Why don't reflective telescopes work with mods or solar viewing?

[etatsolarchat]

So not much if any mods are done with reflective scopes because the images looks like crap, why???

I hear air turbulence in the scope, but refractors are enclosed also?

Anyone have a good explanation?

[swisswalter]

Hi Eric

there are very good WL imager out there who work with reflective systems. It is just not that simple and easy as with a small refractor.

[colinsk]

This seems to be do to diffraction, tube currents, mirror deformations as well as seeing constraints. However I have seen great work done when all of these factors are dealt with.

Heliostats and celostats of less than 6" diameter seem to be more common.

[JohnW]

I tried taking WL images with a glass filter attached to the front of my 8" SCT scope and they were not sharp. I suspect it is the type of optics with large central obstruction (secondary mirror).

[etatsolarchat]

Same here, also did a bunch of other tests on various mak and SCT's, all look bad...

Colin, you say diffraction, how so?

Maybe the central obstruction causes a lot more trouble with bright solar??

I have seen some good looking photo's using a custom reflector that has no mirror coating, also seen a some SCT photos, but in general a major lack of using them and I've had no luck..

[brianb11213]

I tried taking WL images with a glass filter attached to the front of my 8" SCT scope and they were not sharp. I suspect it is the type of optics with large central obstruction (secondary mirror).
Glass filters are far more likely to be a cause of poor definition than the central obstruction - the rather large central obstruction of a SCT actually has minimal impact on images, just a small drop in contrast which is easily corrected in post processing. Try a film filter instead, these usually work far better, especially with larger apertures. FWIW I have seen a few commercial glass filters in my time and none of them have been anywhere close to satisfactory, except for one with a working aperture of only 50 mm. The Zeiss glass filters are OK (but not better than film) and a Zeiss glass filter in a size to fit an 8" SCT will cost about as much as a decent 5" APO refractor ...

The fact remains that reflectors in general have far more issues with tube currents & differential heating than refractors ... even at night when temperature gradients are much reduced compared with solar work. I suspect that, to get a reflector to work well for solar, it has to be enclosed in a very well ventilated slitted observatory with shielding preventing entry of direct sunlight except to the objective, or for the scope to be positioned on a damp grass area - not a concrete plinth - and for the tube to be very well insulated and ventilated.

[solarchat]

not to mention that to turn a large reflector into an H Alpha scope, you would need an ERF in front of the secondary mirror, or basically the size of the objective to keep from melting the secondary. Ever priced a 9.25 inch ERF?

[markthais]

Mirror scopes will work. Most would be used as off axis. Like Stephen said the cost for a 9.25" ERF would be high. You would need it to be at least 1/4 wave on transmission because it is at the objective end of the scope. And it would be about 1" thick.
I used a 3 1/2" Questar with an SO-1 stopped down to 2.5" and extended the filter back to increase the EFL and it worked fine. I didn't have any problems with it. I was about F/24, so it broaden out the filter but the center obstruction didn't seen to matter. I used and RG610 ERF then but now if I where to use this set up I would use one of Baaders 70mm ERF's.
I was told that with a Mak it would be hard to make a telecentric that would work. So the best is to just stop them down.
From a cost point, you can get a low cost refactor and stop it down or add a telecentric and be dollars ahead if you wanted a full disk with an SO-1(20mmCA filter). But from an optical view its hard to beat those 3.5" F/15 Mak's.
If you want to talk mirror solar scope just look at the new scope at Big Bear. What is that 1 meter?
If you are going to use an air spaced etalon in the back, then you will need to be at F/40 or better to keep the bandwidth down if you are just stopping down the scope.
If you are going to try to use a collimated system, I would worry about how big of aperture you would be looking at.
Mark W

[sullij1]

Alan's thread speaks to you question.

http://solarchat.solarastronomy.org/ind ... nd-may-5th

[sullij1]

Also you can do it the way Harold did. His images are second to none.

http://www.unigraph.de/

[colinsk]

Thank you Sullij. I was searching for that link. Reading about his white light telescope evolution is very inspiring.

http://www.unigraph.de/images/Instrumente/dach2.gif

[colinsk]

Look at the extent that he took to eliminate tube currents:

http://www.unigraph.de/images/Instrumen ... folie1.jpg

[etatsolarchat]

Yeah pretty serious DIY scopes...

Also saw Neo posted some excellent images using a C8 SCT.

So, seems white light imaging is possible but takes a lot more effort than the refactor and still not clear as to why, but seems tube currents are worse in reflective scopes..

Will give it a try a few more tries.

[sullij1]

Your welcome. Harald's scope produces some of the most stunning solar images I have seen.

[colinsk]

Can you provide a link to Neo?

[etatsolarchat]

Granulation, or golden nuggets? (5 May)

http://solarchat.solarastronomy.org/ind ... gets-5-may

[sullij1]

Neo is doing a similar thing to Christian VILADRICH. Here is Christian's C14 rig:

http://www.astrosurf.com/viladrich/astr ... ument.html

I want to try this this summer with my C11.

[colinsk]

It is interesting that he is able to use his refractor unprotected but he chooses to do this with the cat.

http://www.astrosurf.com/viladrich/astr ... filter.jpg

[peter drew]

The first incoming light path through a reflecting system is close to the tube wall, a refractor has a converging cone which moves inwards from the tube wall so is less affected by external tube heating.

[etatsolarchat]

yeah, that's the first thing I've heard that makes some sense :cheer:

[sullij1]

It is interesting that he is able to use his refractor unprotected but he chooses to do this with the cat.

http://www.astrosurf.com/viladrich/astr ... filter.jpg

Actally, that looks about right to me. Remember this is CaK/continuum/Red that Christian is doing. When he does Ha he does the TAK with the ERF, Baader telecentric, Daystar Ha combo. If you have the Lunt CaK device you know that it works by just placing it in the rear focuser. No forward mounted ERF necessary (up to 120mm).

My experiments with the Omegas CaK showed they did not "need" an ERF either. Also even Lunt/Daystar recommends that if you go over 120mm you should use some type of ERF. I am certain that is why he employed the Baader film on the CAT. He also uses KG3/IR cut or another filter in the CaK optical train. Valdrich also uses the Baader continuum for white light on the same set-up.

I am certain seeing conditions have to be very good but no reason not to if you fill like lugging out all that equipment and setting it up.

If its the tube current thing U R worried about, in addition to what has already been said RE light cone, the need to cover the CAT in a reflective material is due to the larger surface area soaking up heat coupled with the fact that the Celestron and Skywatcher brands have dark colored tubes that will "really" soak up the heat. Valdrich's CAT is nice in the fact he has the cooling fans incoporated into his design.

[DSobserver]

you can also see here my MAK PST mod : http://solarchat.natca.net/index.php/en ... on-f10-mak

[MapleRidge]

Hi Eric and all...

I had good luck a few years back using the Lunt B1200CaK modue and the LS75/B1200Ha on my 10",f5 Cave newtonian set up with an off-axis mask (roughly 4" wiht the CaK unit). This might be a tough setup for most newtonians, but mine has the mirrors positioned to have a lot of backfocus (required for a long ago project)...still had to use most of my focus travel to get the blocking diaginals to reach focus.

I have an on-going project with a friend who does optical coatings to come up with a way to coat a newtonian mirror to mimic a primary reflection that would be the same as what an ERF would accomplish. He says it can be done...I just haven't shelled out the cash to test it :unsure:

Brian

[etatsolarchat]

I'll put cak on a newt, on my to do list :cheer:

sounds like a interesting project, a mirror that only reflects 656, start looking for some more shells with cash in them

[MapleRidge]

a mirror that only reflects 656...

don't tell me no one else has thought of this LOL

[colinsk]

I have thought this would be a good ERF used as a coelostat. You would have to polish the back and AR coat it or make it a wedge but it would be useful.

[sullij1]

Gee, there is Colin with dem big words again. :blink: Here is a link for those who dont know what a coelostat is (like me ).

http://www.google.com/search?q=coelosta ... 36&bih=799


Good idea, if you all could meke it work you might have a niche side biz.

[sullij1]

Here are some blokes that seem to be putting the foil covered big reflectors and coelostat variations to work for an outreach in far off lands. You will have to page through the photos to see all the different reflective set-ups.

http://web.williams.edu/astronomy/eclip ... roup1.html

[DavidG]

One needs to compare the same size aperture reflector and Cats to refractors before making judgments. Many time the refractor has a smaller aperture which is less effected by daytime seeing.
Having designed and built a number of h-alpha and white light systems, from an optical stand point reflectors have many advantages. One many one is that refractors even APOs have spherochromatism, that is spherical aberration as a function of wavelength. So an objective that is well corrected for visual use in the green, many times is not as well corrected in the red and almost aways is poorly corrected in the deep blue were CaK is done. While a reflectors do not have these problems. Commerical H-alpha and CaK units use specially designed objective that are fully corrected at the wavelength that the filter is designed for.
I built a 4.25" f/10 white-light solar newtonian that gives extremely sharp and contrasty image. http://www.considine.net/dgroski/wlnewt/

- Dave

[brianb11213]

from an optical stand point reflectors have many advantages. One many one is that refractors even APOs have spherochromatism, that is spherical aberration as a function of wavelength.
This is only approximately half of the story - unless you're imaging at prime focus, whatever you're using to increase the focal ratio will have its own aberrations and (given that it's going to contain light transmissive elements) the teleconverter will contribute spherochromaticism of its own.

In practice I've found that spherochromaticism is negligible in my William Optics FLT 110 triplet apo / Televue Powermate combination, at any rate across the visual range from CaK to Ha. When used with a 742nm cutoff infra red pass filter the resolution is less than I'd expect & I suspect spherochromatic aberration may well be a major contributory factor.

[colinsk]

Gee, there is Colin with dem big words again. :blink: Here is a link for those who dont know what a coelostat is (like me ).

http://www.google.com/search?q=coelosta ... 36&bih=799


Good idea, if you all could meke it work you might have a niche side biz.

I just wanted to make sure you got your Google on...

[sullij1]

Always good to learn somthing new Colin . :cheer:

[sullij1]

This IRIS solar scope thing is supposed to go into space on the 26th. Somebody mentioned coated mirrors. This maybe incorporating that kind of tech along with every instrument we have ever discussed and have a forum for. A good read if you are looking for a way to get to sleep.

I guess that answerers the why not a mirror question. Being in a vacuum at subzero probably helps (NOT TO MENTION DEEP POCKETS).

Here is the link with excellent drawings and information.

http://www.spaceflight101.com/iris-spac ... ments.html

[brianb11213]

Very interesting, thanks for pointing this out.

I guess that answerers the why not a mirror question. Being in a vacuum at subzero probably helps (NOT TO MENTION DEEP POCKETS).
Well the vacuum prevents the atmosphere absorbing the UV radiation they're interested in detecting. And helps cut out tube currents ... but forces the integration of technologies we really don't need at ground level, e.g. radiators to cool the optical components.

Very deep pockets are a great help in many ways. I guess the unit cost would come down if these things could be built on a production line & sold in hundreds or thousands instead of being hand built prototypes. Alas, launch costs plus the ground equipment required is going to restrict the market to the order of one binary digit.

[colmic]

Hi,

it's possible to make halpha imaging with a reflector with Big aperture:

- Find an Intes M703 Mak (180mm F/10 with secondary miror not glued)
- Buy a Baader 180mm ERF (about 1200 euros/dollars)
- Use PST etalon

As Vincent made (DSObserver) with full success with his 127mm F/10 Intes Mak, i saw in this mod and it works!

I'm thinking myself about this mod but my question is: will this mod be better than with my TOA130 mod????
180mm reflector versus 130mm APO refractor, that is the question :unsure: :S

[SolarJohn]

Michel - Ha scopes with 200mm mirrors have been made. Lunt started with the LS200, an eight inch reflector using a single etalon in double pass mode. The scope also had an option for CaK viewing/imaging as well. Lunt aborted the commercial project due to costs associated with making it a mass produced telescope, but I believe that an individual can make and tweak one on his own. I know of one individual that has made one, but have lost the web links and cannot find.
Posts on the LS200 may be found here, on CN, and the Lunt Yahoo forum.

SolarJohn

[colinsk]

I ordered a LS60DP before Lunt had delivered a telescope. I was given a LS60DS50 in trade because they could not produce the promised product. I am not sad about it because I have a great solar scope but I only ordered it to sort out the double pass optics.

[etatsolarchat]

Neo, I also tried what you did with very poor results, your WL looks great and others as well so I'll continue down that path, WL seems possible on the scts and other reflectors..

[etatsolarchat]

Neo, I see for WL you use different post filters 35nm ha, continuum, and astronomik red. Do you have any suggestion on which to use when or preference?