Using a laser power meter with thermopile sensor to check ERF filters

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etatsolarchat

Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

So just messing around realized I could do some rough testing of filters and see how much total energy they filter out. I used a laser power meter that uses a thermopile sensor. The sensor has a flat response from UV to far IR.

Using a 127mm telescope measured the total power with various filters

9.25W No filter
6.40W Yellow
5.20W Red
5.00W Red
4.75W B+W 486 UV IR blocker
4.50W B+W 489 IR blocker(KG2)
3.25W IR720nm highpass
3.00W BG39 filter laser safety glass

Interesting to note if you want a cheap camera filter for an ERF a UV IR blocker is better than red.

I tested a couple different safety glass types including the BG39 above and they block a lot of energy but BLUR the image, does anyone know why?? I'm guessing surface flatness but I've seen some great images here that were taken through windows so it doesn't make sense...??


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by swisswalter »

Hi Eric

thank you for that measurement. I would have expected a greater reduction of the load than 66% . But maybe the scale is not linear :blush:


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

Well I checked with my finger and there was still a few W coming through :cheer:

If you mean did it have a log scale, no it had a linear scale plus I never changed the range.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by BelOptik_Oliver »

Hi,
Interesting results Eric.
Yes and the (heat-)power in focus is also dependent from focal ratio, bandpass width of filters and solar radiation spectrum - power maximum at 550nm wavelength.
http://en.wikipedia.org/wiki/File:Solar_Spectrum.png

Oliver


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Spectral Joe »

Were these filters at the objective or near the power meter?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

So if I test at 2 different f ratios the power will be different? I could see the full disc so don't understand how the power would be different.

Some were objective some near power sensor, would that change much? The BG39 that caused blurring was at objective.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Spectral Joe »

The total power is dependent only on the objective area, its transmission and the incident solar radiation at the time of the measurement. The power density (watts per unit of area) is dependent on the above, plus the focal ratio. The location of the filter is important, if the filter is at the objective then any optical defect will affect the image quality. If the filter is near the focus then the power density may be high enough to heat it significantly, causing both physical distortion of the glass and a gradient in the refractive index. Both will cause image quality issues. In addition, since these are absorptive filters, the power they absorb has to go somewhere. The warm (or hot) glass radiates well in the long wave IR, and that radiation is picked up by the power meter. The filter acts as a wavelength converter. The KG2 filter should have an integrated transmission of about 20%, given the published data and a blackbody source of roughly 5800K. Your data shows an integrated transmission of 49%. For a 127mm aperture to have 9.25 watts at the focus the incident solar radiation would be 730 watts per square meter, an entirely reasonable value depending on location, time of day and sky conditions. The transmission you show seem a little high, but I would need to know more about the overall setup to get any idea of why.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

F ratio changing power density I understand.

Hmmm so the objective glass converts some energy, would that energy be the cause for blurring? If so then if another IR filter was inline it would filter out the blur? I checked the image quality with a Cak filter in line so I think it would have filtered out any converted energy. When the BG39 goes in front of objective the image gets blurred :?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Spectral Joe »

More than likely it's not very flat. I've run into this with photographic filters. Once the wavefront is spoiled by a bad surface there's not much can be done to fix it.The BG39 being from laser safety glasses, it would not be required to have a very good figure and I would not be surprised that it would spoil the image.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

That's a very interesting data set.
As Joe says the 125mm objective is the only variable in the "collection" of solar energy.
9.25W is an "average" figure...would probably be higher in mid summer (noon in Arizona/ Australia)
How does the size of the meter sensor compare with the solar disk image?
Have you tried the same experiment with other solar filters?
Baader Solar film
D-ERF
any of the ITF/mini-erf's
It would be interesting to extend the comparisons.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

The sensor is 22mm circle, so I believe it should be bigger than the sun image, 950mm focal length, online calculator says it would be 8.76mm

I have a few solar filters with try out when I get a chance and add to the list.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by sullij1 »

Don't know if this is relevant but the data I have seen here and in other sources indicates that you are right in your assumption of the blurring, is due to surface being less than 1/4 wave. As I understand it that if the 1/4 wave is violated then blurring becomes a problem. Hence the reason for the thickness of a filter glass and glass type is partly to preserve the flatness (1/4 wave) if that’s the term - feel free to educate me (Oliver?).

My past experiments with ~2mm thick photographic filters @ 80a blue for CaK were a disaster. I think because the glass type and thickness could not maintain 1/4 wave under temperature.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

The combo of Red and UV-IR block would be interesting.....


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

A red plus uvir would basically be a real ERF, only drawback is 2 sets of glass surfaces and 2 chances of sub 1/4 wave blurr, I guess?? I'll test that to and compare with a real ERF.

So Joe you say not good figure, same thing as saying sub 1/4wave right?

This is what I thought but how can you get a good image through 2 panes of window glass? Just luck that the cheap glass is super flat :?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

It's only to test the energy transmission of the combo not the optical properties...
BTW do you know which "Red" it is???


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by sullij1 »

Yes luck or deliberately buying glass (KG or other) that is 1/4 wave and thick enough to hold "good figure" under temp. ~3mm + (I think) I also think that diameter plus the proper ERF has something to do with it as larger diameter takes longer to heat and distort. Less heat = less distortion. Just speculation on my part but more educated opinions appreciated. ( I deliberatley try to buy 1/4 wave if I can).

So a proper ERF (of proper wave and thickness) prior to the 2 panes of proper wave and thickness. Make sense?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by sullij1 »

BTW, I tried ~25mm ERF. They heat up too fast and then seem Txfer heat too fast via convection on metal body or filter failure and txfer of heat or both. Smaller diameter seems to not be able to dissipate heat fast enough.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

Hmmm
Tooo close to focus without an upfront ERF


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by BelOptik_Oliver »

Hi,
Low costs photographic glass filters are often manufactured in "fire polishing" or they are plastic/gelatin.
Filters with excellent optical performance are precise optical polished and only made optical (color-) glasses.
That is also important for digital imaging.
Flatness per filter surface may be something worse as L/4. Only important is min.L/4 wavefront distortion for imaging measured in transmission.
All genuine optical glasses must be tempered in glass manufacturing. And free of bubbles, streaks, stress and other inhomogeneities.
The resistance and durability at high temperatures is specified in most glass catalogs.

There can be problems if the filter to fixed mounted in cell. Both, glass and cell materials stretch different under temperature changes.

@Joe
Tempered KG glasses holds up to ~300°C and max. heat should be removed with ventilation or bigger filter area to reduce stress. At BG39 similar. Both glasses absorbs IR radiation between 700nm to 1100nm. They are not use alone exactly in focus.
(But the "UV/IR cut on KG3" is a interference filter with KG glass - reflects UV and IR 700-1200nm, over 1200nm blocks the substrat)

Oliver


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by sullij1 »

Merlin for my CaK test I used the 80a @ 72mm diameter on the objective no uv cut but a 72mm X 2mm thick IR cut over the 80a. Unfortunately both filters were only 2mm thick. The Omega filters were back at the focuser end. After reading Oliver’s post I think the photographic filters were probably low quality glass(not name brand).

Regarding Eric’s post on the photographic UV/IR cut over red glass. I did this with my Omega Ha filters. I used a Hoya 72mm RG630 and UV/IR cut X 2mm on both. This was a Neo suggestion. Although not a proper ERF this combination on the objective for Ha seemed to work well enough. The Omega Ha's were the issue in that system. The 2 pane ERF seemed to hold figure in the experiment. Neo swore by his. If I remember correctly he did buy the Baader D-ERF later.

The small diameter 25mm Inconnel ND filter I am using on the current Cak experiment (prior to the CaK filters) heats up quite a bit as “it” is as far back in the image train as the Lunt module. Regardless of the heating, the filter is holding its figure. It does heat up the filter ring quite a bit but is working well. I noted that when I used the Lunt module the ~48mm ERF also got quite hot to the touch, it did not transfer as much heat to the filter wall. Probably because of the larger filter size (as Oliver notes).


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

Ok, I got a chance to do some more measurements, relisted as percent energy blocked.

Diy red plus ir/uv erf is about the same as a lunt ERF, also checked 4 different objective side solar filters all block ~100%

Using a 127mm telescope measured the total power with various filters

Measured 9.25W No filter

Percent blocked
30.8% Yellow objective filter
43.8% Red 25 objective filter
46.0% Red 25 objective filter
48.7% B+W 486 UV IR blocker
51.4% B+W 489 IR blocker(KG2)
64.9% IR720nm highpass
67.6% BG39 filter laser safety glass objective

43.0% Red 25 camera filter
81.0% Red 25 plus B+W 486 UV IR blocker
82.1% Lunt ERF

~100% Baader solar film, film negative and 2 different glass solar filter all objective side



So cheap camera filters and laser safety glass can have poor flatness and cause blurring, does anyone know if its possible and worth trying to polish a 2mm thick filter to have a L/4 surface, or should I use it as a Frisbee :lol:


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by colinsk »

When measuring my lasers I have a correction for frequency on all of my power meters.

This is a good reference to understand one's power meter.

http://www.repairfaq.org/sam/laserioi.htm#ioilpmt


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

Eric,
I'd love to do a similar power test on some the the multitude of filter I have....
Could you supply exact details of the unit you used to allow me to see if one can be borrowed/ hired over here to do the tests.
Many thanks,


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

Coherent 201 Power Meter, with a LM45 thermopile head.

I can test up to 45W with it, so could attach it directly to my scope, but as long as you have any thermopile with a flat response I would think you might could just test filters without a scope. Most modern laser power meters you have to select what wavelength you want to measure and don't accept full spectrum.

On the flatness discussion, I have filters that blur the image, however I wonder why using solar film doesn't also cause serious blurring, is it becuase they are thin?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

Eric,
Thanks for that. Let's see what I can find.
Re the Solar film...it's not just the thiness, but the surface accuracy of the polymer sheet that Baader use. It's quoted as better than 1/4 wave.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

But the film when installed is not flat at all :?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

http://www.baader-planetarium.com/sofif ... sung_e.htm
Doesn't have to be flat, just very very consistant....
To Quote:
TurboFilm TM - guards your optics -
This is the essence of AstroSolar ™ safety film. A highest precision, crystal clear material (not "Mylar"). It is a monomer film which we found after thoroughly testing more than 200 different film samples on our Zeiss autocollimator. This film was used in german Science Institutes as optical membrane for nuclear and particle physics experiments. The basic substrate has been further refined and homogenized by a proprietary annealing process that was developed exclusively for us. Due to these treatments Turbo Film attains perfect homogeneity and performs equal to a 1/10 wave plane parallel optical window. In fact it has the lowest wedge error of any existing optical window. TurboFilm will protect your valuable optical equipment, close telescope tubes against dust and humidity, greatly reduce air currents - all without reducing the optical quality of your telescope.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

I was hoping to be able to replicate the power transmission testing of various filters and combinations.....
Here in Australia, no one seems to be interested in letting me hire/ borrow a Power meter.
The best so far is a quote for a new instrument - $2500!!
Well outside my budget.....bumma....


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by colinsk »

I use an older version of this for setting up my holography bench. It is not linear for frequency.

http://www.ebay.com/itm/SPER-SCIENTIFIC ... 58a02ec193


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

Hmmm why are so many guys on this forum in Australia anyway, a lot of sun and clear sky, or the average population just more interested in astronomy?

If I could somehow move there you'd be welcome to borrow mine but the one I used goes for cheap on ebay or on ebay check out laserbee, a cheap thermopile laser power meter.. I also have one and will confirm if the readings match.

So the baader film substrate is turbofilm, that make me feel a lot better paying so much for it!


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

Thanks for the link.
http://www.ebay.ca/itm/FREE-SHIPPING-La ... 626wt_1073
Looking at the specs the limitations seem to be 5W and the spectral curve <1200nm.
This wouldn't pick up the IR out to 2500nm ?????


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by colinsk »

The Ophir Head PE50S has an expanded range.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

Colin,
I couldn't find that thermopile on the Ophir website....
Did you mean the L50(150)-LP1-35, part #7Z027265???
(I've asked the local agent for a price)


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

3w laserbee unit read ~56mw when pointed at the sun and putting a filter on top would give relatively similar readings with the original testing. The readings were fluctuating a lot because of wind and clouds so you'd need a very calm and clear day to test but seems doable with just filters as pictured.

I don't know the response curve on the sensor but the original one I used was totally flat from 200nm to 11000nm.




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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Merlin66 »

Thanks for the info.
You mention 56mW, but originally with the 5" scope - no filter gave 9.25W. I thought this was beyond the 3W laserbee capability??
The local supplier has come back with a price, for the sensor only of $1995 and six week delivery time!


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

56mw is just pointing the sensor at the sun, no telescope. the sensor is about 1cm2 and probably not easy to mount to a scope but I guess you could use it on a small scopes..


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by swisswalter »

Hi Eric

thank you for sharing. Interesting stuff


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by etatsolarchat »

Hi,


@Joe
Tempered KG glasses holds up to ~300°C and max. heat should be removed with ventilation or bigger filter area to reduce stress. At BG39 similar. Both glasses absorbs IR radiation between 700nm to 1100nm. They are not use alone exactly in focus.
(But the "UV/IR cut on KG3" is a interference filter with KG glass - reflects UV and IR 700-1200nm, over 1200nm blocks the substrat)

Oliver

So Oliver, how small of an internal ERF do you think is possible for objective? Others hear suggest no less than 1/2 the objective diameter but if they can handle 300C why not push it to the limit?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by BelOptik_Oliver »

Hi Eric,

Now general, pure absorber filters placed behind the main objective lens are possible, when the distance to focus are great. But the absorbed light energy from sun stresst the filter glass and deforms the image wavefront. Possibly crack the glass in mounting ring, if the energy enough.? Critical is only the aperture ratio.

My experience with my coated UVIR cut on KG3 filter glass (2" version) mounted 13cm before the focus of 6"F8 FH refractor for using with prominence viewer are positive. Filter glass and cone baffle keeps cool (10min), after 30min the temperature around 40°C or ~100°F (on cone baffle 46°C/~115°F). Measured with fever thermometer at 21°C air temperature and sun over horizon ~36 degree.

Sorry, I didn't experience with uncoated pure KG filter glass as single sub-aperture ERF. Only in combination with Schott RG630 (#25a) glass in front (B+W091 MRC) and naked KG3 filter (B+W 489) around 20cm befor focus in 80/910mm FH refractor for PST mod. It keeps safe cool, but the image quality with this solution is good but not perfect.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by swisswalter »

Hi Oliver

I can basically confirm your findings. What can we do to get the perfect quality :?


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by BelOptik_Oliver »

Hi Walter,

Perfect image quality in B+W filter solution needs wavefront error <L/4 in transmission. B+W filter min. specification is 1 Lambda or better per surface after coating process. That means if you have good luck and front+back surface has identical wavefront error in same direction, then is wavefront error in transmission smaller as the error per surface. Unfortunately this is unknown when buying - no data :(


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by swisswalter »

Hi Oliver

thank you very much. :-( that means buy and try


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by JohnM »

Hi Oliver

thank you very much. :-( that means buy and try

I think I'm learning that first hand. The Lumicon filter I bought is high quality, but I still can't get close to what I thought I should.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by swisswalter »

Hi John

please tell us more about the lumicon and the results. Maybe pics ;-)


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by JohnM »

Walter,
I've got recent posts on this 4" stage 2 from the last few days in the mod section and in the photos section. You've been replying to some :)


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by daslolo »

Maybe this could work with a heat lamp on the other end
https://www.amazon.com/dp/B07PMS5S12/re ... uEbRP309RR
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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by AndiesHandyHandies »

Hi

I purchased a cheap laser pointer thermometer off ebay fo checking temperatures in a PST mod.

Does anyone know how sensitive it is to the colour of the surface its pointed at, Black2 painted to bare aluminium?

Will it work through a lens or filter? I assume a smaller proportion of the glass and most from the Sun shining on a solid surface behind?

Cheers. Andrew.


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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by Rusted »

AndiesHandyHandies wrote: Wed Feb 26, 2020 9:17 am Hi

I purchased a cheap laser pointer thermometer off eBay for checking temperatures in a PST mod.

Does anyone know how sensitive it is to the colour of the surface its pointed at, Black2 painted to bare aluminium?

Will it work through a lens or filter? I assume a smaller proportion of the glass and most from the Sun shining on a solid surface behind?

Cheers. Andrew.
Hi Andrew,

I have one of these and use it all the time in the observatory for measuring the temperature of surfaces. Accuracy unknown.

For measuring the heat in the H-alpha light cone today I used an inexpensive, domestic, digital inside/outside thermometer.
The "Outdoor" sensor, on a 2m lead, was simply held on a scrap of black camping mattress foam at sharp focus.

Test of my brand new 160mm Baader D-ERF with iStar 150/10 H-alpha objective:
120F within a couple of minutes! Though I was unable to burn my hand with sun's image at focus.

Then I inserted brand new 2" Beloptik KG3 and a 2" Baader CCD, 30cm inside focus:
Temperature dropped to ~55F in 40F ambient.

Those filters must be blocking a lot of heat! Particularly when the sun reaches much higher altitudes in the summer!
Unfortunately I couldn't measure the temperature of the filters with the laser thermal "gun" due to the arrival of constant cloud.

Any thoughts on which of these filters should be placed nearest the objective?
Any point in blowing cooling air at the front filter with a small, computer fan?

P1410163 rsz.JPG
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H-alpha: Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3,
Lunt 60MT etalon, Lunt B1200S2 BF, Assorted T-S GPCs or 2x "Shorty" Barlow, ZWO ASI174.
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Re: Using a laser power meter with thermopile sensor to check ERF filters

Post by AndiesHandyHandies »

Hi

I assume you are adding the Beloptik and the ?Red Baader CCD filter after the Baader DERF?

And measurements are without the Etalon?

After a DERF the Beloptik should only be stopping long IR.

The Red CCD should not be stopping much as I assume it leaks in the long IR like the DERF. And the 100nm pass band of the DERF is in the same red band and a bit narrower.

The Baader DERF allows 600-700nm band through from the published graph and switches back on again below 1400nm so some long IR let through.

I use the Baader 35nm filters as internal ERFs with no external DERF. People say they are the same as the DERF BUT Baaders own graphs show the DERF has a wider 100nm passband. So lets 3x more red light through.

The Baader red CCD filter shows a ?140nm passband, no graph beyond 1200nm. I think Baader are assuming the smaller 35nm and photographic filters do not need showing long IR as CCDs not officially sensitive to long IR. However Mark T reckons the long IR is absorbed by the CCD and reduces contrast. And visually I tend to agree. So the red CCD filter should make no difference after the DERF. Try just with the Beloptik?

Personally I would always use a Baader 35nm, fully illuminated, before the Etalon to reduce heat load on it, either as a stand alone ERF or in addition to a DERF. As its a dielectric filter the energy is reflected rather than absorbed as in the Red CCD so less heating of the filter so less chance of cracking it.

I reckon based on Astrograph using the 2" Baader 35nm in refractors up to 150mm as a internal ERF that the 1.25" Baader 35nm can be used as an internal ERF up to 100mm. I use it in a Vixen VMC 110mm as the internal ERF and the laser thermometer shows nothing much above ambient. I can aim it at the holder for the 35nm through the second eyepiece port on the side of the Vixen.

On my PST mod on the Vixen I put the Baader 35nm after the moved forward PST Collimator lens so that its got the collimated light on it and the refleced energy, as its a dielectric filter, goes back out spread out by passing back through the collimator lens as it came in so no dangerous hot spot from the rejected energy re-focussed.

I have used the 2" Baader 35nm as an internal ERF on my 127mm Meade triplet with no issuues. Brightish reflected light spot on the objective not cracked it yet.

So a Baader 35nm as internal ERF will reduce the red light on the etalon by 33% after a DERF. So should be better than the Beloptik and red CCD in the red.

Using the Beloptik at the back as a backup to protect your eye or expensive CCD.

Andrew.


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