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120-150mm refractor for Cak/Ha imaging?

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#26 MalVeauX

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Posted 14 February 2019 - 09:04 PM

When it comes to measuring how much heat is happening to the clear unobstructed glass in a refractor, sure, the transmission is very high so that energy isn't going into the glass and it's fine. But, what it hits at the end of that refractor is not fine. This is where lots of us have broken things. Also, little apertures don't produce the same load that a large aperture does. There's a big difference between a 60mm, 80mm, 150mm, 200mm, 280mm, etc when it comes to that. A lot of the information is largely based on refractors where you're just passing light through glass that has incredibly high transmission so that glass isn't absorbing that energy, just passing it on.

 

This doesn't work when you apply it to the SCT or reflector design. Point that at the sun, big aperture, without anything other than a little UV/IR cut filter and something is going to get damaged. If nothing else, you can melt whatever is at the end of your imaging train real quick.

 

I've cracked KG3 filters with 150mm aperture, filters made to absorb IR energy. And my eltaon(s) & cameras without a front mounted ERF at 150mm overheat (goes offband). And again, tube seeing is a real thing (same reason planetary imagers don't want tube currents). Having a bunch of heat transferring around in there is bad for seeing. I've experienced that at 150mm with only a UV/IR block filter and the seeing limitations were very apparent (and solved immediately with a front mounted ERF to keep most of it out to begin with).

 

And like your example, pointing a big aperture oil spaced triplet at the sun for a while will not go well likely for a long time.

 

Monday, I will have my filter-cell in hand to setup a SCT with a tri-band 214mm narrowband dielectric filter (serving as a full aperture ERF). I'll do some measurements and compare to my 150mm with it's ERF setup. Currently, my 150mm refractor setup puts a 120F beam (unfocused) onto some paper when I measured it last (not enough to start combustion). And that's what I image with.

 

Very best,


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#27 DennisK

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Posted 14 February 2019 - 09:48 PM

Maybe what we're all saying here is that you need something - whether it be a front-mount ERF or a UV/IR farther down the optical train, to provide "protection" for the system.  Where that protection is, and how good/strong/tight/whatever it is depends on the type and size of the optical train you're using.  I only use refractors and that's what I have experience with. But, when I first went to NEAF and spent time in the quad with the solar guys, they were all using refractors, whether it was Greg with his Solar Spectrum on the A-P or Barlow Bob with his Coronado - I think that was on a Televue Genesis but I'm not sure.  I never "learned" anything else.



#28 BYoesle

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Posted 15 February 2019 - 04:32 AM

Just doing some thought experiments here. While I do not claim to be accounting for all the variables that might come in to play, it seems one must account for the real world amplification (concentration?) of solar irradiance into a much smaller area. This seems what needs to be mitigated to keep these rather expensive filter systems from being over stressed by thermal loading. This makes sense as a function of the area of energy concentration.

 

Regarding the statement about the sun not having a ton of watts of transmission in the IR, it appears that the area under the solar irradiance curve for 600 nm and beyond as passed by the RG630 ERF is substantial – perhaps as much as two-thirds of the total power in Watts of solar irradiance:

 

800px-Solar_spectrum_en.svg.png

Total solar irradiance is nominally specified as 1050 W/m^2, this equals 0.00105 W/mm^2.  The area of an 80 mm diameter objective is 5024 mm^2, so this equals 5.275 W. However, this is enough for a magnifying glass to ignite paper on fire (the famous Fahrenheit 451 degrees).  What’s going on here? I think it has more to do with flux density at the focus of the objective, not the ~ 5 Watts collected by the objective.

 

The area of a 150 mm diameter objective is 17,662.5 mm^2, so this equals 18.545 W entering the telescope. However, this 18.545 W is focused to 93.61 mm^2 in an f8 telescope, which appears to represent a 188.67 increase in the flux density. This then is possibly equivalent to 3,498 W. Again if we look at the spectral irradiance distribution, it appears about two-thirds comes from ~ 600 nm and beyond, or about 2300 W flux density passed by an RG630 ERF at focus.

 

I think if you place your hand at the focus of your 150/1200 refractor, you’ll find the flux density of ~ 2300 Watts at focus pretty uncomfortable, compared to the just ~ 19 watts entering the telescope (e.g. “The palm of your hand doesn't light on fire in the sun and it's not even transparent.”) While the air in the scope might be minimally heated, the filter components themselves – being much closer to the focal plane – may become substantially warmer, giving rise to local thermal currents in the air directly adjacent to these components, and stressing the filter coatings themselves. I believe this is the advantage to use an objective mounted DERF compared to a smaller filter located near the focus.

 

Again, I'm just thinking out loud, and I welcome any further and more informed/detailed analysis others might be able to put forward.


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#29 MalVeauX

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Posted 15 February 2019 - 07:59 AM

I can confirm the discomfort of the focused beam from a 150mm F8 refractor without any thermal mitigation. grin.gif

 

With the 150mm Daystar UV ERF and Baader UV/IR block filter on my 150mm, the load is significantly decreased. I can leave my hand in the beam. Not a suggested test, but hey.

 

I put some paper in the beam and pointed a temperature gun at it and got this:

 

DaystarERF_and_BaaderUVIRblock_Temp_10182018.jpg

 

Very best,


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#30 DennisK

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Posted 15 February 2019 - 09:06 AM

There's a lot of interesting info here but I think we've managed to highjack this thread a little too far away from the original topic.  I personally don't have any more to add.  I retired from engineering 6 years ago and I have no interest in sitting down today to work through thermal calculations on optical systems.  What I have has been functioning for several years without any issues but I will agree that there are other approaches that are just as valid.  Having been out to DayStar for their SolarFest last year, spending time with their staff and seeing their facilities, as well as talking to many other people with interests in this area, I'm comfortable with where I'm at.  I appreciate the exchange of ideas here, it will give me something to mull over the next time I crack open a bottle of chardonay and need something to occupy my time...



#31 MalVeauX

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Posted 15 February 2019 - 09:48 AM

All good, I think it is still relevant in a sense because choosing a 120~150mm aperture refractor, or any optical design, for ultranarrowband solar imaging is the entry point where one has to really start considering large aperture ERFs, which is generally a significant cost which adds to the choice.

 

Very best,


Edited by MalVeauX, 15 February 2019 - 09:49 AM.

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#32 torsinadoc

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Posted 15 February 2019 - 10:17 AM

I started the topic and found the discussion educational


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#33 DennisK

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Posted 15 February 2019 - 11:31 AM

I started the topic and found the discussion educational

GOOD, that's important.  I'm relatively new to CN, I was just concerned that we might be going too far off-topic for the info you were seeking.  I've seen this happen on other forums, sometimes you have to "move" the hi-jack to a thread of it's own.  But MalVeaux's comment above is relavent as well - if you're looking to make the investment in bigger/better/pricier optics, there are other issues that you need to be aware of, things that are less of a concern when you're at a "lower" level in the game.



#34 George9

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Posted 15 February 2019 - 11:57 AM

The thing on this thread that helped me, especially for say a 120mm scope is this: DayStar's comment that wouldn't you rather have a reflective 2" filter inside than a 5" heat-absorbing piece of glass in front. I think yes.

 

For a 150mm, I think a reflective 6" filter like the D-ERF in front is really the right way to do it. I agree with Marty that with a 150 there is just too much energy going in to play with a 2" UV-IR, or than for some testing. E.g., when the scope doesn't track and you are hitting some black thing, perhaps plastic, in the rear of the scope.

 

George



#35 BYoesle

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Posted Yesterday, 06:29 PM

DayStar's comment that wouldn't you rather have a reflective 2" filter inside than a 5" heat-absorbing piece of glass in front. I think yes.

Hi George - yes indeed!

 

I found another plot of the distribution of solar energy that makes me actually more concerned for the need to block IR ahead of the filter system (if not at the front of the OTA).

 

Solar energy distribution.png

 

The statement that "the sun doesn't have a ton of watts of transmission in the IR. The solar transmission watts is very heavy in the visual spectrum," appears inaccurate.

 

52% of the solar radiation at sea level is IR, 43% is in the visual spectrum, and only 5% is in the UV (what the GG and RG glasses are meant to block by absorption).

 

"We moved from Red to yellow in or ERF glass so the clients didn't need to buy new glass for a Sodium or Helium filter. The impact is perhaps 5% more heat, but 5% of a load that is borderline insignificant to begin with anyway." Again this statement does not recognize what is happening near the focus of the instrument where the filter is located. Using a "yellow glass" long-pass filter instead of a RG630 as an ERF is adding insult to injury - you are allowing all the IR and a significant portion of the visible spectral energy through to your filter system. I would estimate this visible and IR transmission amounts to about 75% of the total energy, not the 66% I had stated earlier.

 

It is difficult to see how either the yellow or red glass absorptive filters can do the job "to reduce the thermal load so that the telescope and accessories are not at risk of damage or overheating and poor performance," when it appears what they would absorb is only about a 25% of the total energy. On the other hand, if the filters and their mechanical components are designed purposefully to take the concentrated amounts of thermal energy where they are located, then all is fine. But again this may result in localized heating effects for the components and the air lying next to them, and give rise to undesirable thermals within the OTA. My opinion is that if I can increase the filter component's longevity by spending a little more on a true blocking filter or system of filters (which as noted previously costs way less than $20K), then this makes sense to me. 

 

Regarding the statement "you saw my talk on how much energy makes it into your telescope" and the rationale that there is a minimal amount of energy: As I pointed out earlier, the 1.05 mW per square millimeter entering the objective is not what matters, it is the energy flux per square millimeter of area near the focus for the DayStar and Solar Spectrum filters. I think we all know this.

 

Only 5.3 W is spread out over a 3.1 inch objective; but concentrated at the focus this is enough to ignite paper or burn wood:

 

Magnifying glass.jpg

 

Therefore I would agree that a front mounted ERF consisting of an absorptive colored glass filter that passes all the IR and a significant portion of the visual spectrum is indeed a "dumb" idea. Spending over $1000 for one that is only 0.25 lambda, and then "heats, swells and deforms" is even dumber, and likely is not as homogenous or striae free as optical glass, especially when for about $100 more you can get a true UV/IR blocking DERF with dielectric coatings that prevent heating, swelling, and deformation, applied on optical grade BK7 polished to 0.10 lambda - and then maintains this specification (it does require a cell however).

 

In summary, if you are using an absorptive colored glass ERF, at least add a good IR or UV/IR blocking filter to the nose of the filter or telecentric lens system itself. Indeed, this seems to be a must if you are dealing with an expensive SE or PE grade filter, let alone a entry level Quark. The BelOptik UV/IR KG3 looks to be ideal as it will block IR out to at least 2500 nm. Add a nighttime H alpha filter in front for even better protection from the visible spectrum components which may contribute to heating as well.

 

Doing so could delay or even save you from needing an $875 blocker repair/replacement... waytogo.gif


Edited by BYoesle, Yesterday, 10:27 PM.

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#36 George9

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Posted Yesterday, 10:46 PM

Bob, very good. Does that BelOptic transmit enough? 75% in H-alpha and in the low 80s in CaK. Seems like it should be possible to design one that transmits more. E.g., why not KG2, which would get close to 90% in H-alpha (or at least KG1). The CaK reduction must be related to the dielectric coating as KG3 should transmit there.

 

George

 

EDIT: I see that KG2 transmits 20% in near IR, and KG1 is 5%. So may be too much. But perhaps combined with the dielectric.


Edited by George9, Today, 09:30 AM.



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