D-ERF in converging beam?

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D-ERF in converging beam?

Post by Rusted » Tue Dec 10, 2019 1:58 pm

Hi,

Since I have been using a 90mm internal Baader D-ERF in my 150/8 [+ 1.125 GPC] I have discovered some serious downsides:

A hot exit beam from the empty focuser despite the D-ERF!
This suggests the PST etalon is being baked for hours every day when I am imaging all day. Quite often!
The smaller D-ERF is also being cooked by the same converging beam.

A fierce, re-focused beam just outside the objective. A serious fire hazard with temporary card aperture stops! :oops:
Serious risk to anyone near the telescope. Fortunately I am usually alone in my observatory.
Though even I have been caught out myself when checking the objective for dewing on winter mornings! :roll:

Now, a new unknown has occurred to me:
Does the D-ERF function at its best when mounted in a hot converging beam?
My presumption is that the D-ERF is multi-coated to deal only with incident, parallel light?

I plan to buy a full aperture D-ERF 160mm for the reasons above but have some telescope options:

Keep the old CR150HD 150/8 + PST mods. [+ new 160mm D-ERF]

Buy an iStar 150/10 achromat to better match the PST. [+ new 160mm D-ERF]

Convert my 180/12 R35 iStar to H-alpha with the PST mods. [+ new 180mm D-ERF]
F/12 is not a perfect match for the PST.

I'd really value some expert advice on how best to proceed for H-a imaging with my ASI174MM [& ASI120MC]

Thanks
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Re: D-ERF in converging beam?

Post by marktownley » Wed Dec 11, 2019 9:29 pm

Lots of people use sub aperture ERfs with no issue. However a 160mm DERF will be lots of fun. I think i'd go with the 150/10 Istar.
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Re: D-ERF in converging beam?

Post by Rusted » Thu Dec 12, 2019 7:30 am

marktownley wrote:
Wed Dec 11, 2019 9:29 pm
Lots of people use sub aperture ERfs with no issue. However a 160mm DERF will be lots of fun. I think i'd go with the 150/10 Istar.
Thanks Mark.
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Re: D-ERF in converging beam?

Post by Bob Yoesle » Thu Dec 12, 2019 5:50 pm

I have discovered some serious downsides:
A hot exit beam from the empty focuser despite the D-ERF!
This suggests the PST etalon is being baked for hours every day when I am imaging all day. Quite often!
The smaller D-ERF is also being cooked by the same converging beam.

A fierce, re-focused beam just outside the objective. A serious fire hazard with temporary card aperture stops!
Serious risk to anyone near the telescope. Fortunately I am usually alone in my observatory.
Though even I have been caught out myself when checking the objective for dewing on winter mornings!
Lots of people use sub aperture ERfs with no issue.
Hi Rusted and Mark,

I also had to discover this "the hard way." This is not at all surprising, it simply demonstrates a lack of knowledge about the individual filter(s) being used in DIY projects or how they work, or the intensity of the solar radiation you are playing around with. Often the information is simply not available. DERF's on the other hand have published transmission specs, and we can see what is going on with them.

With regard to the DERF, a lot of people using sub-aperture DERF's (and full aperture DERF's ;-) may not have bothered to simply do the "feel test" as Rusted has. If we look at the solar energy distribution at the Earth's surface we can see a lot of energy comes through the DERF's transmission profile at 600 - 700 nm and beyond 1500 nm:

Solar Energy Distribution.png
Solar Energy Distribution.png (69.77 KiB) Viewed 1608 times
Baader DERF.jpg
Baader DERF.jpg (445.81 KiB) Viewed 1608 times

The incident radiation at the Earth's surface is ~ 1050 W/m^2, or about 1.05 mW per mm^2, this is concentrated as the objective focuses the beam. Placing a DERF closer to the objective than the half-way point to focus should be just fine, but will focus the reflected radiation back out of the objective. The energy reaching focus can be calculated as a proportion of the reflected or transmitted radiation as follows:

The area of a 150 mm diameter objective is 17,662.5 mm^2 = 18.545 W. This 18.545 W is focused to 93.61 mm^2 in an f8 telescope = 188.67 times increase in the flux density. This is therefore equivalent to 3,498 W - think about how a magnifying glass works to ignite a piece of paper. If the final overall transmission of the DERF is 10% for all transmitted wavelengths, then about 350 W reaches the focus - and as Rusted has discovered, you can indeed feel this. So even though the best place for any ERF is ahead of the objective, the DERF allows a significant amount of solar radiation to come through, and additional filtering is required if you don't want to "bake" your etalon, especially as it lies relatively close to the focus.

Therefore, I would employ secondary filtering ahead of any etalon I wish to keep it at near ambient temperatures. A good combination would be the Beloptik 2 inch UV/IR on KG3 for additional IR blocking to from 1500 - 2500 nm, combined with the Baader 2 inch 35nm nighttime H alpha filter to reduce the 600 - 700 nm that is passed unimpeded through the DERF. These filters work through reflection and absorption (KG 3 for long IR, Baader 35 nm H alpha for the the 600-700 nm visible), and may have limited lifetimes in the harsher solar telescope environment they're employed in. But they are less costly than possibly having to replace an etalon.
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Re: D-ERF in converging beam?

Post by Rusted » Fri Dec 13, 2019 10:19 am

Bob,

Many thanks for your valuable insight into the thermal conditions within our modded H-a solar telescopes. Vital information to keep us [and our equipment] as safe as possible. From reading Baader's own claims, the purchase of the full aperture 160mm D-ERF should provide a cool beam within the telescope. You seem to suggest the complete opposite.

The simple fact that so much heat was passing through my PST etalon was becoming a serious worry. I often track the sun literally for all the sunshine hours in a day. Usually leaving the drives running while I process videos on the spot just to see how I'm doing on seeing, etc.. I regularly capture and process at the same time. For hours on end when it is sunny.

Each donor PST [etalon] is now around £500 at European secondhand prices. With a distinct degree of pot luck on etalon quality. I am not aware of any commercial PST-like etalons available separately in this price range but I haven't looked.

The option exists to go for a Quark but getting a good one also seems to hang on the drop of a dice. Which is completely unacceptable at these prices. So my existing PST etalon remains the obvious choice and it must be protected as much as possible. Your protective filter suggestions will be ordered.

Now I am seriouly beginning to wonder whether I wasn't cooking my old ASI120MC when it began to show a film of solar-surface like artefacts in the field of view. Though it seems unlikely given the Maier ITF and PST IBF5 in series. Is this likely given the extreme exposure in my case?

The cost of building a complete new 150mm, H-alpha dedicated scope from scratch is becoming even greater than I had imagined. I must count myself lucky that I have got away with it so far. Natural caution and endless experimentation, in trying to improve my imaging results, has probably helped. Now I have a 25" monitor and an ASI 174 "visual" is no longer of much interest. Though [fortunately] I never detected any visual discomfort with the binoviewer.

I have a thermal "pistol" for reading surface temperatures at a distance. As I intend to build a skeleton OTA I could place a blackened, thin metal target at vital component points in the focused beam to check "real life" temperatures. Not so easy to do in my present, closed tube. So I can't try it yet.

Let's be careful out there, people! ;)
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Re: D-ERF in converging beam?

Post by Merlin66 » Fri Dec 13, 2019 3:24 pm

Bob,
I think there's an error somewhere in your maths.
18 W incoming is 18 W based on 1050 W/ m^2
The intensity will vary but surely not to 3.5 KW -????
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Re: D-ERF in converging beam?

Post by Bob Yoesle » Fri Dec 13, 2019 4:01 pm

Now I am seriouly beginning to wonder whether I wasn't cooking my old ASI120MC when it began to show a film of solar-surface like artefacts in the field of view. Though it seems unlikely given the Maier ITF and PST IBF5 in series. Is this likely given the extreme exposure in my case?
I doubt it, as the BF should have taken care of excessive UV/IR reaching the sensor.
I think there's an error somewhere in your maths.
18 W incoming is 18 W based on 1050 W/ m^2
The intensity will vary but surely not to 3.5 KW -????
Hi Ken,

I'm certainly not infallible here, and I will simply share that I calculated the flux density by dividing the area of the objective by the area of the Sun's image at the focal plane: 17,663 mm^2 / 94 mm^2 = 188. So the energy concentration is 188 times the energy falling on the objective. 18.5 watts x 188 = 3,478 watts, so this is how I came to the resultant flux density in watts. I then just made a rough guess that about 10% of the solar irradiance is passed through the DERF to the focus to surmise that about 350 watts would be found at focus. Of course further up the light cone the flux density decreases based on the area of concentration, until you get to the objective where it once again becomes about 18.5 watts.

I'd be happy if additional or alternative methods of calculation would clarify the issue...
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Re: D-ERF in converging beam?

Post by Rusted » Fri Dec 13, 2019 4:37 pm

I am mathematically dyslexic so will have to keep this simple.
All figures are approximate:

Area of objective is 177cm^2. [7.5x 7.5 x Pi]
1m^2 = 10,000cm^2 [100x100] = 1000W m^2 at sea level.
1200cm FL = 1.1cm solar image at focal plane. Or [say] 1cm^2 image area.

10,000 / 177 [cm^2] = 57W total infall on the surface of the objective.
Concentration by area alone = 177/1 [cm^2] = 177x

177 x 57W = 10,089W at focal plane!
10,089 / 10 [D-ERF @ 10%] = 1009 W at focal plane.
Seems rather unlikely. :mrgreen:
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Re: D-ERF in converging beam?

Post by Merlin66 » Fri Dec 13, 2019 9:00 pm

The area of the objective determines the energy input.
18W input means that there's always only 18W to play with.
There's no concentration in terms of the input energy, it will always be 18W.

What does change is the intensity i.e. W/mm^2.
As the area of the beam changes towards the focus, the intensity (W/mm) will increase but it's still 18W / area of beam.
The area of the solar disk at focus (roughly 1/100 the focal length) will, I think, determine the max intensity, in W per mm^2.

Each element, ERF, etalon, ITF, Blocker etc either reflects or absorbs this input energy, reducing it as we move through the system.

I did an energy balance sheet for a typical solar scope many years ago....need to re-find it to show what happens.
Elements which absorb light (absorption filters) will heat up and re-emit the energy at the temperature blackbody wavelength.
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Re: D-ERF in converging beam?

Post by Merlin66 » Sat Dec 14, 2019 1:25 am

Looking at the energy distribution by wavelength is sometimes better.
Solar_Spectrum.png
Solar_Spectrum.png (36.25 KiB) Viewed 1532 times
I did some rough calculations which would indicate the D-ERF would pass about 26% of the total incoming solar energy. 130 W/m^2 at Ha, and 130 W/m^2 in the IR above 1500 nm.
The etalon reflects about 90%, the same (?) for the ITF and the final blocking only allows a 1 A bandwidth at Ha to get through.
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Re: D-ERF in converging beam?

Post by Rusted » Sat Dec 14, 2019 8:15 am

I'm usually completely out of my depth when it comes to the simplest maths but let's try this again: :D

Simplifying our energy intensity problem at focus: Using 1000W/m^2 at sea level.

Lens area 15cm Ø = 176cm^2. Image area 1cm^2. Difference in area = 176:1 = Amplification factor.

1000w/m^2 at sea level = 0.1 W/cm^2. [Just matching area terms.]

0.1x 176 = 17.6W/cm^2 Energy falling on objective in cm^2. This is not the total energy but intensity per unit of area.

17.6 x 176 = 3100W/cm^2. Energy falling on 1cm ^2 solar image in cm^2.

For the bare objective and empty OTA we have an intensity of 3100W/cm^2 at the solar image ignoring all losses.

Assuming 10% passed by the D-ERF this still equates to 310W cm^2 at the focal plane ignoring the etalon's effect.

Can the PST etalon accept continuous baking by ~300W/cm^2 ?

If the etalon is 2cm in diameter it sees an increased beam area.

So 300W x 3.142 = ~1kW total heating effect after the D-ERF!!

Please tell me this isn't true! :mrgreen:
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Re: D-ERF in converging beam?

Post by Merlin66 » Sat Dec 14, 2019 8:29 am

There’s no amplification ratio.
In your example you have 17.6 W input to a 15cm objective.
At the focus, no filters the energy at the focus will be 17.6 W.
If the solar area at the focus is 1 cm^2 then the intensity is 17.6 W/ cm^2
The D-ERF with a throughput of 260 W/ m^2 , 0.26 W/ cm^2 would have this intensity at the etalon.
To answer - this isn’t true - yes it is not true.
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Re: D-ERF in converging beam?

Post by Rusted » Sat Dec 14, 2019 9:38 am

EDIT: I have deleted my original post as nonsense.

The total energy available is only 1000W/m^2 or 0.1W/cm/2. 0.1W/cm^2 x area of lens of 176cm^2 = total of 18W at the objective.

Paper burns at 233C. It does not spontaneously combust in the brightest sunlight even at he highest recorded air temperatures on earth. [56.7C] I still believe there is an amplification or concentration factor = area of objective/area of the solar image. How else do we explain the rise in temperature at the solar image and all the way along the converging beam of any positive lens, or concave mirror?
Last edited by Rusted on Sat Dec 14, 2019 10:43 am, edited 1 time in total.
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Re: D-ERF in converging beam?

Post by GreatAttractor » Sat Dec 14, 2019 10:36 am

Here's the mistake: 1 m² = 100 cm × 100 cm = 10 000 cm². Assuming irradiance of 1000 W/m² (0.1 W/cm²), a 15 cm-diameter objective gathers only π × 7.5² cm² × 0.1 W ≈ 17.7 W.

(Or in other words: if the unfocused irradiance is 1000 W/m², you can't have a mere 15-cm lens gather > 3 kW ;)).
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Re: D-ERF in converging beam?

Post by Merlin66 » Sat Dec 14, 2019 10:44 am

Your misunderstanding starts here:
""0.1x 176 = 17.6W/cm^2 Energy falling on objective in cm^2. This is not the total energy but intensity per unit of area.""

0.1 W/ cm^2 x 176 cm^2 = 17.6 W. When you do a units check the cm cancel out and leave just W.
The sensation of heat (energy) is due to the area being illuminated - focus that 17.6 W say on an area of 1mm^2 will give an intense feeling of heat.
The effect of this intense energy/ heat and the final temperature relies on the specific heat of the receiving surface .......Thermodynamics 101.

Consider the area of your face when you look towards the sun, say 20 x 20 cm, the absorbed energy would be 40 W - you feel hot, maybe ....
now consider placing your finger on a 40W bulb, area may be 1 cm^2....hot? definitely!!!
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Re: D-ERF in converging beam?

Post by Rusted » Sat Dec 14, 2019 10:55 am

GreatAttractor wrote:
Sat Dec 14, 2019 10:36 am
Here's the mistake: 1 m² = 100 cm × 100 cm = 10 000 cm². Assuming irradiance of 1000 W/m² (0.1 W/cm²), a 15 cm-diameter objective gathers only π × 7.5² cm² × 0.1 W ≈ 17.7 W.

(Or in other words: if the unfocused irradiance is 1000 W/m², you can't have a mere 15-cm lens gather > 3 kW ;)).
Agreed. :D

Do you agree that the concentration factor of our objective lens = area of the lens/area of the focused solar image?

Then how do we calculate the rise in temperature along the converging beam of our OTA?

Note: By enclosing the converging beam in a tube we are further concentrating solar energy by reducing losses.
Much like a solar oven on steroids. Even a plain piece of glass, in place of the obj.lens, would raise internal temperatures if pointed at the sun! :cool:
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Re: D-ERF in converging beam?

Post by Rusted » Sat Dec 14, 2019 11:16 am

Merlin66 wrote:
Sat Dec 14, 2019 10:44 am
Your misunderstanding starts here:
""0.1x 176 = 17.6W/cm^2 Energy falling on objective in cm^2. This is not the total energy but intensity per unit of area.""

0.1 W/ cm^2 x 176 cm^2 = 17.6 W. When you do a units check the cm cancel out and leave just W.
The sensation of heat (energy) is due to the area being illuminated - focus that 17.6 W say on an area of 1mm^2 will give an intense feeling of heat.
The effect of this intense energy/ heat and the final temperature relies on the specific heat of the receiving surface .......Thermodynamics 101.

Consider the area of your face when you look towards the sun, say 20 x 20 cm, the absorbed energy would be 40 W - you feel hot, maybe ....
now consider placing your finger on a 40W bulb, area may be 1 cm^2....hot? definitely!!!
Thank you for your continuing patience Merlin.

I hope you will forgive my continuing with this discussion until we reach a satisfactory conclusion.
If only to gain a better understanding of the thermal stresses on the vital components in our H-alpha OTAs.

There does seem to be some disagreement even amongst those far more gifted than I in maths and science.
Fortunately [for me] not all H-alpha imagers share your considerable knowledge on this subject.
So we must all gain our snippets of useful information at the feet of those who do.

I hope we all agree that increasing the aperture will increasingly stress the "innards" more than a smaller 'scope.
How can we quantify the increase in thermal stress on any component in the enlarged beam?

A filter will absorb the focused heat if it cannot reflect it efficiently at its first surface.
What about the etalon? Does heat pass right through? Or does it absorb heat? It looks clear to the naked eye.
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Re: D-ERF in converging beam?

Post by Bob Yoesle » Sat Dec 14, 2019 4:27 pm

Hi Ken,

You are of course correct in that 18 W in remains the same (conservation of energy), it is the energy density per unit area which has increased ~ 188 times at the focus. Therefore the 18 W falling over the surface area of the objective can be converted to heat through warming would be equivalent to many more W at the focus. The concentration ratio is simply the area of the collector versus the the receiver. In this case 188. The actual amount of heating depends on the properties of the material and its ability to absorb, reflect, and re-radiate this energy.

If you hold your hand up at the location of the objective, and then place your hand at the focus with the full aperture of the Sun shining through, you will indeed see that the energy density ("intensity") has increased significantly. My calculation was just an attempt to show what the equivalent amount of warming could be... of course for a different material, the energy distribution might be quite different.
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Re: D-ERF in converging beam?

Post by Rusted » Sat Dec 14, 2019 6:44 pm

Thanks Bob. If only the sky would clear I could be taking temperature measurements at vital points in the beam.
We have been in a long run of overcast skies in one of the the wettest years and autumns on Danish record.

The summer was fine and I spent hours, almost every day, "testing my solar kit to destruction."
Almost literally so, when I set fire to a temporary aperture stop on the 6" and then blamed my wife.
The strong smell of burning might well have been due to Her Ladyship lighting the wood stove, but wasn't.
The smoke [and flames!] were literally all my own work. With only a little help from the 18W Sun. :D

It would just be nice to have some solid data to compare with the "theoretical models." ;)
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Re: D-ERF in converging beam?

Post by GreatAttractor » Sat Dec 14, 2019 9:38 pm

Rusted wrote:
Sat Dec 14, 2019 10:55 am
Then how do we calculate the rise in temperature along the converging beam of our OTA?
There's certainly some of that, but probably less important than one would think. The air (over the kind of distances we have in telescopes) is after all quite transparent to solar radiation.

Consider this: Lunt CaK modules are specced for use with unfiltered refractors of up to 100 mm aperture. A module ends with a small ERF, which reflects the converging beam from the objective back towards the front. The ERF is quite close to the focus, so the reflected beam converges somewhere inside the OTA. Still, the resulting "hotspot" is apparently not very problematic (Lunt is not telling people "use only with open-tube strut refractors, otherwise tube currents will make it unusuable").

Now let's say we use a 200 mm telescope with a full-aperture ERF transmitting 10% of energy. The energy gathered is 4x that of a 100 mm refractor, but just 0.4x after the ERF. So the internal tube heating is probably still acceptable.
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Re: D-ERF in converging beam?

Post by Merlin66 » Sat Dec 14, 2019 11:02 pm

https://en.wikipedia.org/wiki/Autoignition_temperature

Looking at the formula...
For paper:
k = 0.05 W/m K
Density = 1201 Kg/m^2
c = 1336 J/Kg/K
T(ig) = 246 deg C
T(o) = assume 20 deg C
q" = Heat flux in W/m^2

The two unknowns - t(ig) - the time to ignite, and q'' the Heat flux.

Entering the above data, we get
t(ig) = 63000 * 0.05 * 1201 * 1336 ((246-20)/(q"))^2

Looking at the earlier example, the input energy is 18W. Assume the solar image at focus is 1 cm^2. The with no ERF etc. the Heat flux at the focus would be 180000 W/ m^2
Putting this in the above equation gives t(ig) to be approx 0.1 sec. Instant ignition!
You can change the input energy and solar image size to reflect your actual situation.

OK, what about when we have an D-ERF in the system, followed by an etalon (which passes 1 A every 10 A = 10% transmission, the rest is reflected back towards the objective.)
The D-ERF as stated previously transmits 26% and reflects 74% of the incident energy ( Note: with a sub diameter D-ERF this means the Heat flux at the external focus in front of the objective is x3 the Heat flux at the telescope focus!)

18W * 0.26 = 4.68 W

then the etalon:
4.68 * 0.1 = 0.468 W

Let's image that energy to an area of 1 cm^2, = 4680 W/m^2 . Using the above formula we get:
t(ig) = 146 sec = 2.5 min (approx)
This means (in theory) it would take 2.5 min to ignite (ie burst into flames, not just smolder) a piece of paper held at the focus behind a D-ERF and PST etalon.
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Re: D-ERF in converging beam?

Post by Merlin66 » Sun Dec 15, 2019 1:18 am

OK, update.
I made a spreadsheet to confirm the above...
The main issue is that a PST mod gives an f10 system - for the example 150mm aperture and assuming it is a f10 system, the min solar diameter at focus is 1.5 cm.
See the attached screenshots.
( this is why a magnifying glass burns paper - the short focal length gives a very small solar image and hence bigger Heat flux)
By the way " No ants were harmed in the establishment of this calculation" ;)
EDIT: I made an error in the formula, now corrected.
I'm beginning to question this formula....when direct sunlight is used (1000 W/m^2) it says paper will burst into flames after 54 mins - obviously never the case!!! I'll continue searching for a better, realist formula.
Solar ignition_no DERF_01.JPG
Solar ignition_no DERF_01.JPG (72.93 KiB) Viewed 1422 times
Ignition time < 1 second
Solar ignition_with DERF_01.JPG
Solar ignition_with DERF_01.JPG (71.05 KiB) Viewed 1422 times
No allowance for etalon etc. Ignition time 5 seconds
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Re: D-ERF in converging beam?

Post by Astrophil » Sun Dec 15, 2019 4:20 am

Interesting calculation on paper burning. Has it been tried and timed in this application? I did not see any reference to the color of paper, relating to its reflectivity at the wavelengths passed through the system nor its thickness which would relate to total absorption (Extinction length). Was a blocking filter used before the focused solar image? Was it part of the calculatios?

Just curious, I love it when formulas work.

Phil

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Re: D-ERF in converging beam?

Post by Merlin66 » Sun Dec 15, 2019 6:08 am

Phil,
Paper colour is not mentioned as a factor in the text. No allowance for ITF or blocking filter at this stage.
Unfortunately the formula I used doesn't seem to be accurate.
Still investigating....
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Re: D-ERF in converging beam?

Post by Rusted » Sun Dec 15, 2019 10:12 am

GreatAttractor wrote:
Sat Dec 14, 2019 9:38 pm
Rusted wrote:
Sat Dec 14, 2019 10:55 am
Then how do we calculate the rise in temperature along the converging beam of our OTA?
There's certainly some of that, but probably less important than one would think. The air (over the kind of distances we have in telescopes) is after all quite transparent to solar radiation.

Consider this: Lunt CaK modules are specced for use with unfiltered refractors of up to 100 mm aperture. A module ends with a small ERF, which reflects the converging beam from the objective back towards the front. The ERF is quite close to the focus, so the reflected beam converges somewhere inside the OTA. Still, the resulting "hotspot" is apparently not very problematic (Lunt is not telling people "use only with open-tube strut refractors, otherwise tube currents will make it unusuable").

Now let's say we use a 200 mm telescope with a full-aperture ERF transmitting 10% of energy. The energy gathered is 4x that of a 100 mm refractor, but just 0.4x after the ERF. So the internal tube heating is probably still acceptable.
Thanks. Out of interest, I once pulled the etalon adapter from the 2" tailpiece extension of my H-a 6" while tracking the sun.
To see a tight vortex of brightly lit particles rising up the red beam into the telescope tube as if drawn upwards by convection.
Possibly anodizing "dust" rubbed off by friction between the closely fitting parts? It seemed quite dramatic at the time.

I doubt a continuous convective current is even possible in a tightly closed refractor OTA. No chimney effect without a draught.
So the vortex was probably, only the result of the sudden withdrawl of a sealed item. Like pulling a cork on a bottle.
Made highly visible only by my activities in the relatively dark dome while tracking with a relatively large instrument in H-a.
The vast majority of imagers are probably working out of doors so extremely unlikely to ever see such a "smoke" test. :D

Any question of tube currents during solar work should be readily exposed using a clear plastic, or glass tube for the OTA.
Or even a clear plastic window to save money. Get a smoker to blow some smoke into the main tube.
Close up and wait to see if the cloud moves in the converging beam.
The objective [and D-ERF] can provide suitable illumination for the test.
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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