Got ERF, going for a Frankenscope design

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Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

I managed to get a very nice tri-band (H-alpha, solar continuum, and CaK) dielectric ERF of 85mm diameter and 8.25mm thickness. I am getting a mount made (or perhaps ask a colleague whether I can use his lathe) for my APM 80mm scope as a first stage in making a sort of Frankenscope using my LS35 as a basis. I sometimes think going towards a "Franken-Quark" might be the best solution, using the 12mm blocking filter I have followed by a telecentric lens (4x, either the Baader or a PowerMate), and placing the etalon (with tilt tuning system) behind it. This should be able to provide full-disk views of the sun, and have a comparatively large sweet spot, due to the use of a telecentric system. I would then have an 80mm solar scope (although I will need a bigger camera than the DMK21). The nice thing of this configuration is that I simply replace the "Franken-Quark" by my Herschel wedge, and I am ready for viewing in white light (without needing to remove the ERF).

Am I right in thinking that replacing the H-alpha blocking filter by a CaK unit should in principal give me an 80mm CaK scope? After all, the etalon passes a "comb" series of spectral lines, and the blocking filter selects the one required. The tri-band ERF should transmit CaK, and the CaK module selects the correct order from the etalon.

Please tell me if these ideas are crazy :D


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by swisswalter »

Hi Michael

not at all crazy. We can confirm, that that triband works very well at all three wavelenght (Ha, continuum, CaK)


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Re: Got ERF, going for a Frankenscope design

Post by marktownley »

Yes, the etalon will transmit caK too. The triband is great, I have 2 of them!

Not sure how successful an external etalon will be behind a telecentric, we had quite a discussion about it some time ago on here, with quite a few trials too...


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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Thanks for the feedback. The only reason I can see that a front-mounted etalon might not work internally, and in particular behind a telecentric lens, is if it has some form of central obstruction (like the Coronado ones). However, the Lunt LS35 etalon seems to have a perfectly clear aperture (and as far as I know it is used internally in LS60s, but I may be wrong). I discussed a different LS35 mod on this forum before, and people advised against it, but lately I have seen some great results on solarchat with an essentially identical set-up ( using a bigger scope as a Galilean scope in front of the LS35). I think I will go ahead. I will root around for a second hand 4x powermate, and build a test set-up. If all goes pear-shaped I can always sell that powermate at little loss. Long term project though, don't expect results any time soon.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

BTW, those great results with an LS35 mod were here:

http://solarchat.natca.net/viewtopic.php?f=9&t=11271


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by swisswalter »

Hi Michael

go for it, you will be ending up with a fine mod. Don't forget to report about it


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Thanks Walter. I will most certainly report on it once I get everything sorted (whether it works or not)


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Might have got myself a TV 4x Powermate plus T2 adapter. Just a few more bits and bobs and I might get it working :D


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by swisswalter »

Hi Michael

good news. Looking forward to seeing the first light


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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Don't hold your breath, it will take a while. I have to get a mounting for the ERF, and design a housing and tilt mechanism for the etalon still. If I get this done before the end of October I will be well pleased


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by swisswalter »

Hi Michael

take your time, there is no need to hurry up


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

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Re: Got ERF, going for a Frankenscope design

Post by Astrograph »

Hi

Apart from the satisfaction of tinkering with something and making it all work, can I ask why you would not just use a Quark with a UV/IR cut on your APM 80 for H-Alpha? I had one setup for an event last week and you get almost a full disc with good detail. It would just seem like a lower cost route to getting something that will work.

The scope works great with a 1.25" Wedge for White light and with a K-Line filter in the back of that you have a basic Ca-K scope.

The Quarks 4.2x is optimised for the distances it is from the etalon and the eyepiece / camera is from the back of the etalon. It also has a blocking filter in it and would work a whole lot better than a 4x PM.

Just wondering....


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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Of course you may ask that, and I certainly see the use of a quark on my 80mm F/6. I noted that I had two parts of the quark available: Blocking filter and etalon, which meant I needed just a telecentric system, and a means to fit them all together. This got me thinking about a mod. Furthermore, I have two problems with the quark. First, its blocking filter is not quite big enough to fit the entire disk, which I feel is a bit of a pain, in my proposed set-up, I can use my B1200 blocking filter (overkill on the LS35, I know, but I got it for 5 euro more than a new B600), which combined with a 4x telecentric system leaves me able to handle scopes up to 800mm focal length and still get the entire disk.

Second, the temperature tuner of the quark requires a start-up time, which can be a hassle. I often take my little Lunt LS35 to work, and can set it up during lunch or tea break in a matter of seconds, to catch those brief sunny spells and make the most of my break. I much prefer a pressure or tilt tuner as the response is instant. I think I can build a little container with T2 thread on either side which will house the etalon with a tilt-tuner (this is currently the main issue). Behind a telecentric lens, a tilt tuner should not have too much of a sweet spot issue.

The rear end of the powermate can be unscrewed to make room for a casing for the etalon, so I do not worry too much about the working distance (and I should get the PM with its T2 adapter, so it should fit into the etalon housing). I would prefer the Baader Telecentric 4x (200mm space to play with (double stacking ??)), but that costs more than twice as much. If my current experiment works, I can always sell the powermate with little or no loss, and replace it with a Baader. Replacing the B1200 filter with an appropriate CaK module gets me a CaK scope as well (thanks to the tri-band ERF), and I can simply insert my wedge to get a white-light view (again, thanks to the tri-band ERF). I can later use the same set-up with e.g. a 6" F/6.5 achromat for even more aperture (and use the ERF internally).

Hope this clarifies my frankenquark idea


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

BTW, I am not sure the 4.2x telecentric of the quark is necessarily better than a 4x powermate. In the final version I intend to put a helical focuser behind the etalon, so I can optimize the distance to the telecentric lens. I will have to see how things work out


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by Astrograph »

The 4.2x Quark telecentric is actually designed to be optimal with the working distances involved with the Quark, plus as you say, it has the blocking filter. The systems really designed to work as a single unit. Its not just a small version of the ION/Quantum with a barlow on the front.

While the PM 4x is one of the more telecentric Powermates, I doubt it would work any better than the Quark. Suck it and see I guess.

To operate properly, telecentrics should really be made work with a specific focal length of scope. The Baader 4x is designed (I understand) for an 800mm F8.

A friend of mine has actually been playing with double stacking a Quark, using a front mounted Etalon as the second filter, apparently with some good results.

Enjoy your tinkering!


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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

The telecentric element in a Quark also has to contend with a range of focal lengths and focal ratios, so faces similar problems to any PowerMate or telecentric barlow. I chose the 4x first of all to get close to F/25 (although quarks seem to be quite happy at F/21 when combined with F/5 scopes), and second because it is more telecentric than e.g. the 5x (which is worst in this respect).

I am not expecting to beat a quark of course, I am expecting (or at least hoping) to get something rather better than my current LS35 (and I intend to be able to put the LS35 back together again if desired). I have seen some LS35 mod results using a Galilean scope in front of the front etalon of the LS35 which showed little sweet-spotting and humongous detail. Something close to that is what I am aiming for. A telecentric solution (even if imperfectly telecentric) should have even less of a sweet-spot issue.

BTW, I did consider using a 2" UV/IR cut reflecting filter in my 80mm scope, but when I could get the tri-band dielectric ERF I could not resist. I also prefer to keep as much heat as possible out of the scope. Besides, if it works I can use it to scale up to a 5" or 6" with internal ERF


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Meh, 4x PM deal fell through. Will go for a Baader Telecentric 4x now. The difference in price after adding shipping costs from the UK were just too high to justify the price difference with getting a new Baader Telecentric. Oh well, at least the Baader specifies an optimal back focal distance, which allows me to design the etalon housing more precisely, and I might be able to double stack with 200mm distance to the rear focal plane.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by Bob Yoesle »

Hi Michael,
I sometimes think going towards a "Franken-Quark" might be the best solution, using the 12mm blocking filter I have followed by a telecentric lens (4x, either the Baader or a PowerMate), and placing the etalon (with tilt tuning system) behind it. This should be able to provide full-disk views of the sun, and have a comparatively large sweet spot, due to the use of a telecentric system... Oh well, at least the Baader specifies an optimal back focal distance, which allows me to design the etalon housing more precisely, and I might be able to double stack with 200mm distance to the rear focal plane.
Unfortunately, you are not very likely to achieve very good telecentric full-disk performance in you proposed system, even with the much-better Baader telecentric:

1. Air spaced etalons have a narrower acceptance angle than solid etalons.

2. Unless the telecentric lens system is designed for a focal length pretty close to your objective, it wont provide the required field and instrument angles to the filter for good contrast performance. The Baader telecentric systems are designed for an 800 mm FL objective. Therefore your optimum scope would have an ~ 800 mm FL and a ~ 3200 mm EFL, making full disk viewing and imaging difficult.

3. Unless the resultant f ratio light cone angles are at or under the acceptance angle, the bandpass will broaden across the filter. Double stacking to reduce bandpass further narrows the filter system acceptance angle, and you will need an even narrower light cone presented to the filter system - likely closer to f 50 or greater than f30 - to achieve good contrast performance. You will either have to reduce aperture or increase the effective f ratio when DSing.

Overall, you might have more success attempting a collimator based system. I would use a collimator with at least 1/2 the objective'd focal length - especially if DSing -- in order to keep field angle magnifications as low as possible.

Good luck and best wishes,

Bob


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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Thanks Bob, I will take this in consideration. I was considering the option of a collimator as well, might make a test rig with the ST80 I have lying about for that purpose. One other option I am considering is computing a telecentric system specifically for the focal length of the scope I am using. I have a couple of pretty decent achromatic doublets lying around (remnants of binoculars) with sufficient back focal length to give me a good working distance. I will see if I can calculate a telecentric system for my F/6 scope (I should be able to do that, after all what else is an MSc in Astronomy good for ;) ). I might even get in tought with some friends at the Kapteyn Astronomical Institute across the road (I work in computer science) and see whether their students might build a system as an assignment (after all, what else are students good for :mrgreen: )


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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Re: Got ERF, going for a Frankenscope design

Post by Bob Yoesle »

Sounds like you have a good start Michael; you might find this thread of interest: http://solarchat.natca.net/viewtopic.php?f=9&t=2483

And this paper: http://home.comcast.net/~g2baraff/Solar ... g/Tele.pdf


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Re: Got ERF, going for a Frankenscope design

Post by michael.h.f.wilkinson »

Thanks Bob, very useful. I was already veering towards a negative meniscus lens for the first component, so this confirms that. Selecting long focal lengths is also a good way to keep aberrations down (although problems with flexure may increase as the length of the system increases). We will see. Somebody at Astron says he is willing to run designs through Zemax for me. I think I will take him up on that offer. In the mean time the simpler colimator lens experiment is beginning to take shape. I can get a negative meniscus (coated) of approximately -175mm focal length which gives me a collimated beam just under 35mm and magnification of about 2.3x with my ST80. I will have this mounted this is a 2" holder fitted in a 2" helical focuser which replaces the ST80 focuser. The main issue then is to align the ST80 with the LS35 (should be feasible with the guide-scope rings holding the LS35) and providing a sleeve between rear of ST80 and front of LS35 that keeps light out. As this is much simpler I can get this done quite quickly.


Solar kit: GP-C8 with Thousand Oaks Solar filter, APM 80mm F/6, Lunt Herschel Wedge, Solar Spectrum 0.3Å H-alpha filter, Beloptic Tri-Band ERF (80mm free aperture), Thousand Oaks 90mm ERF, Coronado SolarMax II 60mm with Double Stack Unit. Lunt straight B1800 Ca-K module.
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