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What do you think is the most versatile high resolution solar telescope design?

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What do you think is the most versatile high resolution solar telescope design?

Post by MalVeauX » Fri Sep 21, 2018 3:26 pm

Hey all,

I'm curious what everyone thinks is the most versatile high resolution telescope design for solar imaging application.

There are solar newtonians, SCT's and of course the refractor (for common air spaced and mirror designs). However, I'm curious about which design is ideal for different wavelengths for solar imaging, mainly paying attention to 393nm, 430nm, and 656nm. My understanding of an SCT is that it's not very good at 393nm. I don't know much about a solar newtonian (decoated?). I'm very familiar with refractors for solar. Some of these designs would use a single ERF that passes the appropriate frequencies. And some designs could use several different ERFs for different applications (such as a truss Refractor, to allow internal ERFs specific to the frequencies that can be interchanged).

I'm also looking at the relative cost of each system, excluding the actual narrowband filters and associated blocking filters (if applicable). Mainly just looking at the telescope design and the thermal handling (in the form of the ERF for it).

Of course there is also the question of "what is high resolution?" with respect to aperture and scale for solar imaging. I think we can all agree on large apertures, however, I'm curious what the smallest aperture of each instrument design would still provide what one would consider high resolution? I think we all have an understanding that there are practical limits with respect to our seeing in various places and that there's a point where you'll not benefit from a large scale system, and of course, how do you find that limits without already having such a system?

For simplicity, let's have a thought experiment around the 8 inch aperture. I think we can agree this is fairly high resolution for solar application. It's a relatively common and affordable aperture in all three telescope designs. And it's overall dimensions and weight can be handled on fair mounting equipment. And there are different means to apply energy rejection, both external full aperture, external partial aperture and internal filtration approaches.

Thanks for your time! :bow2

Very best,

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by marktownley » Sat Sep 22, 2018 9:41 am

For me the Baader Tri-band SCT offers great value for money and in a range of sizes to suit local seeing.

http://astrograph.net/epages/www_astrog ... Telescopes
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by george9 » Sat Sep 22, 2018 12:27 pm

I notice that Baader does not use the Edge version of the Celestron. Whereas AiryLabs does. And of course, with a front-mounted ERF, you can do what you want. Anyone notice a difference in performance? Especially at 393nm?

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by christian viladrich » Sat Sep 22, 2018 6:06 pm

My 10 cents :

With a 8-inch SC :
- with the appropriated filter : photosphere is OK from about 450 nm and beyond,
- with the appropriated filter : chromosphere is OK in Ha.
In a nutshel, a C8 (EdheHD or not) is useless for K-line ou Ca K imaging because it is not diffraction limited below about 450 nm.

With a 8-inch Cassegrain, Dall-Kirkham or Newton :
- with the appropriated filter : photosphere is OK at whatever wavelengths (depending on the surface accurary of the optics),
- with the appropriated filter : chromosphere is OK in Ca K and Ha.
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by Valery » Sun Sep 23, 2018 6:46 pm

ARIES' solution for SCTs at 430nm and 393nm.

download/file.php?id=34233
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by MalVeauX » Fri Sep 28, 2018 1:48 pm

I've been looking at different designs and ERF options to see what's versatile and does the job, but also with a budget in mind, and relative ease of mounting (as to not require a mount far more expensive just to carry the thing of course).

I think a large refractor is very versatile, and easier to install energy rejection filters since they can go internal a little behind the front objective. 8 inch lenses are around $2k. The only issue is of course the tremendous moment arm of a long large heavy refractor (even at F6.5~F7). This sort of instrument would require a lot of mount just from the moment arm perspective, even a "shorty" 8 inch refractor is long and heavy (that's also assuming a truss refrator design with modular ERF options internally).

I've been trying to learn more about solar reflectors, with dealuminized mirrors to act as herschel wedges but with monstrous aperture. Also interesting to consider and obvious versatile from the standpoint of being a fairly inexpensive means to get a big aperture that is full spectrum and already has a lot of thermal load reduction, so it could potentially be fairly cost effective, from the aperture stand point, for what it is. The problem becomes the size and weight putting a lot of stress on a mount, with a huge moment arm and being a wind catcher. Then again, a truss design might help there perhaps. I'm still looking up information on this as I don't know enough about how the dealuminizing process is done and whether it's anything more than a white light instrument, or if it at all can be used for other frequencies like HA, etc.

The SCT is a very attractive design from the standpoint of being very compact, light weight, and fairly inexpensive to get too. The ERF is where the cost and consideration changes things big time if wanting full aperture. I guess I'm still ignorant to the difference of a SCT mirror and a reflector mirror with respect to 393nm and resolution and I need to research a little more on this subject. It seems like this design would be very vesatile and cost effective and really attractive from the mounting standpoint (being small and compact and light weight, it could comfortably ride lesser mounts and even potentially have a little 60mm full disc scope with it for a very nice pair to cover wide FOV and narrow FOV). But I'm still curious about not being diffraction limited for calcium.

Curious indeed!

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by christian viladrich » Fri Sep 28, 2018 5:37 pm

Hello Marty,

The field of use of an uncoated mirror is the same as with an Hershel wedge. This is "white light" observing and imaging.

The main benefit of an uncoated mirror is the level of transmission compared to an Astrosolar d3.8 full aperture filter. This makes possible the use narrow band filters from about 1 nm FWHM (and larger) with very short exposure times.
The range of filters I use with my 300 mm Newtonian teelscope is : 396 nm FWHM 8 nm, 430 nm FWHM 1.9 nm, 500 nm FWHm 12 nm, 656 nm FWHM 12 nm.

There is no enough light to use filters narrower than about 1 nm FWHM.

Back to the SC... They suffer from spherochromatism (because of the Schmidt plate). This means that the spherical aberration depends on the wavelength. There are designed for red or green light. They are no longer diffraction limited below about 450 nm.
Here is the Strelh ratio for various Celestron SC. The optics is no longer diffraction limited when the Strelh ratio goes below 0.8 :
Image

So the SC is definelty not for Ca K observation, except if associated with corrector for spherical aberration designed for Ca K.

Hope this helps.

Best regards
Christian Viladrich
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by christian viladrich » Fri Sep 28, 2018 5:53 pm

BTW, more information on the SC optical performances for HR imaging :
http://astrosurf.com/viladrich/astro/in ... sis-SC.htm

And analysis of telescope designs :
http://astrosurf.com/viladrich/astro/in ... alysis.htm
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by MalVeauX » Fri Sep 28, 2018 6:25 pm

Thank you Chrisitian, extremely helpful! :bow :bow

Very best,

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by langleif2 » Sun Sep 15, 2019 7:25 am

christian viladrich wrote:
Fri Sep 28, 2018 5:37 pm

Back to the SC... They suffer from spherochromatism (because of the Schmidt plate). This means that the spherical aberration depends on the wavelength. There are designed for red or green light. They are no longer diffraction limited below about 450 nm.
Here is the Strelh ratio for various Celestron SC. The optics is no longer diffraction limited when the Strelh ratio goes below 0.8 :
Image

So the SC is definelty not for Ca K observation, except if associated with corrector for spherical aberration designed for Ca K.

Hope this helps.

Best regards
This is somehow confusing because another expert I've written with says

"The data of spherochromatism must be wrong, because it is a mirror system with SCT plate for correction of the spherical aberration. The chromatic aberration with this thin SCT plate is lower than a super apo!"

"I never heard spherochromatism problems from users. The chart shows a big aberration. How is the data created? I know star friends who does Venus pictures in UV light with UV band pass filter." As seen below these pictures was recorded with a standard C11:

uv-venus-17-1.jpg
uv-venus-17-1.jpg (53.45 KiB) Viewed 839 times

26xBL6Dv5sjAelG3C.gif
26xBL6Dv5sjAelG3C.gif (18.2 KiB) Viewed 839 times

There are 50 minutes between the clouds in the gif picture. The somewhat blurred images of CaK with big aperture is connected more with the rather bad seeing conditions in the blue end of the spectrum when imaging the Sun.
Last edited by langleif2 on Mon Sep 16, 2019 8:48 am, edited 1 time in total.

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by christian viladrich » Sun Sep 15, 2019 11:54 am

Hello langleif2,

Spherochromatism in Schmidt-Cassegrain is nothing new. You can open any good books on telescope optics to get some information on it. This is just basic optics.

You can read for example the two books I co-authored with some friends (unfortunately in French language for now) ;-)

There are also "Telescope Optics" by Rutten and "Telescope, Eypieces and Astrographs" by Smith and all. These textbooks are really worth the money. I recommend them to all

On the web, you can have a look at my webpages :

http://astrosurf.com/viladrich/astro/in ... -Plate.htm

I giae information and data on the OSLO model I used.

And of course there is Vlademir Sacek web page, which is the reference :

https://www.telescope-optics.net/SCT.htm

Some additional information :

- the Schmidt cassegrain telescope has its spherical aberration only corrected for one wavelength (and not both ends). In the old time, the Celestron SC were corrected for red light, now they seem to be corrected for green light.

- given the dispersion in manufacturing, it might happen that the wavelength of best correction is shifted a litlle bit the blue, or to the red.

- the OSLO model used for the simulation gives results consistent with actual measurements made with an HASO,

- the Venus image taken with the C11 is pretty good for a C11, but far from what can give a 300 mm Newtonian. But mostly, Venus images in UV are much dependent on seeing

- For comparison here is an example of Venus image taken in UV with a 150 mm refractor :

http://astrosurf.com/viladrich/astro/pl ... 44mnUT.jpg

- amateur for Holland made a Barlow lens specially designed to correct the spherochromastism in UV for imaging Venus in UV. It worked quite well. I think I have thelink somewhere on my PC.

- UK optical expert Es Reis designed a corrector of spherochromatism for the C14,

- It is relatively easy to observe spherochomastism of a Celestron 8 at the eyepiece. Still, this is not an issue for visual observation and most photograph observation

BTW, Maksutov telescopes also suffer from spherochromastism, but to a lower extend compared to SC.

Basically, spherochromastism comes from the fact there is a refractive element is the optical system (Schmidt plate, meniscus, etc.).

Hope this helps
Christian Viladrich
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by bart1805 » Sun Sep 15, 2019 12:13 pm

I think this is the link you are referring to Christian.
https://www.astrosystems.nl/uv-barlow-

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by christian viladrich » Sun Sep 15, 2019 12:38 pm

Yes, indeed !
Thanks Bart
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by langleif2 » Sun Sep 15, 2019 3:36 pm

Hello Christian,
I see you linked to the reference, the telescope "bible", Telescope-Optics.net. Very well, there you can read on SCT spherochromatism: "In general, SCT spherochromatism is low. For a typical commercial ~ƒ/2/10 version, with both mirrors spherical, k~0.25, σ~0.4 and m~5, relative corrector power ~0.72, and 0.866 neutral zone placement, the red (C-line) and blue (F-line) geometric blurs are still within the Airy disc."

I'm not going to argue about a SCT is perfekt in CaK, it's not, but I think the graf you are showing for SCT spherochromatism in the blue end is way too exaggerated as you pointed out your self in: "But mostly, Venus images in UV are much dependent on seeing". The seeing is just right bad in the blue end in contrast to the red end. You also see it in planetary imaging in blue with mono camera and filters and that with all kind of instruments. But of course, some are better than others.

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by Valery » Mon Sep 16, 2019 4:57 am

Here is the image in UV continuum at 393nm taken by me with a large SCT. No special balow UV corrective lens.

download/file.php?id=34233

Anyone have any notice about the sharpness of the sun limb at this image?


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Re: What do you think is the most versatile high resolution solar telescope design?

Post by krakatoa1883 » Mon Sep 16, 2019 8:26 am

langleif2 wrote:
Sun Sep 15, 2019 3:36 pm
I think the graf you are showing for SCT spherochromatism in the blue end is way too exaggerated
for optical engineers "blue" means the F line that is 486nm. At this wavelength SCs are still usable as one can see from the graph posted by Christian as well as in spot diagrams for SC. However CaK is 393nm, a very different beast, Strehl ratio falls rapidly beyond the F line and there are actually very few commercial telescopes - other than pure reflectors, that are diffraction-limited in CaK because most of them are designed to perform best around the yellow-green lines.

Of course this doesn't not mean SC can't be used for CaK imaging.
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by langleif2 » Mon Sep 16, 2019 8:38 am

Hey, that's a fine picture, Valery, but it's not sharp even when using sharpness algorithms. There are some spherochromatism and it's getting severe quickly below that F-line. That's why a Newton and a classical Cassegrain is way better below 450nm. It can be done to some extent as you pictured, but your H-alpha shots are way better.

Now, there must be a kind of "work-around" to introduce some undercorrection to a SCT to make it a little better around 400nm without using an expensive custom made UV corrective lens. Or simply buy a cheap 8" GSO f12 classical Cassegrain!

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by langleif2 » Mon Sep 16, 2019 8:44 am

krakatoa1883 wrote:
Mon Sep 16, 2019 8:26 am
langleif2 wrote:
Sun Sep 15, 2019 3:36 pm
I think the graf you are showing for SCT spherochromatism in the blue end is way too exaggerated
for optical engineers "blue" means the F line that is 486nm. At this wavelength SCs are still usable as one can see from the graph posted by Christian as well as in spot diagrams for SC. However CaK is 393nm, a very different beast, Strehl ratio falls rapidly beyond the F line and there are actually very few commercial telescopes - other than pure reflectors, that are diffraction-limited in CaK because most of them are designed to perform best around the yellow-green lines.

Of course this doesn't not mean SC can't be used for CaK imaging.
Thanks, I'm aware of that, you answered a little faster than my answer to Valery. :)

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by Valery » Mon Sep 16, 2019 10:06 am

langleif2 wrote:
Mon Sep 16, 2019 8:38 am
Hey, that's a fine picture, Valery, but it's not sharp even when using sharpness algorithms.

Leif,

It is not sharp??? Hah, take a look and see what is the scale there! There the theoretical resolution has been reached. It can't be sharper even a same size reflector will be used.

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by christian viladrich » Mon Sep 16, 2019 4:43 pm

Hi,
I would like to go back to the question of spherochromatism which is inherent to the Schmidt- Cassegrain design. As I have already said, this is nothing new, optics beeing optics.
Let's see what Celestron wrote in their White Paper :
https://s3.amazonaws.com/celestron-site ... _final.pdf

Page 2 is to be read by those who still believe there is no spherochromstism in the SC design.
Page 17 : the C8 EdgeHD is diffraction limited at 485 nm, 546 nm, and 656 nm
Page 18 : the same is true with the C9.25", but the spot diagrams are much better compared to the C8 because of the longer F/D ratio of the primary. Too bad Celestron did not plot the Strehl ratio. Have a look at my website ;-)
Page 19 : the C11 is diffraction limited at 546 nm and 656 nm, they made no claim for 485 nm, still the spot diagram is getting bigger.
Page 20 : the C14 is diffraction limited at 546 nm. Spots in blue light get rather bad.

These simulations are consistent with the simulation I made with OSLO. This is not a big surprise ..The White Paper refers to one of the books I gave as a reference, book from which I took the OLSO model of the C8 EdgeHD...One of the authors of this book worked on the design of amateur commercial telescopes ...This is a small world ;-)

Note that the simulation goes down to only 485 nm. As pointed out by Krakatoa, the spherochromatism builds up very fast which shorter wavelengths.

On a practical side, I have two C8, one C14 and I occassional used a few C11. My TOA150 outperfoms my C14 by far in UV. My point here is just to give information on the pro and cons of each design. Unfortunately, there is no one-fit-all answer. This is why my garage is packed with a number of telescopes while my car stays outside ;-)
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by Valery » Mon Sep 16, 2019 6:16 pm

I found no one who don't believe that SCT has a huge sphero-chromatism in a blue-violet-UV and in red, far red and IR.

Yes, the best high res universal solar telescope is a pure reflector. For me - a Cassegrain or better a Ritchie-Chretien like our friend Jean Pierre Brahic has (14" Cassegrain with a 12" ARIES DERF).

Never liked Newtonians - too big physically for their focal length and not easy focus position.


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Re: What do you think is the most versatile high resolution solar telescope design?

Post by christian viladrich » Tue Sep 17, 2019 8:14 am

Hello Valery,
Going back to the initial question of this post, and after the tests recently done à St Véran, I agree that the combination of a large ERF and a reflecting telescope is the best HR solar telescope in large diameter.
The Cassegrain configuration is indeed very attractive.
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Christian
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Re: What do you think is the most versatile high resolution solar telescope design?

Post by langleif2 » Wed Sep 18, 2019 7:59 am

No one seems to make a suggestion for a "work-around" solution to the sphero-chromatism inherent in the Schmidt plate. Of course, custom solutions are there, but rather too expensive. In my stores I found a so called apo Barlow I never found any use for. It's a triplet with an air gab, a cemented part and a third lens. The air gab is tiny so there's really only one option - to increase the distance between the two lens arrangement.

And you know what? With the modified barlow and looking on a star test in a refractor optimized in green as they usual are, I found a "nice" undercorrection. Not doing any calculations (I can't) I don't know how much, but I can increase the distance a little step by step and further experiments will show how much better or worse. There's no astigmatism seen so far. I need to do the same experiments with the SCT, I don't see why it shouldn't work.

With the refractor and the barlow mounted it's now optimized in the blue end and less blue blurr to be seen at focus compared to normal. Maybe it's imaginary and it shouldn't have an influence on chromatic aberration but I did see this again and again to be sure shifting forth and back with prism and star diagonals and eyepieces. So now I'm very intrigued. Of course it's not easy to compare the results with a camera with and without the barlow because of the ever changing seeing conditions especially in the blue end where bad seeing is ahead.
Last edited by langleif2 on Thu Sep 19, 2019 5:46 am, edited 1 time in total.

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Re: What do you think is the most versatile high resolution solar telescope design?

Post by MalVeauX » Wed Sep 18, 2019 1:09 pm

So it's a been a year since I posted this question, figured I'd follow up with the recent chatter. I've done three approaches overall with three scope designs. I've used the 150mm clear aperture refractor. 200mm F6 reflector with D-ERF. 200mm F10 SCT with D-ERF. This is completely anecdotal on my part so I cannot say one thing is definitely better than another, plus copy variation among it all.

I started with a 150mm refractor. It was actually really good for everything. It was very well corrected in CaK wavelength and of course handled anything upwards of Ha wavelengths. Overall very satisfied with what a 150mm refractor can do when seeing permits. I had a full aperture ERF on mine and it made a big difference in local seeing within the tube, as it was not an open tube (an old synta 150mm F8 achromatic doublet). I honestly should have never tried to go bigger, looking back. The 150mm really did the job most days and my seeing supported it pretty often. But, I had to go bigger... overall my biggest compliment to using the refactor is that it really is ideal for multi-wavelength imaging and handled near UV wavelengths great which is hard to come by in other optics I'm finding. So overall, I think a 150mm refractor is one of the ideal instruments for multi-wavelength solar imaging. Unfortunately bigger ones are just crazy as I'd love a 200mm achromatic doublet! Maybe one day...

Image


So when I was looking into how to go bigger, knowing I'd need a big D-ERF to go with it, I started looking at scope designs and ergonomics. Everything seemed to point to the SCT as a compact big aperture and ergonomics are great since it's rear imaging train like a frac, light weight, compact, easy to handle and easy to mount. I got a C8 Edge and it really did handle HA great. I was pretty content at this point with the aperture and scope overall for this purpose and I got an Edge because I use it for all-imaging purposes (DSO, planets, lunar, etc) so I wanted the overall better SCT if able and found a good used one. The one snag I found was it did not handle short wavelength. This particular scope was not happy at 393.4nm wavelength and I couldn't get a decent focused image in CaK for the life of me. After looking into it more, I was finding that it was common for these scopes to have strehl ratios less than 0.4 in near UV and so I'd never be diffraction limited and it was just very rough. I tried masking the aperture to help it, but it wasn't worth it as at that point my 150mm refractor had that aperture and did the job very well with sharp results. Overall apparently these F2.2~F2.4 mirrors just cannot be corrected excellently through the whole spectrum and a longer focal ratio mirror would be needed if I wanted that.

Image

I started exploring another option to try and get a better mirror that was better for short wavelength so I started looking into long focal-length quartz mirrors after talking with a few people. I sourced and gambled on a 200mm F6 Quartz primary & secondary reflector and put it to work. At first it seemed fine on everything, it handled lunar high resolution surface fine in narrowband, it handled planets great, and it showed excellent star tests and collimation tests. But every time I pointed it at the sun, even with a full aperture D-ERF, it had problems. The focused image had a nervous twitch to it, so everything was constantly blurry, despite being star collimated and full aperture D-ERF. Didn't matter what wavelength I used. I used it at 610nm and 742nm on the lunar surface and it focused fine and produced lovely high res results. But on the sun, the image instantly was twitchy and nervous looking like it was being pinched in some way. I couldn't figure it out. I never really got to see if it would handle short wavelength any better since it couldn't do what I needed even in HA. Sadly. So now, I still have this scope, but I never got to test it on 393nm. I did however test it on 430nm and I did get convection cells one day with ok seeing. But it too had a nervous twitch to the image that I couldn't explain, so it wasn't the narrowband itself. I'm not sure what it was. I even collimated it with the D-ERF in place on a star the night before, it held collimation. So I cannot explain the problem. I had high hopes for a F6 newtonian mirror to be best for short near UV wavelength and up to HA, but ultimately ergonomically it was a bear to use due to size and weight, focuser was way out of the way and made it hard to manage and ultimately I couldn't do anything with it for solar work, despite it doing excellent on lunar/planets. So it is shelved for now. I use it for visual at night these days. I will revisit it one day maybe.

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So at the end of the day, I went back to the C8 Edge and it does the job for everything except 393nm & 430nm. For short wavelength, I just go back to my 150mm refractor and it does the job better though not at the resolution I was hoping to get to.

Overall, I think the refractor is the best way to go. I understand a good long focal ratio mirror would also be good. Ultimately I hope to one day get a 200mm achromatic doublet (istar optical) and have an open OTA made for it and fit my D-ERF to it. But I will need a new mount to handle that! So that's not happening any time soon. So the SCT still is the "best" ergonomically and for simplicity for HA. Does great for HA. It just doesn't do near UV unfortunately well.

And that's where I am these days!

Very best,

langleif2
Oh, I get it now!
Oh, I get it now!
Posts: 47
Joined: Sat Jun 30, 2018 8:32 am

Re: What do you think is the most versatile high resolution solar telescope design?

Post by langleif2 » Thu Sep 19, 2019 6:11 am

Hello Marty
Thanks for the update. What you are experiencing with the long Newton is most likely thermal turbulence. You are letting in radiation in a long tube that at near focus will be around 70 deg Celsius at least! That's my temperature measurement with the DERF and a SCT at the exit hole nearby focus before entering the PST etalon. The best telescope short in length and with easy handling mastering allmost all wavelengths is a CC - classical Cassegrain or even an RC. You can buy a cheap one fx 8" AT f12 or 8" Orion f12 in your country - and sell your SCT ;-)

Yeah, a SCT is not designed for short wavelengths below F line - 486nm. I'm working on a triplet Barlow for introducing some undercorrection so it can handle the shorter wavelength better. So far so good with a f5 refractor. Another beast is the SCT f10, only time will show if results are getting better. But seeing at my latitude is the real limiting factor at that CaK wavelength.


Regards

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