Different Ca-K options, differences?

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Re: Different Ca-K options, differences?

Post by Valery » Wed Jun 14, 2017 6:14 pm

Mark,

For Ca K and Ca H the best refractors are TOA 130 and 150. All other refractors regardless of manufacturer are poorly corrected at these wave lengths.

If you can use a 6" F/12 achromat, you will be OK with crisp images at Ca K/H. But such a scope is not quite transportable and not easy useable.

Another, best (IMHO) solution is a Marksutov telescope.

Any coating is OK.


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Re: Different Ca-K options, differences?

Post by Astrograph » Wed Jun 14, 2017 10:25 pm

Valery, where is the basis for TOA scopes being so wonderful at sub 400nm? They are just pretty ordinary APO's using ordinary glass. Nothing more.

Any coating is not OK. For proof of this see this wavelength test between the Pierro Astro ADC and the cheap ZWO. The PA has fused silica prisms which pass sub 400nm light better and coatings that are designed to compliment this. The difference is quite obvious.

In any case we are getting away from how this thread started.
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Re: Different Ca-K options, differences?

Post by Merlin66 » Thu Jun 15, 2017 11:27 am

Does it really matter?
The current CCD cameras dramatically fall in sensitivity in the UV.
The shortest wavelength I've been able to record with the spectroscope is just below 3700A.
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Re: Different Ca-K options, differences?

Post by Valery » Thu Jun 15, 2017 7:08 pm

Astrograph wrote:
Wed Jun 14, 2017 10:25 pm
Valery, where is the basis for TOA scopes being so wonderful at sub 400nm? They are just pretty ordinary APO's using ordinary glass. Nothing more.
Rupert,

Please, no offence. You are not a pro, nor a specialist in the optics as far as I know. But I am.

The basis for TOA scopes being so perfectly corrected is in the design of these objectives. TOA states for Triplet Ortho Apochromat = triplet with equal (close to this) apochromatic correction across spectrum. They are not only corrected for secondary spectrum (common focus for wide spectral range) but they also corrected for spherical aberration for almost entire spectral range they are purposed for. In the ordinary apochromats spherical aberration is corrected only for a single wave length. Usually this is 540nm and near it. In the TOA objectives spherical aberration corrected for ALL wave lengths. Therefore images at 400nm is same crisp as at, say, 540nm and even crisper because of lower diffraction influence.
Similar designs (with widely spaced lenses) are well known for optical designers for a loooong time. The only disadvantage is a poor temperature equilibration with enviromental air. This is a key factor and most designers prefer lower corrected objectives, but with much faster thermal equlibration. Somewhen in a past we have discussed these questions with Roland and Yuri. They too are not fans of orthoapochromatic objectives because of this mentioned above disadvantage. But for a Ca K imaging they are almost a perfect choice in such a size.

Again, for 400nm +/- several tens of nanometers, all coatings are about similar performance +/- a few % in transmission. Of course if an objective consists many lenses with lower transmission in UV, then a final transmission will be about 20% lower - absolutely not critical for imaging there.


Valery.
Last edited by Valery on Sun Jun 18, 2017 11:50 am, edited 1 time in total.
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Re: Different Ca-K options, differences?

Post by Astrograph » Thu Jun 15, 2017 11:12 pm

Valery. I take a pride in helping people to try and understand the reality of what is out there rather than back up marketing BS. Many things look great on paper. In the real world what is delivered is often nowhere near. That applies to Tak as much as any other. TOA's, regardless of their design strategy set no new heights for performance. Being replaced by customers for something better is not uncommon. I don't dispute that in theory it might be better but there are many designs out there that promise much and deliver little new, if anything. Design is one thing, implementing it is another and doing it consistently is another still. I see this all the time. It is constantly frustrating that something that seems so simple as some shaped and polished glass put together can vary so widely in performance on a case by case basis with the same telescope. I see this because I inspect all I sell. What becomes clear is that who makes it is much more important than how or what it is made from.

I am well aware of your own experience and qualifications with optics. I feel no need to question that. You know nothing about me so please don't insult me. I run a professional astronomy business.

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Re: Different Ca-K options, differences?

Post by marktownley » Fri Jun 16, 2017 8:54 am

Good discussion chaps.

I've certainly tried all of the CaK options above, and my preferred one is a double stacked CaK PST filter of my own design. If I had to pick from an 'off the shelf' unit I would go for a CaH quark, if cost was no objective I would go with the Lunt B3400 or maybe a custom job from Solar Spectrum.

In terms of scopes i've only ever used 'budget' cheap refractors with CaK and there is significant variance in light throughput (different scopes, same focal ratios, same camera, same day), this can only be glass types or coatings causing the variance. When I reviewed the CaH Quark there was significant variance in brightness visually using different eyepieces. These are just my observations.

As i've become more experienced with my solar imaging i've learned to see the results of spherical aberration when imaging at CaK wavelengths, and, as above there is significant variation between scopes.

I certainly don't have the experience of using the higher end scopes that both Valery and Rupert have, but, being a nerd I do enjoy reading test reports and so I take something from these (and I do appreciate a test report isn't everything!).

For me, in the less than ideal seeing I have a lot of the time, a dream scope for me would be a 5-7" frac and a tri-band (full aperture) ERF, but it would have to cut the mustard at 393nm. To get to that point though I do need to win the lottery! :D
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Re: Different Ca-K options, differences?

Post by MalVeauX » Sat Jun 17, 2017 9:59 pm

Hi all,

Just a little update.

I'm tinkering with a Skybender with a 380~395nm pre-filter inside, IR rejection filter on the nose, and two 393nm filters stacked on the camera nose. I used a focal reducer to do the full disc. And I used a 3x barlow for the details. I did some white light too just to compare (white light was done with a baadar solar film & solar continuum filter).

Image

Image

Image

Image

Image

What do you guys think? Close? Or is it mainly just white light that looks like it, but not really calcium?

++++++++++++++++++++++++++++

The skybender itself and the filters:

Image

Image

Image

Very best,

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Re: Different Ca-K options, differences?

Post by marktownley » Sun Jun 18, 2017 8:10 am

Hi Marty. One of those 393 filters needs to be mounted internally in the skybender - at the moment all it is doing is acting as an extension tube. Apollo has had some success with these setups in CaK.
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Re: Different Ca-K options, differences?

Post by MalVeauX » Sun Jun 18, 2017 12:17 pm

Hi Mark,

The filter inside the tube is a 380~395nm pre-filter. Then the two 393nm filters. Do you think the pre-filter should be exchanged to one of the 393 filters and then just one 393 on the nose of the camera?

I can see through the 393 filters, but I cannot see through the 380~395, it looks like a mirror on both sides when I take it out and look at it.

Very best,

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Re: Different Ca-K options, differences?

Post by marktownley » Sun Jun 18, 2017 12:36 pm

I would try a 393 on the 380~395 filter, but, if apollo recommends the configuration it is in go with it.
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Re: Different Ca-K options, differences?

Post by MalVeauX » Sun Jun 18, 2017 1:56 pm

Yea, I'm only setup this way because that's how he suggested it be used. I didn't notice much difference tilting though. I'm curious if putting the 380~395 on the nose with the IR rejection and then putting a 393 in the tilter and a 393 on the camera would be more appropriate?

I'm trying it out as I didn't buy it, just seeing if there are alternatives to the $1k options (Lunt, Quark) since there really isn't a lot of Ca options out there really, compared to HA. Really bummed the PST CaK is no longer made, as that would have been ideal for me I think for cost.

Long term, I can't decide if a Quark CaH or a Lunt CaK BF would be ideal. I think ultimately I will stay within refractors of 6 inches or less for this, simply due to cost of moving into a big aperture SCT and the ERF required, etc. It's just too costly. I can however manage 6 inch refractors. I'm just not sure if the Lunt BF1200 would be best, if I put a 3x or 5x barlow on the 150mm refractor (one day at least), or if a Quark CaH unit would be more appropriate for something like that. I would like to be able to do full disc and high mag with the device. The Quark CaH still seems to need long focal ratios but at least lacks the barlow so I could do full disc with that. But the Lunt is attractive as well. Just not sure what would really be ideal for imaging (I do 1% visual, with a PST, just to look for morning proms basically).

Very best,

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Re: Different Ca-K options, differences?

Post by Montana » Mon Jun 19, 2017 6:59 am

Wow! I think these images are fantastic! lovely and crisp and lots of plage seen around the spots in the centre of the disc. That is impressive! what aperture is the scope and what are the 393 filters?

:hamster: :bow

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Re: Different Ca-K options, differences?

Post by Astrograph » Mon Jun 19, 2017 9:21 am

Personally I think you would be better off with the Lunt CaK module is you can just use it with no extra costs and it will work. The Daystar CaH will require an ERF of some kind. Although I have used it on an F6.5 scope visually and it looked 'ok' it really needs to be at least double that. An important consideration with the Daystar is back focus. If you add a telecentric then things can be made to work because the focal point gets moved further back. If you want to use it on its own then your scope must have enough back focus to accommodate the Quark and still allow focus. On many Chinese scopes there is not enough back focus (and hence inward movement on the focuser) to reach focus. This is not an issue with the Lunt as it is fitted internally.

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Re: Different Ca-K options, differences?

Post by MalVeauX » Mon Jun 19, 2017 1:24 pm

Montana wrote:
Mon Jun 19, 2017 6:59 am
Wow! I think these images are fantastic! lovely and crisp and lots of plage seen around the spots in the centre of the disc. That is impressive! what aperture is the scope and what are the 393 filters?

:hamster: :bow

Alexandra
Thanks!

Scope is 120mm refractor (Celestron Omni XLT 120; F8.3). The 393nm filters are just 1.25" filters that I received with the Skybender. I'm not sure about their make/origin. Maybe Semrock?
Astrograph wrote:
Mon Jun 19, 2017 9:21 am
Personally I think you would be better off with the Lunt CaK module is you can just use it with no extra costs and it will work. The Daystar CaH will require an ERF of some kind. Although I have used it on an F6.5 scope visually and it looked 'ok' it really needs to be at least double that. An important consideration with the Daystar is back focus. If you add a telecentric then things can be made to work because the focal point gets moved further back. If you want to use it on its own then your scope must have enough back focus to accommodate the Quark and still allow focus. On many Chinese scopes there is not enough back focus (and hence inward movement on the focuser) to reach focus. This is not an issue with the Lunt as it is fitted internally.
I have no issue with backfocus; I use straight-through, no diagonals, and my focuser is linear bearing and I use assortments of extensions to get focus with my current telecentric barlow in my Quark no problem.

Interesting, didn't realize the Quark CaH would need an ERF. I read it just needed the standard UV/IR cutter just like the other Quarks for heat rejection, but beyond that, just had to be at F7 or greater and it was good to go. Unless my understanding is incorrect?

Ultimately I too think the Lunt is the way to go in the long run for CaK for me, if I get to that point. I will keep using this Skybender setup for a little while and see what I can get out of it before moving to something else. I'd love to have both just to compare and see if one really is superior with my setup or not. I like my Quark Chromosphere, but I definitely appreciate the idea of not needing another powered device to manage.

Do you think the B1200 would be a limitation with a 3x or 5x barlow on a 1,000mm scope (120mm aperture, F8.3) using an ASI174MM?

Very best,

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Re: Different Ca-K options, differences?

Post by Astrograph » Mon Jun 19, 2017 2:18 pm

Hi,

To clarify the back focus comment. A telecentric adds backfocus. You need a telecentric (not a barlow) to allow an etalon to operate correctly and onband. The Quark Calcium does not have a telecentric as its bandpass is wide and scopes from F8 should be OK. The problem is that inserting the Quark Calcium into the light path uses up what back focus you have. With some scopes you will not get focus. You don't need extensions, you need a scope whose focuser retracts further inwards. This is not a factor with the Lunt as it has a collimating lens inside.

The Quark Calcium has a block in it nose which allows no ERF below about 60mm. Above that you can use an UV/IR cut but a 120mm scope needs some better. A Baader blue filter is best as this passes the calcium wavelength. It is also dielectric so reflects energy. The Lunt needs nothing as its ERF is built into the front of it.

I have used a B1200 (this refers to the 12mm size of the block) with an 800mm scope. That was OK. The diagonal of your 173 is just over 13mm so you would get some vignetting. Just use ROI. Don't wast money on a B1800

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Re: Different Ca-K options, differences?

Post by MalVeauX » Mon Jun 19, 2017 2:58 pm

Hi thanks, makes sense. The Quark Calcium sounds more fussy than the Quark Chromosphere!

Lunt sounds more simple. I don't mind a little vignetting. Thanks!

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Re: Different Ca-K options, differences?

Post by christian viladrich » Tue Jun 27, 2017 6:41 pm

Hi to All,

It is nice to have such an extensive post on Ca K observation :-)

As I have been observing in Ca K since the last 17 years, and using a Taka TO150 for Ca K observations for more than 7years, I would like to put forward some information.
- The TOA 150 mm is of top optical quality at 396 nm. I have somewhere on my computer the spot diagrams calculated by Taka at 396 nm. There are simply perfect.
- The AR coating does absorb a significant amount of light. The total transmission (for the six air surfaces) at 396 nm is about 60-70% (I have the data on another computer). This is not a big deal.
- In real life, and this is all that matters, the TOA performs splendidly in Ca K.

Here are two examples of images taken at full aperture :
Image

Image

Please note the structure of the chromosphere "surface". What you see is the solar granulation which appears "inverted" (or in negative) compared to visible light. Please note the size of the solar cells. Some cells are darker than other. There is no noise on these images, only actual solar details.

As for the optical performance of Schmidt-Cassegrain. This optical design does suffer from spherochromatism which degrades optical performance in UV. Here are the Strelh ratio versus wavelength of various popular SC:
Image
Remember that to be diffraction limited, an instrument should have a Strelh ratio greater than 0.8.

In other terms, a SC would need a chroma corrector to perform correctly in near UV light.

In a nutshell, all is a matter of pro and cons (and also cost ..).

Best regards
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Re: Different Ca-K options, differences?

Post by marktownley » Tue Jun 27, 2017 9:15 pm

A nice contribution there Christian. If you are able to post the data you have on your TOA150 from your other computers on here I would love to see it :)
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Re: Different Ca-K options, differences?

Post by krakatoa1883 » Wed Jun 28, 2017 3:42 pm

Astrograph wrote:
Mon Jun 19, 2017 2:18 pm
The problem is that inserting the Quark Calcium into the light path uses up what back focus you have. With some scopes you will not get focus. You don't need extensions, you need a scope whose focuser retracts further inwards.
Quark-Ca is short, with its 2" nose it requires only 62 mm backfocus, most refractors will have no problem to accomodate it, mine goes to focus even with Quark inserted in a 1.25" diagonal.

The Quark-Ca has an advantage over the Lunt module: it can be used on reflectors, in fact a solar newtonian of medium or even large aperture would be a very powerful scope for imaging in Ca-H. Mine works wonderfully with Quark while I couldn't use it with a Lunt unit.

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Re: Different Ca-K options, differences?

Post by Astrograph » Wed Jun 28, 2017 3:57 pm

You are fortunate that it focuses. It is not always the case.

Have you been using your Calcium Quark on a Newtonian with no ERF or are you talking about one with a mirror with no coating?

Lunt also offer a version of the CaK module as a diagonal. Obviously that only suits certain scopes too but reflectors must have ERF's (if they are normal) and Calcium ERF's are rare.

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Re: Different Ca-K options, differences?

Post by krakatoa1883 » Wed Jun 28, 2017 6:08 pm

It is a newtonian with the primary mirror uncoated (secondary is fully reflective), it still transmits enough light for Quark. The scope is a f/5 but I use a barlow lens that brings it to f/10.

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Re: Different Ca-K options, differences?

Post by Valery » Fri Jun 30, 2017 5:43 pm

krakatoa1883 wrote:
Wed Jun 28, 2017 6:08 pm
It is a newtonian with the primary mirror uncoated (secondary is fully reflective), it still transmits enough light for Quark. The scope is a f/5 but I use a barlow lens that brings it to f/10.
Enough for imaging or for visual?

If you mean for imaging what is the equivalent F/D at the camera chip? Remember, that for full resolution realization
one need to use for Ca an F/D 1,67x greater than diffraction resolution for H-a. I doubt that Quark Ca H transmits enough light for such a scale if to use it wih one uncoated mirror.


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Re: Different Ca-K options, differences?

Post by krakatoa1883 » Sat Jul 01, 2017 3:05 pm

No problem to image at 3760 mm that corresponds to about f/29 at the image plane (sampling for 393nm). And mine is a 130mm only, a larger newtonian can do even better, the problem is not light, is the seeing.

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Re: Different Ca-K options, differences?

Post by mdwmark » Sun Jul 02, 2017 4:06 am

Wow, a lot about K-line. This wavelength really doesn't sell to many filters. Only the younger people can really see anything normally. I like K-line, in the days of film it worked fine. With the cameras today and processing you can make the image look like it was shot with a narrower filter.
1) refactors are not usually corrected for that wavelength. If it is an air spaced you can increase the spacing to correct for 393.4nm but you loose the longer wavelength. The old MgF2 AR is still the best low cost AR for a K-line scope. Most common AR today used TiO2 as the high index. TiO2 starts to absorbing at 400nm. It is usually not used for UV optics.

2) SCT's you have already seen from a earlier post that they are not designed for 393.4nm. Even in Celestron's notes,they say the scope is corrected to the green line.

3) Newtonian's, should do better. I would keep to a longer Focal ratios.

The main thing I would worry about is heat. You put a piece of blue glass in a F/8 beam and it will get so hot you can't touch it. Do the same with red and it's not a problem.

All that said, Valery is right about using a corrector. It can be made, but one size does not fit all. And you will need to worry about off axis coma showing up.

The different filters you have talked about. The Lunt's is a single cavity, probably with a low index spacer. So it broadens out fast depending on the focal ratio. I would keep it in the F/20-30 range for best contrast.
The Quark uses a 5 Ang HW two cavity filter. It should be used in the same focal ratio also. This HW should show some plage.
The 5Ang filters is soft coated ,so the wavelength will move red with the extra heat. If the 2.4Ang is soft coated then it will move off K-line even easier. So on hot days you may want to tilt the filter to better center the bandpass.

You really need to be close to 1Ang to get that good contrast your looking for in K-line.

This is my 2 cents
Mark W.

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Re: Different Ca-K options, differences?

Post by mdwmark » Sun Jul 02, 2017 5:35 am

One more cent,
With refactors, if you stop them down .This will help with correcting for K-line.
Even 50mm at a F/20 or so, does quite well with K-line. And with these HW's you can get away with using a barlow.
Mark W.

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