TOA-130 temperature / focus management strategies? - Forums

Topic starter

Dec 29, 2024 · Edited: Dec 29, 2024

Finally have my TOA-130 up and running at DSP remote in New Mexico. TOA-130 is definitely very sensitive to changes in temperature. I have a moonlight nightcrawler 3.5" installed on it. I was looking at my imaging history last night in NINA, and the focuser position started the night a 19623 and finished 16147 on the same Luminance filter!! Each step size is 0.269 microns so that is nearly 0.9mm change in focus.

I am using NINA advanced sequencer. Currently I have Autofocus trigger by both HFR increase of more than 5% with a sample size of 10. I also ask it to autofocus with a delta temperature change of 1c. This was completely arbitrary choice not rooted in any data. This resulted in 23 autofocus runs over a period of 11 hours. Each autofocus run takes about 2 minutes which means nearly 40+ minutes wasted imaging time.

NINA is relying on the built in temperature sensor inside the nightcrawler, which I am not sure how accurate it is. I am thinking of sending my Nightcrawler temp probe. Here are my questions to the experts here

1) Are you using a temperature probe with your night crawler? How are you attaching it to the OTA?

2) Are you using any sort of temperature compensation strategy instead of autofocus for temperature change? What is your experience with it and how did you set it up?

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Sam Berrad

Dec 29, 2024 · Edited: Dec 29, 2024

Hi

Good question - I have been using my TOA130NFB with an OPTEC focuslynx and more recently with an Esatto 3.5LP.

You are right about the large change in focus - what is interesting is that the focuser comes inward while the scope is cooling, which is counterintuitive- the reason is that although the tube is contracting, the lens is changing shape at the same time.

As well, I find that the focus change is not linear with ambient temp - it seems to change more in the first 3 hours as the scope cools towards ambient.

Overall, I find that refocusing every 50-60 minutes works very well to keep the focus within the sweet spot - so about 10 times for a 10 hour session is enough.

Both the OPTEC and the Esatto have their own temperature probes which are not perfect but work well enough to approximate ambient.

Sam

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Bill McLaughlin

Dec 29, 2024 · Edited: Dec 29, 2024

I no longer have the system but here is some temp vs focus data from 2013 when I had a TOA-130 with a nightcrawler.

TOA Temp vs Focus

FYI, this was with the external nightcrawler temp probe taped to the OTA....

I calculated the depth of focus and used the amount of temp delta to move it 1/2 of that as the "focus by temp change" number. In those days it was with SGP but I do the same in NINA with my other systems these days. I do not use "focus by change in HFD/FWHM" because if the seeing deteriorates that will give lots of new focus runs that will gain you nothing.

I don't remember the number I used for the TOA but these days for my FSQ I use .5 C. The TOA is slower so the DOF is bigger but the tube is also longer so would move the focal plane more per degree.

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Tony Gondola

Frequent contributor

Dec 29, 2024

This makes me wonder if refractors are more sensitive overall to temperature then newtonian reflectors. Just going by HFR it's rare that I have to refocus my steel tube 6" f/6 newt. within a 6 or 7 hour winter imaging run.

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Sam Berrad

Dec 29, 2024

Yes indeed - refractors generally move a lot more but there are exceptions;

I have a CDK 12.5 which moves very little after the initial cool down.

The TOA130 is a big mover with temp like many other refractors.

However I have a smaller williams optics GFT81 which has a built-in 5 element petzval flattener - this refractor mover very little compared to the TOA130 (about 10%).

So it depends on the lens design.

The TOA130 is a spectacular instrument but the focus change is something that we have to compensate for. Like anything it is important to compensate sufficiently without overdoing it - this is why I use the simple and pragmatic 50-60 min refocus

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Niko Geisriegler

Dec 29, 2024

I have a Starlight Feathertouch 3” OAZ with a QHY QFocuser (High precision Version) on my TOA130NS.
The whole thing has been in use for 1 year now.
I work with NINA, the focus is triggered after 10% HFR.
Only the LUM filter is focused, because I have set this as a reference with the filter offsets.
The temperature drift can also be measured in NINA and then used for compensation, as far as I understand it. But I have to do that first.

Without the temperature compensation, my HFR values are between 2 and 3, depending on the seeing.

Hope that helps you!

CS Niko

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Ashraf AbuSara

Topic starter

Dec 29, 2024

Thanks everyone for the replies and input.

Sam Berrad:
Overall, I find that refocusing every 50-60 minutes works very well to keep the focus within the sweet spot - so about 10 times for a 10 hour session is enough.

The issue I have with focusing based on time is that you maybe under focusing in the early part of the night when the temprature drops quite drastically, and over focusing in the later part of the night when things get more stable. For example between 7pm and 8:30pm, my focal point drifted from 19623 to 17991. Then spent another 4 hours before it dropped to 17000.

I think the ideal solution is to just compensate for temperature change to always have the adequate amount of autofocus routines. But how is the question.

Bill McLaughlin:
I no longer have the system but here is some temp vs focus data from 2013 when I had a TOA-130 with a nightcrawler.

TOA Temp vs Focus

FYI, this was with the external nightcrawler temp probe taped to the OTA....

I calculated the depth of focus and used the amount of temp delta to move it 1/2 of that as the "focus by temp change" number. In those days it was with SGP but I do the same in NINA with my other systems these days. I do not use "focus by change in HFD/FWHM" because if the seeing deteriorates that will give lots of new focus runs that will gain you nothing.

I don't remember the number I used for the TOA but these days for my FSQ I use .5 C. The TOA is slower so the DOF is bigger but the tube is also longer so would move the focal plane more per degree.

Thanks for the data! I agree I would like to get away from focusing by HFR change.
How did you tape the external probe to the OTA? Which part of the OTA did you tape it to? Were you attempting to measure the OTA temperature itself and insulate the probe with a tape from ambient temp?

Tony Gondola:
This makes me wonder if refractors are more sensitive overall to temperature then newtonian reflectors. Just going by HFR it's rare that I have to refocus my steel tube 6" f/6 newt. within a 6 or 7 hour winter imaging run.

Oh definitely. The TOA is the most sensitive instrument I have used so far. My AG Optical FA12 and Epsilon are far more stable in terms of their focal point.

Niko Geisriegler:
I have a Starlight Feathertouch 3” OAZ with a QHY QFocuser (High precision Version) on my TOA130NS.
The whole thing has been in use for 1 year now.
I work with NINA, the focus is triggered after 10% HFR.
Only the LUM filter is focused, because I have set this as a reference with the filter offsets.
The temperature drift can also be measured in NINA and then used for compensation, as far as I understand it. But I have to do that first.

Without the temperature compensation, my HFR values are between 2 and 3, depending on the seeing.

Hope that helps you!

CS Niko

In my backyard I often had HFR in the 1.6-1.8 range. Oddly my remote location actually has worse seeing conditions, but much much darker skies and far less humid. HFR there ranges in the 1.8-3.0 range. Mostly sitting in the low 2s.

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Bill McLaughlin

Dec 29, 2024

Ashraf AbuSara:
How did you tape the external probe to the OTA? Which part of the OTA did you tape it to? Were you attempting to measure the OTA temperature itself and insulate the probe with a tape from ambient temp?

I used gaffer's tape so it would come off as needed and put a bit of foam over the probe under the tape so that it measured as much OTA and as little ambient as possible. I also put it about a foot forward from the focuser so as not to be influenced too much by that mass of metal.

Bill McLaughlin

Dec 29, 2024

Ashraf AbuSara:
My AG Optical FA12 and Epsilon are far more stable in terms of their focal point

Agreed. My Epsilon is more stable and the CDK even more so.

Willem Jan Drijfhout

Dec 29, 2024 · Edited: Dec 29, 2024

Like the others, my TOA-130 also changes focus quite a bit with temperature. What I understood it is especially the refractive index of the glass that changes, making the focus distance shorter as temperature goes down. In general reflectors are much less sensitive to temperature changes. My CDK14 barely changes focus when temperature drops.

I have always used a 1°C delta to trigger autofocus. Temperature reading has come from both environmental monitors and scope-based probes, but found very little difference between them. As long as you have a steady reading. It's not the absolute temp value you're interested in, just the change.

It is important to keep the power to your dew-strip constant (if you use dew-heater). So no dynamic dew-control or so. As soon as the dew heater changes power, the focus changes (which is logical). Set it at some default value at the start of the night (e.g. 50% for normal nights, and 100% if you expect it to get more humid). Then the temp of the scope will fluctuate together with ambient.

Never used HFR as trigger. Not sure what the additional benefit would be, as it is not only dependent on focus, but also on sky/weather conditions.

How often autofocus runs is dependent on your local situation. Some areas are notorious for serious temperature drops when the night falls. Then you need more. 23 seems like a lot. I can usual do with about 5-10 or so, but I'm living in a moderate climate.

In Ekos/KStars, there is a new algorithm, that makes small changes to the focus position in between focus-runs, after it has established some level of relation between temperature and focus position. That is never 100% correct of course, but better than nothing, and at each focus-run it recalibrates again. Quite nifty. I have made graphs in the past and find that relation between temperature and focus position for my setup is linear, but I never took the jump and adjusted blindly, just based on that relationship. The software does allow it though. If you don't see a linear relationship, and focus positions change rather randomly with temperature, you may want to check your focuser for any slippage.

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Shannon Foye

Dec 29, 2024

I'm curious as to how much the change in focus is due to the change in temp of the tube vs. the temp of the lens. As the temperature dropped in October and November, I ran out of inward focus with my TOA and NiteCrawler using a new Precise Parts drawtube extension that I mistakenly made a few millimeters too short. I based the length of the custom extension using the summer focus point.

Before I received a longer drawtube extension from Precise Parts, I compensated by running my dew heaters manually at 100% instead of the "Auto Dew" setting in the Pegasus' power box app. Heating the lens with the dew heaters when the temps dropped to the 30's and low 40's allowed me to keep the focus point at around 1500 to 3500 steps on the NightCrawler. Otherwise, I would run out of inward travel with the dew heaters set to Auto Dew.

I like the suggestion of securing the external temp probe against the OTA and having some type of insulating material (foam or foil tape) covering the probe to get a more accurate reading of the scope's change in temp. And I'd be curious to see if doing made a significant difference in the accuracy of a change-in-temp approach to focusing.

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Tim McCollum

Dec 29, 2024

I have a WO FLT-132 with nightcrawler and have to refocus it every .7c of temperature change.
I have it trained in SGP and it will sometimes change the focus between subs.
I live in N.E. Georgia

Stargazers SA - Andrew

Dec 30, 2024

We have an Askar PHQ130 which is quite similar and have the same focus issues. I set the autofocus at 30min intervals for the first 2hrs and 60min from then on. This seems to work quite well although in the first hour you can see minor changes in focus after 20min.

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Ashraf AbuSara

Topic starter

Dec 30, 2024

Thank you everyone for the feedback. I found a number of things this evening that I might try to implement. Going to attempt some form of temperature compensation as described in this video.

https://www.youtube.com/watch?v=61ZKmtCBVDU

I'll keep everyone posted, as this might be helpful to not just TOA users, but large refractor users in general.

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Alex Nicholas

Dec 30, 2024

TOA users have it worse than most large refractor users though… This is the price you pay for an almost flawless optical design.

The TOA is an air spaced triplet lens design, and the air gap between those lenses is quite substantial. A scope like my Askar 120APO is also an air spaced triplet lens, however, the air gap between the lenses is quite small..

The boundary air gaps in the TOA design act as insulators, so while the telescope is cooling, the front element is going to cool fast as its directly exposed to the cold air. Then the air between the front element, and the secondary optic needs to cool, in turn, cooling the secondary optical element - then another airgap needs to equalize before the third lens cools.

Oil spaced triplets cool faster again, as the oil film separating the lens elements conducts heat better than an air gap does..

This temperature equalization of the optical elements is the reason you see more pronounced focus shifts in the first few hours of imaging… you're dealing with the thermal equilibrium of the OTA, the optical elements, any flattener/reducer elements from the rapid temperature drop from day to night time temperatures… For this reason, I make sure my telescope is outside at least a few hours before dusk so its at least at ambient air temp as quickly as possible.

I will admit it seems very odd (though it is what I see with my refractor too) that as the temperature drops, the focal point seems to move inward, rather than outward.

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Mark McComiskey

Dec 31, 2024 · Edited: Dec 31, 2024

Ashraf AbuSara:
Thank you everyone for the feedback. I found a number of things this evening that I might try to implement. Going to attempt some form of temperature compensation as described in this video.

https://www.youtube.com/watch?v=61ZKmtCBVDU

I'll keep everyone posted, as this might be helpful to not just TOA users, but large refractor users in general.

Let me know how that goes. It worked poorly for me. The problem I have is that the relationship between temperature and needed change seems hard to calculate. The change in the air temperature and the change in the tube/focus seem to have some kind of lag factor that I was never able to account for properly, so getting a meaningful R2 on a plot of focus position versus temperature has eluded me. I have months of data, literally thousands of data points, but the R2 are terrible. Presumably for the reasons outlined above by Alex. I am sure that there is some reliable relationship that could be determined, but I settled for focusing every 0.3 degrees, which does indeed consume a lot of time.

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Ashraf AbuSara

Topic starter

Dec 31, 2024

Mark McComiskey:
Ashraf AbuSara:
Thank you everyone for the feedback. I found a number of things this evening that I might try to implement. Going to attempt some form of temperature compensation as described in this video.

https://www.youtube.com/watch?v=61ZKmtCBVDU

I'll keep everyone posted, as this might be helpful to not just TOA users, but large refractor users in general.

Let me know how that goes. It worked poorly for me. The problem I have is that the relationship between temperature and needed change seems hard to calculate. The change in the air temperature and the change in the tube/focus seem to have some kind of lag factor that I was never able to account for properly, so getting a meaningful R2 on a plot of focus position versus temperature has eluded me. I have months of data, literally thousands of data points, but the R2 are terrible. Presumably for the reasons outlined above by Alex. I am sure that there is some reliable relationship that could be determined, but I settled for focusing every 0.3 degrees, which does indeed consume a lot of time.

Interesting point Mark. Did you put a temprature probe on the OTA body itself instead of ambient temperature to use as a source for your model?

Mark McComiskey

Dec 31, 2024

Yes.