What to do with the focuser temperature probe? - Forums

Topic starter

Mar 26, 2025

Hi,

I just received my Moonlight NiteCrawler WR35 for my TOA-130. In the box there's a super long temperature probe cable with what I'm guessing is a sensor on it's end.

Where are you supposed to mount this on the telescope? Why is the cable 6' long? It's longer than the telescope by almost a factor of 2.

How is this temperature probe different than the one I got with my Pegasus Pocket Power Box Advance Gen 2?

All you NiteCrawlers out there, what did you do with your temperature probe?

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Richard

Mar 26, 2025

Temperature fluctuations can cause shifts in focus, and for refractors, this occurs at the objective lens at the front of the telescope. Some controllers can compensate for focus based on shifts in temperature so the ideal place to put the probe is near the objective lens.

That being said, as a user of a 120mm refractor and Pegasus Focus Cube, I don't use a temperature probe at all. Instead, within my image acquisition software (NINA) I only perform an auto focus routine upon filter change, or when the Half Flux Radius (HFR) of my captured subs increases above a certain threshold. That way, I don't need to think about temperature and just go by the quality of my subs.

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Quinn Groessl

IOTD Staff

Mar 26, 2025

It's not much different than the one on your powerbox, it's just that your focuser isn't going to use the temp from the one on your powerbox. I have no idea why the cable is so long. What I do with mine is bunch all the wire up and zip tie it all together and plug it in.

Darryl Ackerman

Mar 26, 2025

Software like N.I.N.A. use the focusers temperature probe to read the temp to decide if refocus is needed based on the change you've defined in a trigger.

The powerbox temperature is not referenced for that task. You don't need the external probe, the NiteCrawler has a built-in probe and works just fine.

I have 2 triggers setup for focusing. After temperature change (+/- 2) or every 90 minutes.

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Kyle Goodwin

Mar 26, 2025

Attach the temperature probe to the side of the telescope tube using insulating (foam) tape. What you want to measure primarily is the temperature of the longest metal thing which will affect the focus of the scope. Generally that will be the telescope tube. The nightcrawler could be on a larger scope, so it has a long cable. If your scope happens to be carbon fiber and doesn't expand/contract much you might want to attach it to any metal flange around the focuser, but it probably won't make much difference where you put it in that case. The temperature is used to let your acquisition software know how much the temperature has changed so you can refocus when it changes too much.

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Marcelof

Mar 26, 2025

If the Pegasus sensor is already doing its job, just put this one in the drawer full of accessories you are not using.

The thing is that one of the parameters to decide when to re-focus is the ambient temperature changes and you have to tell the capture program you use where to get that information, no matter if it is the Pegasus or the Moonlite focuser.

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Bill McLaughlin

Mar 26, 2025

I agree that it should go on the part of the scope that is the most affected by temperature change and be insulated from the air. Much better than one that just measures air temp.

John Stone

Topic starter

Mar 27, 2025

Bill McLaughlin:
I agree that it should go on the part of the scope that is the most affected by temperature change and be insulated from the air. Much better than one that just measures air temp.

Okay, so press the end of the temperature probe again the alu tube of my refractor near the lens cell and then tape it over with, painter's tape?

Bill McLaughlin

Mar 27, 2025

John Stone:
Bill McLaughlin:
I agree that it should go on the part of the scope that is the most affected by temperature change and be insulated from the air. Much better than one that just measures air temp.

Okay, so press the end of the temperature probe again the alu tube of my refractor near the lens cell and then tape it over with, painter's tape?

I would use gaffer's tape so it can be removed but maybe put a bit of foam under the tape but over the sensor so as to isolate it from ambient.

Alex Nicholas

Mar 27, 2025

mine dangles near the front of the dovetail bar, I did have to bunch the cable up considerably and tie it to the OTA, however it reads temperature in a location that is close to the objective lens, but far enough from the dew heaters such that they do not impede its reading..

Honestly - if you're autofocusing based on filter change (or using filter offsets) and on HFR/FWHM increase, temperature compensation really isn't required.

I did in the past on my FSQ85EDX with the reducer, set my robofocus up with full temperature compensation, it would adjust the focuser based on temp shifts (sometimes only 4 of 5 steps) between nearly every sub, as the FSQ with the reducer is VERY sensitive to temperature shift. My experience with the TOA tells me that you'll have a somewhat dramatic shift in focus a number of hours into the run, as the spacing between the elements in a TOA creates a sizeable air-gap that acts as insulation, encapsulating the warmth in the inner most element for a very long time…

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Kyle Goodwin

Mar 27, 2025

Alex Nicholas:
mine dangles near the front of the dovetail bar, I did have to bunch the cable up considerably and tie it to the OTA, however it reads temperature in a location that is close to the objective lens, but far enough from the dew heaters such that they do not impede its reading..

Honestly - if you're autofocusing based on filter change (or using filter offsets) and on HFR/FWHM increase, temperature compensation really isn't required.

I did in the past on my FSQ85EDX with the reducer, set my robofocus up with full temperature compensation, it would adjust the focuser based on temp shifts (sometimes only 4 of 5 steps) between nearly every sub, as the FSQ with the reducer is VERY sensitive to temperature shift. My experience with the TOA tells me that you'll have a somewhat dramatic shift in focus a number of hours into the run, as the spacing between the elements in a TOA creates a sizeable air-gap that acts as insulation, encapsulating the warmth in the inner most element for a very long time...

You really don't want to measure the ambient temperature or the temperature of the lens cell. You want to measure the temperature of the thing expanding/contracting changing the focus point. That's the metal of the telescope tube. It will certainly be influenced by the ambient temperature, but it's not going to be the same unless the ambient temperature has been stable for a long while.

I'm not using temperature compensation, I'll just do an autofocus run when the temperature changes enough to be close to exiting the CFZ of the scope. On my AP 130 GTX that's about 2 degrees C. I don't recommend a refocus trigger on HFR/FWHM since there are so meany other reasons (most of which, seeing) which will cause unnecessary and useless refocusing.

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John Stone

Topic starter

Mar 27, 2025 · Edited: Mar 27, 2025

Has anyone successfully utilized temperature compensation vs refocus on temperature change?

Once setup how trustworthy is the temperature comp curve?

How do you create such a curve for all temperatures?

Anyone got such a curve for a TOA-130 that they can share?

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Alex Nicholas

Mar 28, 2025

Kyle Goodwin:
Alex Nicholas:
mine dangles near the front of the dovetail bar, I did have to bunch the cable up considerably and tie it to the OTA, however it reads temperature in a location that is close to the objective lens, but far enough from the dew heaters such that they do not impede its reading..

Honestly - if you're autofocusing based on filter change (or using filter offsets) and on HFR/FWHM increase, temperature compensation really isn't required.

I did in the past on my FSQ85EDX with the reducer, set my robofocus up with full temperature compensation, it would adjust the focuser based on temp shifts (sometimes only 4 of 5 steps) between nearly every sub, as the FSQ with the reducer is VERY sensitive to temperature shift. My experience with the TOA tells me that you'll have a somewhat dramatic shift in focus a number of hours into the run, as the spacing between the elements in a TOA creates a sizeable air-gap that acts as insulation, encapsulating the warmth in the inner most element for a very long time...

I'm not using temperature compensation, I'll just do an autofocus run when the temperature changes enough to be close to exiting the CFZ of the scope. On my AP 130 GTX that's about 2 degrees C. I don't recommend a refocus trigger on HFR/FWHM since there are so meany other reasons (most of which, seeing) which will cause unnecessary and useless refocusing.

Fwhm/hfr is fine if it's an average.. I take the average hfr from the past 4 subs and if the new sub is 10% worse than average it focuses. I find if the seeing is that bad, I'm not imaging anyway hahaha.

Himanshu Pandey

Mar 28, 2025

John Stone:
Has anyone successfully utilized temperature compensation vs refocus on temperature change?

Once setup how trustworthy is the temperature comp curve?

How do you create such a curve for all temperatures?

Anyone got such a curve for a TOA-130 that they can share?

Yes, it works very well. I recently got my temperature compensation working through NINA's advanced sequencer and I was amazed to see almost an entire night of subs below 2 hfr. The focus change for my scope is somewhere near 16.5 points/degree C and using that value in NINA I see the hfr of the frames consistently below 2 all night. I do an autofocus every two hours. The temperature drop across a night is not consistent and so you cannot exactly model what will happen but you can use a good estimate to keep you close and true it up now and then with an auto focus run.

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Kyle Goodwin

Mar 28, 2025

Himanshu Pandey:
John Stone:
Has anyone successfully utilized temperature compensation vs refocus on temperature change?

Once setup how trustworthy is the temperature comp curve?

How do you create such a curve for all temperatures?

Anyone got such a curve for a TOA-130 that they can share?

Yes, it works very well. I recently got my temperature compensation working through NINA's advanced sequencer and I was amazed to see almost an entire night of subs below 2 hfr. The focus change for my scope is somewhere near 16.5 points/degree C and using that value in NINA I see the hfr of the frames consistently below 2 all night. I do an autofocus every two hours. The temperature drop across a night is not consistent and so you cannot exactly model what will happen but you can use a good estimate to keep you close and true it up now and then with an auto focus run.

What method do you use to set it up so it only moves in between subs? I've considered doing this a few times, but never actually done it.

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Himanshu Pandey

Mar 28, 2025

Kyle Goodwin:
Himanshu Pandey:
John Stone:
Has anyone successfully utilized temperature compensation vs refocus on temperature change?

Once setup how trustworthy is the temperature comp curve?

How do you create such a curve for all temperatures?

Anyone got such a curve for a TOA-130 that they can share?

Yes, it works very well. I recently got my temperature compensation working through NINA's advanced sequencer and I was amazed to see almost an entire night of subs below 2 hfr. The focus change for my scope is somewhere near 16.5 points/degree C and using that value in NINA I see the hfr of the frames consistently below 2 all night. I do an autofocus every two hours. The temperature drop across a night is not consistent and so you cannot exactly model what will happen but you can use a good estimate to keep you close and true it up now and then with an auto focus run.

What method do you use to set it up so it only moves in between subs? I've considered doing this a few times, but never actually done it.

I use the *Move Focuser by Temp* in the target instruction list of advanced seq.

Rick Krejci

Mar 28, 2025

Kyle Goodwin:
You really don't want to measure the ambient temperature or the temperature of the lens cell. You want to measure the temperature of the thing expanding/contracting changing the focus point. That's the metal of the telescope tube.

Actually, the objective lens temperature change is a main factor in focal point shift. Been mentioned in a lot of Aluminum vs Carbon Fiber Tube discussions like this:
https://www.cloudynights.com/topic/919035-carbon-fiber-ota-yea-or-nay/

Kyle Goodwin

Mar 28, 2025

Rick Krejci:
Kyle Goodwin:
You really don't want to measure the ambient temperature or the temperature of the lens cell. You want to measure the temperature of the thing expanding/contracting changing the focus point. That's the metal of the telescope tube.

Actually, the objective lens temperature change is a main factor in focal point shift. Been mentioned in a lot of Aluminum vs Carbon Fiber Tube discussions like this:
https://www.cloudynights.com/topic/919035-carbon-fiber-ota-yea-or-nay/

I’m not sure. I need to refocus my aluminum tube refractor every 2C or so. My carbon fiber tube CDK almost never needs to be refocused over night after one about an hour in to the session after the rapid cooling of the observatory has been accomplished. Perhaps there are a lot of variables here that make it difficult to say, but that’s my experience.

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Jon Bryan

Mar 28, 2025

I wrapped my tube with a strip of velcro up near the objective and sandwiched my EAF temperature probe in between the overlapped ends. It cools down very fast and seems to give me good, stable temperature readings.

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John Stone

Topic starter

Apr 1, 2025

I've built up my scope with all the electronics and I've taped the moonlight temperature probe just behind the dew shield of my TOA-130 and covered it with a bit of foam tape.

Now, the last step is put on my Dew Strap, but I just realized the temperature probe is right where I put the dew strap?

So, where do I put the temperature probe again?

Bob Lockwood

Apr 1, 2025

I don't use one, but since what you want to do is monitor temp of the tube, I'd put it on the tube closer to the focuser. There's no reason it needs to be close to the objective lens and it won't do anything but interfere with the heat any dew straps attached will produce.