Temperatures at focus of 6" f/10 H-a telescope:

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Rusted
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Temperatures at focus of 6" f/10 H-a telescope:

Post by Rusted » Thu Mar 26, 2020 2:32 pm

I have checked the temperatures at focus between 12.45 and 14:40[CET] today to update an earlier discussion on this subject.

I set my iStar 6" f/10 H-alpha [1500mm] tracking the sun with the full aperture Baader 160mm D-ERF in front of the objective.

First I managed to draw smoke from a plastic coated "outdoor" sensor of a domestic, digital In/Out thermometer. The smoke was not the greatest problem. It was the 65C, 150F max allowable measurement. I had badly underestimated the temperatures at focus!

So then I moved onto a 130C spirit thermometer in my collection. By pressing the bulb against a piece of "Scotchbrite" red/brown, abrasive fiber, I measured 122C. By which time my arms were tired from holding the fiber pad and the thermometer bulb in exact focus. It took some minutes to reach the maximum temperature.

Then I used a modest "Pistol" digital thermometer. As much seen pointing at foreheads in the present pandemic. Again I used the Scotchbrite pad as my thermal target. It quickly registered over 130F with the distinct smell of burning rubber at focus! Eek!

After repeated tests with the "naked" D-ERF, I replaced the "protective" UV/IR filters in their normal position about ~250mm inside focus. The 2" reflective, Baader 35nm H-a first. Followed by a 2" Beloptik KG3 just behind. The temperature at focus was now down to 47C!

It is a good job I read about these protective filters on this forum! Now I know why my ZWO ASI120MC developed a permanent solar surface: An image overlay by thermal printing! Imagine if that happened to my ASI 174! I'm fairly sure these cameras aren't designed to soak up 130+C for hours on end! Of course the PST etalon, ITF and blocking filters lie between the camera and the rest of the telescope. At 15:30 it is too late in the day to measure the temperature after these.

I let the protective filters "soak" for a few minutes while the telescope tracked the sun. Then manually slewed away and took a quick reading off the back of the KG3 at a quite modest 18C. Though I am still very happy it was suggested here to put the KG3 behind the reflective Baader 35nm H-a.

Imagine the temperatures reached at focus on a hot summer's day with the sun high overhead! This is the late middle of March with the sun no more that 35° high here at 55N. Ambient temperature was only 10C/50F in the shade today. Baader's claim for a "cool" optical train is rather misleading IMO. ONe can easily burn one's skin at focus.

I'm not sure how efficient the Scotchbrite is as a thermal target. It registered between 35C & 45C when lying in the sun [in the shelter of the dome] when measured with the pistol. I'd imagine it has low thermal capacity and quite reasonable absorption to have provided my temperature readings. Please suggest a better idea for a thermal target if you have one.
http://fullerscopes.blogspot.dk/

Baader 160mm D-ERF, iStar 150/10 H-alpha, Baader 35nm H-a, Beloptik KG3, PST etalon, Lunt B1200S2, ZWO ASI174.

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Re: Temperatures at focus of 6" f/10 H-a telescope:

Post by marktownley » Thu Mar 26, 2020 2:37 pm

Great information, many thanks.
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Re: Temperatures at focus of 6" f/10 H-a telescope:

Post by AndiesHandyHandies » Thu Mar 26, 2020 6:38 pm

Hi

The simple glass front ERFs do pass a lot of energy. I would always put a Baader 2" or 1.25" 35nm behind one of those.

I have been taking readings on my Vixen VMC110 Mak, been using for 2 years. Santel M180 Mak and a 90mm triplet recently. I will write a report.

Using the cheap laser pointed yellow remote thermometer pointing it up the eyepiece I normally see about 24C, and also on a big refractor with Omega Bobs Filters, 40nm ERF and two 1.5A dichoric filters. So if you have one a simple safety check before you observe.

The Laser thermometers assume a surface with 95% emisivity so pointing at matt black telescope surfaces should be accurate.
Looking at the front of glass companents just taken out of the telescope should be accurate as well as glass is the same.
Where I am pointing the Laser pointed thermometer up the system, after the Baader 35nm as the full white beam is too much, I am taking it as a Black Body equivilant reading, it does show the reduction in energy as you pass back through the filters and etalon.

A DERF dichoric filter near the front of the system is good.

On my new Daystar QUARK Chromosphere I have added a dichoric red filter after the ZWO UV-IR which came with it to protect the QUARK.

And as always I have a Beloptic KKG3 on the eyepiece, which appears to reduce some component of the image.

Andrew.

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Re: Temperatures at focus of 6" f/10 H-a telescope:

Post by Rusted » Thu Mar 26, 2020 7:09 pm

AndiesHandyHandies wrote:
Thu Mar 26, 2020 6:38 pm
Hi

The simple glass front ERFs do pass a lot of energy. I would always put a Baader 2" or 1.25" 35nm behind one of those.

And as always I have a Beloptic KKG3 on the eyepiece, which appears to reduce some component of the image.

Andrew.
Hi Andrew,

Did you mean reduce some component of "the image" or reduce some component of the energy?

Somebody here has mentioned they don't like a KG3 for imaging but do use one for visual.
I'm afraid I don't remember who said that.

I removed my 2" KG3 and found absolutely no difference in the image.
So it went straight back in again as extra insurance for my camera.
Unlike many solar observers and imagers I spend many hours tracking the sun.
Which must increase the risk of long term damage.

We are promised lots of sunshine tomorrow so I shall measure the temperature after the etalon and after the blocking filter.

I was unable to get a stable reading from my thermal pistol on black painted metal.
Hence the Scotchbrite abrasive fibre as a target. Which was the nearest object at the time I was testing.
I have no real idea if it is accurately reporting temperatures but they are repeatable.
http://fullerscopes.blogspot.dk/

Baader 160mm D-ERF, iStar 150/10 H-alpha, Baader 35nm H-a, Beloptik KG3, PST etalon, Lunt B1200S2, ZWO ASI174.

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Re: Temperatures at focus of 6" f/10 H-a telescope:

Post by AndiesHandyHandies » Fri Mar 27, 2020 9:35 am

Hi

I mentioned Mark T used a KG3 for imaging as he feels the long IR affects the image.

I also put a Beloptic on my original PST for visual and it seemed to help for me.

My Beloptik KG3 on the eyepiece does reduce the thermometer reading out the back on the Maksutovs, but not on the refractor as BK7 does absorb long IR as I recall.

If you point the thermometer up the optical train it does give smaller readings as you come back down from the ERF, white light is too much. But how accurate I do not know.

And I take out bits starting from the back quickly and take the temperature of the front element in that bit and the wall temperature of the holder.

I compared white light and the red from the front Baader 35nm ERF using the spot on a black aluminium cover.

Andrew.

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Re: Temperatures at focus of 6" f/10 H-a telescope:

Post by Rusted » Sat Mar 28, 2020 6:42 am

Thanks Andrew.

I was so busy capturing proms yesterday that I completely forgot to check the temperatures in the light path until it was late afternoon. :oops:

Ideally I wanted to do it around lunch time to ensure I was checking the highest potential temperatures.
Albeit in late winter/early spring here at 55N. -4C at night with 50F a "heatwave" during the day. :lol:

I might try a quick spray of matt black paint onto cooking foil as a better target. Or even candle smoke.
Something thin enough to rapidly absorb heat and conductive enough to get a quick temperature reading.
http://fullerscopes.blogspot.dk/

Baader 160mm D-ERF, iStar 150/10 H-alpha, Baader 35nm H-a, Beloptik KG3, PST etalon, Lunt B1200S2, ZWO ASI174.

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Re: Temperatures at focus of 6" f/10 H-a telescope:

Post by AndiesHandyHandies » Sat Mar 28, 2020 8:59 am

Hi

As most bits in a scope are alloy tube perhaps a bit of alloy sheet, with an air gap and then another bit of alloy for internal bits. Both anodised or painted black.

The cooling effect is greater in free air to an enclosed space. You do not want extra radiation or convection off the open back.

Andrew.

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