My Focus Secrets Revealed

[El Paso Eric]

In the past, I've mentioned a system that I use to help me maintain focus.  My scope, being f5, has a pretty short CFZ, of around 50-60 microns.  So maintaining focus is a real challenge, with an aluminum tube.  I didn't want to use the typical robofocus and have to spend time slewing to a star, focusing, slewing back to my target, plate solving and then starting the next exposure.  This system allows me hands-off focus management for more than 20 degrees F of temperature drop.  It's not the prettiest system, but I've been using this system nearly 2 years with exceptional results. 

I was thinking of trying to market the system, but decided just to reveal how it works.   Since I'm saving for some Astrodon filters, the payment from the publisher helps a lot.  There's a short video (very rough) and also some build instructions with a detailed outline of the build.  It has a lot of background info, as most reading this are not likely astronomers.  I've been really happy with how it works and never go out without it. 

I hope someone else finds it useful, or at least interesting.  Here's the URL:

http://tinyurl.com/d92fv92

[Bob Star]

The link isn't working for me Eric.

[El Paso Eric]

Sorry, I updated the link, but here's the full URL:

http://www.designnews.com/author.asp?section_id=1362&itc=dn_analysis_element&doc_id=247356

[David (djrLX90)]

That's awesome Eric, it that a heating pad with a Watlow style thermal controller hooked up to it!!!  I love it!
I decided to give Starlight Instrument's new Temperature Compensating focus Boss II system a try at NEAF and so far it keeps real tight stars, over massive temperature swings, with just a 2 point temperature calibration!!!!

[Les (t_total123)]

Cool idea Eric . I myself focus  every 30 mins normally

[Bob Star]

A truly original solution to focus control Eric!!! You are one talented guy.

I've been looking into the various focus control systems and haven't found one that fits my current budget. Is this something that someone with a non-engineering background, like me, can tackle? If its just soldering components to a project board I might be able to handle it   ;-)

Bob

After fully reading the detailed instructions...that I missed before, it looks do-able. Might give it a shot while waiting for clouds to clear!

[El Paso Eric]

Hi Bob,

It works great for me and my widefield scope.  I was initially worried about thermal gradients inside the tube, and near the objective, but I don't seem to have these issues, or if I do they are below the resolution of my system.  I'm still able to get pixel-width stars, with only a single focus session. 

Although there is a parts list from Allied, You can save A LOT of money buying parts off ebay.  I'd be glad to give you pointer, if you want to give it a try. 

[MikeM]

Very novel idea, nice work.

[El Paso Eric]

Thanks for taking a look, David, Les, Bob & Mike.  I appreciate the comments...

[El Paso Eric]

BTW, if any one wants to give this a try, but is not in the "Build mode", shoot me a PM.  I have a ready-made scientific system that I bought after I built the one for my telescope.  All you'd need are the heating pads.  Everything else is ready to go.  It's this unit:

http://www.glascol.com/product/subproduct/id/218

I'd not normally sell this unit, as I use it occasionally for work.  But since I'm still trying to raise money for my Filters, I thought I'd offer it up for sale. 

[jehoener]

Eric:

I've thought of this approach, too, and have experimented briefly with an off-the-shelf heating pad on a C8; just haven't followed through.  Makes perfect sense to me to fix the dimensional-change focus problem at the source by eliminating the thermal change.

I emailed Dew-Not asking if they could manufacture a large version of their thin film dew heaters, large enough to wrap an OTA, but they didn't reply.  The biggest drawback to the heating pad approach is the bulk of the pad, and that the pads seem never to be the right size.

While thinking about the heating pad approach, I (and apparently several others) wrap my scopes in a thin WalMart yoga mat or bubble wrap as an insulator in order to at least slow the OTA's radiation of heat into space and resulting dimensional change.  It seems to help.

-Joe

[El Paso Eric]

Hi Joe,

Thanks for sharing your experience.  You're definitely correct about the difficulties in finding the right heating pads.  I had looked at many different approaches, before ending-up with the Walgreens heating pad.  I really needed my pads to be longer, but this was the best I could do.  I also used 2 pads in parallel, in order have the wrap extend around the tube enough.  The idea that I though had the most promise was the film heaters used for in-ceiling heating installations.  But I ended-up using the heating pads, for their ability to wrap around the mounting rings, and other protrusions.  A friend also pointed out that the random nature of the heating pads, coupled with the blanket probably provided a good compromise for the application, and may end up being more uniform than a flat film-type heater.

Although I keep my eyes open for longer heating pads, I find that the ones I'm using currently provide enough capacity to allow me to image with only 1  focus session.  Since I've started using it, I've only had one time that I can recall needing to refocus, before the night was over.  Since that's not the norm, it probably had to do with something to do with the wrap being installed poorly.

It would be interesting to see if there were any thermal gradient effects in a scope as large as a C8.  If you ever decide to give it a try, I'd be interested in the feedback.

[jehoener]

Eric:

FWIW, there are  heating pads made for pet beds, available in all kinds of sizes and shapes, and available from many online sources.  They are thinner than the usual heating pads and fit a curved OTA better.  They provide a much gentler heat than people heating pads. 

-Joe

[El Paso Eric]

Interesting Joe.  I'll take a look at these pet bed heaters. 

As for the gentler heat,  it's really irrelevant, since the system is a closed-loop control.  The controller regulates the temperature to the set-point value, fed back from the thermocouple.  Once auto-tuned, they will hit the set-point, with very little overshoot.  That's the the beauty of the system and being able to set the temperature at various values.

Thanks for the tidbit...

[David (djrLX90)]

I have used these in the past and they are excellent.  The larger ones are 12*12 inches.

http://www.omega.com/ppt/pptsc.asp?ref=KHR_KHLV_KH&ttID=KHR_KHLV_KH&Nav=

[El Paso Eric]

I have used these in the past and they are excellent.  The larger ones are 12*12 inches.

http://www.omega.com/ppt/pptsc.asp?ref=KHR_KHLV_KH&ttID=KHR_KHLV_KH&Nav=

Yep, Looked at those, and have used those at work. 

Similarly, I looked at these, which were available in larger sheets.

http://www.infraredheaters.com/ceilings.html

[Jason C]

Very cool....or rather hot! I remember touching on this design a little while back. I used the Kapton units in the TEC modified 350xt's I did (mine and Greg's)for anti chip fogging.

The Kapton heaters are the most flexible (flexible in power arrangements). The ceiling designed radiant heating panels look nice too, but would be totally unpractical for a "field" system with power requirements. At 240 AC, they draw 4.2A.....at 120v AC 8.4A which is just more than half the capacity of the house outlet circuit. At 12v dc they would be better suited temperature wise, but would draw all a "field" battery could supply.

[El Paso Eric]

Thanks for all the comments, folks.  I really appreciate you taking a look at this.

@Jason - The great thing about using a temperature controller is that when the set-temperature is low (around Dew Point), the average power draw is really, low.  The change in temperature is small so the temperature loss is slow.  In addition, it's just trying to maintain the scope temperature, not to really heat up the tube significantly.  With the set-point being so low the controller cycles only a couple times a minute for very brief on-times.  I'd need to use a logger or kWh meter to calculate the average power, but it's very low.   

With the film heaters, as well as nearly all other heaters, it's a function of resistance.  So for example the ceiling units are 240V at 4.2 amps.  That equates to a resistance of about 57 Ohms and a power draw of about 1000W, which is way too much power for a system like this.  At 120V, the power would be half that or 500W - 2.1A.  This is more manageable, but still a lot of power.  If a 12V control system was used, the power would be only about 50W  My heating pad units would provide about 144W, if wide open.  So there's always options, depending on how things are wired and powered. 

But I think the key to reducing power is the use of a good temperature controller.

Thanks for the comments...

[jehoener]

It would be interesting to see if there were any thermal gradient effects in a scope as large as a C8.  If you ever decide to give it a try, I'd be interested in the feedback.

Eric and others:

Last night I ran an experiment with numbers, using my C8 wrapped with a doggie-bed heating pad.

For the purpose of conserving battery power while imaging in the field, a controller is no doubt the best idea.  But since I was working from A/C in my ROR observatory, I just depended on the heating pad's internal thermostat.  I didn't think to measure the temperature the pad heated the OTA to, but will check that tonight, and will report here.

Ambient temperature during the 6-hour  imaging run started at 61F, and dropped to a low of 54F at dawn.
After opening the observatory, turning on the heater and letting temperatures equalize for more than an hour, I focused once with a Bahdinov mask and started the run.

Here are CCDStack's FWHM measurements of the 20-minute subs, from beginning to end of the run:

3.19
4.28
4.8
4.72
4.57
4.58
4.39
4.5
3.95
4.55
4.68
4.51
5.06
4.22
4.42
3.98
4.08
4.05
4.36

I suspect the rather high numbers and the swings in value are the result of some star trailing because of poor guiding, and my low-end mount.  My C8 doesn't have really great optics, either. The interesting thing is that there's no apparent pattern of focus degradation during the session.  There wasn't a big swing in ambient temperature, but the heating pad apparently helped maintain focus.

I don't know about the effect of tube currents caused by the heating pad, or how I would measure them.

-Joe



 

[El Paso Eric]

Hi Joe,

Thanks for the info.  I think using the internal thermostat is probably not going to give you the best results, as it's going to heat the tube well beyond the dew point.  Too much heat in a system like this will certainly cause image quality issues.  But using a temperature controller, you can set the temperature just above dew point and get the best results.  Many times, it won't even feel warm, but is still maintaining the temperature.  That being said, you data doesn't seem to have any large ramps or other strangeness. 

I think if you wanted to evaluate the "seeing" caused by tube currents, you'd need a second camera taking FWHM star data at the same time as the main scope.  If you plot this data together, and the paths are parallel, the tube currents are small.  But if you plot the 2-scope data and the paths are different, tube currents could be the cause.  There's a lot of variables with trying to test this, as the focus would also need to be perfectly maintained.  Seems to me to be a tough challenge, to get good test data.  The best test is to shoot images with the scope and check the results.  I figure if I'm getting pixel-sized stars, I probably couldn't do any better.


Keep us posted on your results...

Eric