assume this is because the OTA has not reached equilibrium sufficiently after being heated during the day.

And there are other issues as well. As temperature drops (or raises in less frequent cases) different components in the OT will have different coefficients of expansion (contraction) - for example the glass of the lens or mirror as well as the tube. Clearly there is no law that says they will expand or contract in step with each other to result in a linear & predictable focus point particularly if the temperature happens to drop more rapidly that when one graphed the change in focus vs temp. The slope of the line could be quite different (and non-linear) depending on how quickly the lens and tube react to the dropping temperature.

Peter

[Bassnut]

The Optec temp compensating focuser http://optecinc.com/astronomy/catalog/tcf/17670.htm has a "training" routine that by nature takes into account all non linearities. I think some other focusers do this too.
I have one, its very good and once homed provides "absolute" positioning (its magic with focusmax), although I have not tried the temp tracking feature.

[bugeater]

I think in some cases you may be able to analyse a non-linear relationship, but yes it could be very complex and difficult (or impossible) and frankly just autofocusing is probably easier. Things like the rate of change of temperature having an effect could make things very complex.

Oh well, once I get my bits for my Arduino focuser I can see how complex it gets for myself :)

[E_ri_k]

Thanks guys, the kit arrived yesterday. I will need to modify the bracket to fit onto the focuser body, but it shouldn't be a huge problem....

I had a read of SGP's autofocus routine. Looks pretty straight forward. I'll fit it all tomorrow and test it out.

I'll graph the results and see what it looks like.

Erik

[PRejto]

Fred,

Are you sure about this? I spoke at length with Jeff Dickerman and Steve Brady at AIC (San Jose) recently on this very subject. My strong memory is that both said the plot of temp vs focus position had to be linear. And I just read the instructions on the Optec site re temp compensation where one is instructed to get a beginning position and end position after a 5 degree temp change. 2 points define a line, no?

Peter

[Bassnut]

mmm, on a read of the manual, it only has a start and end point, not multiple points. It appears a line between 2 points might indeed be pretty straight.

[gaston]

The OPTEC temperature compensated focusers are based to a strictly linear relationship. I am not aware of any solution which can handle non-linear dependency between focus and temperature, or other factors, for amateur scopes.

Classical scopes, such SCT with aluminum OTA, exhibit focus shifts from mainly the tube expansion/shrinkage, which is very linear with temperature (tube thermal inertia is much smaller than the mirror glasses). Other effects are less important, therefore a linear temperature compensation scheme does work well most of the time/nights.

Carbon fiber scopes and opened tubes, such as more advanced designs like RCT, do not have much tube length variation with temperature anymore (at least with well made carbon designs and related focusers), however mirror (lenses) radii changes are one of the main factor of focus shift.

For instance departure form spherical mirrors, such as hyperbolic mirror based scope designs, can be extremely sensitive to shape and alignment. Unless made of Zerodur glass, Pirex/quartz have enough left over volumetric thermal expansion coefficients to create a significant focus shift.
Some refactors are also very nonlinear in focus versus temperature dependency. When an nonlinear focus behavior is experienced this usually means a more complex process is at work here, and current temperature is likely not the only parameter, inertia plays also some role.

It should be remembered that for an RCT (or some other more advanced optical layouts) the best focus zone is usually tighter than the classical CFZ. RCT designs for instance trade coma for astigmatism, stars are round only in a very tiny focal plane located between the sagittal and tangential focal planes where a star would be elongated (due to astigmatism).
The CFZ is geometric in nature and does not account for the scope layout (SCT, RCT, ...).

When focus shift is due to the optics (radii, shapes, ...) the process may be highly nonlinear and unpredictable. For a reflector usually the primary mirror is much bigger than the secondary and have a significant larger thermal inertia leading to mirror figures evolving at different rates.
The resulting effect is not only a matter of temperature but also its rate of change, making any focus shift prediction solely based on external temperature challenging.
Using the temperature to infer the focus shift is a proxy which works well in some situations (such for SCT closed aluminum OTAs), but it does not provide any direct measurement of the actual focus.

As an example in this education page you can see the evaluation of the focus shift with temperature for a good popular carbon fiber 10" RCT:

http://www.innovationsforesight.com/education/real-time-autofocus/

This typical non-linear behavior usually does not repeat form night to night, unless the temperature profile (such as rate of change/derivative) over time is very similar.

In such situations periodic refocusing or continuous real time auto-focus may be the only effective solutions.

[E_ri_k]

This is the kit fitted. The supplied bracket wouldn't fit, so I had to add a couple of holes in the bottom cover of the focuser. Good news is it works I guess! However the "in" and "out" buttons operate in reverse? I checked the polarity of the supply power and it's correct, so I'm not sure what's going on there??

Going to have a play tonight as it's going to be clear!

Erik

[Chris85]

Funny that, mine was reversed too. I had to go into the focus settings in SGP and click the "reverse command" or whatever it was called button to get auto focus working.