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Quark on Newtonian?

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#1 UniversalMaster

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Posted 18 June 2020 - 11:41 PM

Hi!

 

Newtonians are quite cheap for larger apertures and work great with white light films. My 10" give excellent views of the sun with a full-aperture Baader film. But for a Daystar filter (e.g. quark), they state on their website that

 

"Note:  Newtonian Telescopes do not lend themselves easily to DayStar applications."

http://daystarfilters.com/SCT.shtml

 

Why is that exactly?

 

Best Regards,

Søren



#2 Astrojensen

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Posted 19 June 2020 - 01:45 AM

You need a front mounted energy reduction filter. These are currently not serial produced in apertures larger than 180mm (to my knowledge). ARIES Optics in Ukraine can make a large one, but you'd better be sitting down, when you hear the price.  

 

 

Clear skies!
Thomas, Denmark


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#3 sg6

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Posted 19 June 2020 - 02:04 AM

A Newtonian collects the light/energy over the full aperture and with a quark there is no front filter, so you collect say 10" diameters worth.

Then you focus/concentrate that energy down and the first item along the optical chain is the secondary.

So you are concentrating a lot of energy on to the secondary - a full 10" worth of IR (heat), then consider that a 2" lens will focus the sun (exactly what you are doing) enough to set fire to material.

 

So you end up almost guaranteed to cook the secondary, the adhesive and the general full assembly. Replacing secondaries at regular intervals takes various amounts of $'s and time.

 

Does make you wonder how a focuser on a refractor survives, however I suppose that the light/energy should never contact the focuser assembly, just pass through the hole.

 

The "problem" of the Newtonian is that secondary mirror up towards the top, it has to be taken into account.



#4 Astrojensen

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Posted 19 June 2020 - 02:52 AM

A Newtonian collects the light/energy over the full aperture and with a quark there is no front filter, so you collect say 10" diameters worth.

Then you focus/concentrate that energy down and the first item along the optical chain is the secondary.

So you are concentrating a lot of energy on to the secondary - a full 10" worth of IR (heat), then consider that a 2" lens will focus the sun (exactly what you are doing) enough to set fire to material.

 

So you end up almost guaranteed to cook the secondary, the adhesive and the general full assembly. Replacing secondaries at regular intervals takes various amounts of $'s and time.

 

Does make you wonder how a focuser on a refractor survives, however I suppose that the light/energy should never contact the focuser assembly, just pass through the hole.

 

The "problem" of the Newtonian is that secondary mirror up towards the top, it has to be taken into account.

And that is why you need a front mounted energy filter...

 

 

Clear skies!
Thomas, Denmark



#5 AJamesB

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Posted 19 June 2020 - 03:15 AM

 

but you'd better be sitting down, when you hear the price.

Last I checked, the 8" was $2,000 US dollars, and the 11" was about $4,000.  They even make a 14inch, but not sure about its cost.  There are some amazing images though from people with c8/c11's and that have the seeing to support such focal length.



#6 tholan

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Posted 19 June 2020 - 07:42 AM

This is a bit "off the wall' but would it be possible to place a smaller diameter ERF up near the secondary to protect it. Probable way too hot in that location.



#7 UniversalMaster

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Posted 19 June 2020 - 08:02 AM

Sorry, maybe I should have been more clear.... I was not talking specifically about my 10" Newtonian, but Newtons in general. I will never get the money to but a front-mounted DERF in that size. But a 6" should be OK with only a UVIR filter:

http://www.daystarfi...-or-Front-mount

 

However, Daystar states quite generally that Newtonians are no good with their filters:

"Note:  Newtonian Telescopes do not lend themselves easily to DayStar applications."

http://daystarfilters.com/SCT.shtml

 

My question was simply, "Why not Newtonians for Daystar filters?"



#8 UniversalMaster

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Posted 19 June 2020 - 08:04 AM

A Newtonian collects the light/energy over the full aperture and with a quark there is no front filter, so you collect say 10" diameters worth.

Then you focus/concentrate that energy down and the first item along the optical chain is the secondary.

So you are concentrating a lot of energy on to the secondary - a full 10" worth of IR (heat), then consider that a 2" lens will focus the sun (exactly what you are doing) enough to set fire to material.

 

So you end up almost guaranteed to cook the secondary, the adhesive and the general full assembly. Replacing secondaries at regular intervals takes various amounts of $'s and time.

 

Does make you wonder how a focuser on a refractor survives, however I suppose that the light/energy should never contact the focuser assembly, just pass through the hole.

 

The "problem" of the Newtonian is that secondary mirror up towards the top, it has to be taken into account.

 

Is that much different than what the diagonal mirror experiences used in many refractors? Also pretty close to focus...



#9 MalVeauX

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Posted 19 June 2020 - 09:35 AM

Heya,

 

In general, I do no suggest a newtonian for Ha/Cak/etc solar. It's good for photosphere imaging. Fantastic for that and a very inexpensive way to get into high res photosphere imaging in solar. But it's not a good choice for HA or CaK, or any other ultranarrowband imaging. The mirrors of the longer focus newtonian are superior to F2 SCT mirrors, but the design of the Newtonian has issues with thermal handling and this is why I cannot suggest it for Ha/Cak, even with an appropriate D-ERF that is full aperture.

 

I have that setup that people are talking about. And using the Quark or something else doesn't matter. It has nothing to do with Daystar nor another etalon product being used there. It has everything to do with that secondary mirror design instead and the issue is thermal handling, even with a D-ERF. More on that in a minute.

 

Full aperture D-ERF on a Newtonian with a Quark. It works. I don't recommend it.

 

32881881387_d889580dce_b.jpg

 

33948346358_e8f517e94b_b.jpg

 

32881881627_40e35b6f3e_b.jpg

 

So naturally, why? Right? It works. But why not recommend it. A couple of us have done D-ERF's on Newtonians and the outcome is the same in terms of finding there's an issue with thermal handling at the secondary mirror. Not sure exactly how its happening, but it is. Christian has some great examples with a heat camera to show the concentration on the secondary mirror and how even with a 2nd D-ERF in front of that one is very difficult and still doesn't quite tolerate the heat. It's strange because SCT secondaries handle it. I'm not sure if it's because the mirror on the secondary of a SCT is behind plastic completely or what. But SCT designs handle this heat very well, so a SCT with a full D-ERF and a 2nd final D-ERF in the focuser eliminates virtually all heat and remains high transmision and works great, and I have this setup too (C8 Edge + D-ERF + Baader Red CCD-IR Block and throw a Quark on the back or any etalon you have and you are in business and the heat is handled). But this is not the case on a Newtonian. The primary mirror is fine and with the full aperture D-ERF it handles heat there ok. But the secondary mirror collects heat for some reason. Maybe it's due to it being struck by the energy passing by on the way to the primary mirror and then reflecting or something like that. Maybe it's the spider. Maybe its the whole apparatus. I'm not quite sure. But several people trying this setup came up with the same issue, heat issues, even with full D-ERF, and it was always at the secondary. And even with a 2nd D-ERF suspended (at great pain) in front of the secondary, it still was getting too hot.

 

The results are the secondary mirror has major thermal issues, it gets hot. Even though I have a D-ERF in front of it completely and a second D-ERF in the focuser after the secondary, the heat coming out is still significant. Way hotter than SCT (even though you'd think logically the designs are not too terribly different, but they really are significantly different from a thermal stand point). The result is the image under heat twitches like its nervous and bends and fluctuates, you can't get focus and keep it as it drifts constantly as heat fluctuates. It's not seeing conditions, it's heat on the mirror, not even heat in the tube, but heat on the secondary mirror itself. The only way to stop this would be a D-ERF in front of the secondary mirror, but that proves to be highly challenge to mount and still doesn't work perfectly. So again you can do this, but the heat is there, and your image will not be clear, it will twitch and spasm oddly, even with excellent seeing conditions. So it's no good for imaging and likely not healthy for your expensive imaging filters.

 

SCT are inexpensive too, up to 8 inches. So for a high res system of 8 inches, I would certainly pick SCT over Newtonian for this application (but specifically only for HA, not CaK). The SCT is also fine for photosphere imaging. A newtonian can be used for photosphere imaging great with solar film for cheap, any size. But for HA, the needed transmission requires a D-ERF, not even photograde film, and the D-ERF allows too much heat on that secondary. And CaK is in the same spot as HA in terms of that setup, since its ultranarrowband, a D-ERF is needed, not film. And SCT F2 mirrors are awful for CaK, where Newtonian longer focal-ratio mirrors are much better for CaK. So there's no perfect instrument for this, outside of a huge refractor. Refractor designs are ideal for solar narrowband imaging of all wavelength, simply super inconvenient in terms of size and cost. But far easier to thermally handle. SCT comes next. Newtonian last unfortunately.

 

Small aperture off-axis stuff works on Newtoians, but .... it's small aperture, so why bother, just use a refractor.

 

Anyhow, I would love to learn something new about newtonian design with respect to thermal regulation. I have the D-ERF. I have the Newt. I have HA filters to cycle it with of several sorts. But the heat is a problem even with the D-ERFs on Newts. Otherwise, they would be a super option for high res! Especially shorter wavelength options. I literally bought my F6 quartz 200mm newtonian for solar for this reason. The newt is fantastic for photosphere imaging (solar film and you're set!). But not ultranarrowband where you need a D-ERF due to transmission.

 

Very best,


Edited by MalVeauX, 19 June 2020 - 09:39 AM.

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#10 vincentv

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Posted 19 June 2020 - 04:28 PM

Marty,

I'm sure you have checked this, but my hypothesis is that you're seeing the convection currents from the spider and secondary holder. That's the only difference I see between newt and SCT. The secondary does cast doubts but then the spacing between holder and ERF is different.

Have you tried placing a cardboard/paper shade on top of the ERF? Just enough to shade the secondary and spider vanes.


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#11 MalVeauX

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Posted 19 June 2020 - 05:42 PM

Marty,

I'm sure you have checked this, but my hypothesis is that you're seeing the convection currents from the spider and secondary holder. That's the only difference I see between newt and SCT. The secondary does cast doubts but then the spacing between holder and ERF is different.

Have you tried placing a cardboard/paper shade on top of the ERF? Just enough to shade the secondary and spider vanes.

Heya,

 

This is a good point and assumption. I can test this, and if my weather improves I will try and get some data on it. It would be nice if it were as simple as that for sure. I've seen and talked with others who are attempting this with large newts and so far, nothing is a good working model for thermal stability. The spider arms would hold heat and transfer it to connections so that may also contribute. All of it can contribute. The SCT lacks that and the mirror is shielded behind plastic coupled to a new plastic piece with air between the two, so there's insulation. But with newts is metal touching metal touching glass, so heat transfer is more efficient in that system.

 

Very best,



#12 bandazar

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Posted 19 June 2020 - 05:53 PM

Wonder if a partial type of coating could be applied to the primary.  I know some people have been taking off the coatings off of the primary to view white light viewing.  Maybe a coating that reflects only a part of the rays based on aperture?   Is that possible?   I don't know if it'd be cheaper than a D-ERF though.  But it would be lighter.  You'd still want a quality mirror, imo.  And of course  your local seeing conditions would have to warrant such a large aperture use.  In my area, maybe only 1 or 2 days out of  month are good enough for my 150mm solution. 



#13 Great Attractor

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Posted 20 June 2020 - 01:38 AM

Marty, I'd be very interested to hear about your results when you attempt this. Perhaps in addition to a circular cover on the secondary holder, you could also try putting thin white paper (e.g.) strips on the arms.

 

 

Maybe a coating that reflects only a part of the rays based on aperture?   Is that possible?

Yes. Check out Alexandre's thread here: he's been using a 250 mm truss Newtonian with ERF-like Hα coating on the primary mirror.


Edited by Great Attractor, 20 June 2020 - 01:41 AM.

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#14 UniversalMaster

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Posted 20 June 2020 - 03:29 AM

Thanks for all the replies. I still quite don't understand why the secondary in a Newtonian is in any different situation than the diagonal mirror in a refractor though. But I guess you can't argue with empirical data :-)

The guy in the link, Alexandre, seems to use a truss Newton in addition to his special mirror. Maybe that will help keep the optics cool and help eliminate tube currents?

One could also try to move the primary as far as possible up the tube to defocus the sun some more near the secondary.

#15 vincentv

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Posted 20 June 2020 - 02:43 PM

The linked scope is fully open, any convection currents will be broken up by the slightest breeze. Compare that to Malveaux's fully closed setup. Also painting the upper side of the vanes white does not risk reflections since the ERF is effectively at the other end.

 

Another test is to add a fan on the upper end of the OTA to avoid stratified air. It's a similar approach to what people with big dobs use to break air currents near the primary. Adding a shade is simple, but the fan is probably the best longterm fix since it doesn't add diffraction and/or alignment errors.

 

Moving the primary is valid. Curiously it puts you in the same position as any other newt designer: do you use a large secondary to deal with the heat (normally it would to get a wider FoV) or use a small one to limit diffraction effects. And of course it doesn't help if the actual cause are the spider vanes.

 

I'm following with interest. A ~114-120mm long focus newt would be a terrific all around solar scope.


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#16 MalVeauX

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Posted 20 June 2020 - 02:46 PM

A ~114-120mm long focus newt would be a terrific all around solar scope.

A 120mm achromatic long focal-ratio refractor already does this much better, without any need for front aperture D-ERF, making it tremendously easy to do all wavelengths from 393nm to 656nm with highest throughput for the aperture. A simple internal sub-aperture D-ERF via a tiny 50mm filter (such as a Baader Blue or Red CCD-IR Block Filter series) will handle all of it. Much more simple than a newtonian. Same aperture range. Inexpensive. Much easier focuser placement. Higher transmission for aperture too.

 

I do this with a 120mm F8.3 refractor and a 150mm F8 (often masked to 120mm F10) refractor.

 

I think this is also true up to 150mm with refractors (inexpensive 150mm achromatics are widely available).

 

Where the Newtonian starts to become attractive for solar for cost is at 200mm and larger, where a refractor is no longer easy to source and becomes bigger and heavier than a typical Newt does.

 

Solar Newtonians with big apertures for photosphere imaging are the best cost per aperture too (and I recommend them over SCT as SCT has fast F2 mirrors which are terrible for short wavelength, if you want to attempt short wavelength imaging for higher angular resolution to maximize the aperture's potential). F6 or longer mirrors are superb for this, even when not custom or specially ground down for this purpose. Solar film makes it possible without any changes to a commercial mirror or OTA which is great, and inexpensive.

 

Solar_Newt_200mmF12_BaaderND38_03132020.jpg

 

Very best,


Edited by MalVeauX, 20 June 2020 - 02:55 PM.

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#17 Great Attractor

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Posted 20 June 2020 - 03:06 PM


Solar Newtonians with big apertures for photosphere imaging are the best cost per aperture too (and I recommend them over SCT as SCT has fast F2 mirrors which are terrible for short wavelength

To be precise, the problem is not due to f/2 itself, but the additional corrector(s) in the optical path (spherochromatism). An all-mirror system will perform equally good (or bad) for all wavelengths. (But of course to have a usable system with a f/2 primary, some form of corrector is required).


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#18 MalVeauX

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Posted 20 June 2020 - 03:18 PM

To be precise, the problem is not due to f/2 itself, but the additional corrector(s) in the optical path (spherochromatism). An all-mirror system will perform equally good (or bad) for all wavelengths. (But of course to have a usable system with a f/2 primary, some form of corrector is required).

That is true and thank you for the precision point. The problem is finding a commercially available corrector that handles the issues for this system and this mirror figure for 393nm and 430nm. No corrector is needed at all for 540nm or 656nm, but 393nm and 430nm requires it. This makes it a problem. Meanwhile, a F6 or longer mirror can do 430nm without a specific corrector. But a F2 SCT struggles here.

 

Very best,



#19 gnowellsct

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Posted 20 June 2020 - 06:29 PM

Is that much different than what the diagonal mirror experiences used in many refractors? Also pretty close to focus...

Most people using dielectric diagonals in their refractors these days.  On the quark rig, in theory, a good dielectric makes the UVIR unnecessary (so I have read here).   And I confirmed it when very noobish I forgot to put the UVIR in place and did not melt my etalon.  I use the Astro-physics diagonals.  

 

But ever since then I have used the UVIR even if unnecessary.  In any case to the point: most Newtonian diagonals have either standard (89% reflective) or fancy reflective secondary diagonals (95-96%), but they do not have dielectric diagonals.  

 

The fancy dielectrics like Astro-physics are 99% reflective.  If you had a budget newt with an 89% reflective diagonal the 11% non-reflective might lead to heat management issues.  Not catastrophe, just heat management issues.

 

Greg N




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