Does F10 Matter in CaK?

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Re: Does F10 Matter in CaK?

Post by brianb11213 » Sat Nov 03, 2012 9:31 am

There the same amount of energy it's just spread out farther. That is one of the problems I see using a fast system. A lot of heat on the bandpass.
I don't know about your filters but the commercial CaK units from Lunt and Coronado both feature a "hot mirror" (dichroic interference filter) at the front of the device which reflects very nearly all the visible & infra red straight back up the tube, so it doesn't warm the filter to any significant amount. The way I understand the 4" aperture safe limit on the Lunt unit, it's because of differential heating between the "hot mirror" and its mount when the defocused solar image is hitting both which might possibly cause damage. The focal ratio doesn't actually come into the equation except for the fact that the defocused solar image will be smaller / more intense in proportion to the focal ratio i.e. there is more energy per unit area in the 3" inside focus beam of a 4" f/15 scope than there is 3" inside focus in a 4" f/5.

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Re: Does F10 Matter in CaK?

Post by sullij1 » Sat Nov 03, 2012 4:57 pm

I understand it as brianb, that is where the confusion creeps in for "everybody". Which is it????????

That is why I am asking all the questions!!!!!!!!!
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Re: Does F10 Matter in CaK?

Post by markmanner » Sat Nov 03, 2012 4:57 pm

I find it strange that I had an easier time focusing at F10 rather than F7. I guess the slower the better. .

Pat, the critical focus zone on telescopes is wider at larger F/ratios, so an f/10 scope is easier to focus than an f/7. For example, for a 4" scope at f/7, the critical focus zone in microns is approximate 108, and for a 4" at f/10, it is 220.

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Re: Does F10 Matter in CaK?

Post by marktownley » Sat Nov 03, 2012 7:57 pm

A very interesting thread Joe! Thanks for creating it :)

I think, the biggest variable here is the scope being used; people appear to get good and bad results in CaK with a variety of scopes at a variety of f-ratios. My unbranded 70mm f6 achromat performs very well at f6, my Tal100R @ f10 gives really nice contrasty images, however my Meade 127mm AR5 @ f9.7 gives the worst results of the 3 scopes. I think it's down to the objective lenses themselves...

Just my 2c... :whistle:
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Re: Does F10 Matter in CaK?

Post by sullij1 » Sat Nov 03, 2012 8:39 pm

Hi Mark,

Thanks for poping in. Is your AR5 the newer ED/APO version or the Achro version? (again the objective question)

Never mind - I see you did say Achro,
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Re: Does F10 Matter in CaK?

Post by sullij1 » Sat Nov 03, 2012 9:15 pm

[quote] The focal ratio doesn't actually come into the equation except for the fact that the defocused solar image will be smaller / more intense in proportion to the focal ratio i.e. there is more energy per unit area in the 3" inside focus beam of a 4" f/15 scope than there is 3" inside focus in a 4" f/5.

Not that this may matter for image as much as thermal cycle on the filters but I still dont get the idea.

For example Merlin66 has pointed out in the past the the solar disk in mm is roughly 1/100 of the focal length. So- 1/100 of 1000mm gets you "approximately 10mm concentration of the disk/heat" on the primary bandpass filter. (actually 10mm at the focal point/sensor.)

Granted as we discussed for the Lunt's the hot mirror and dichroic coated (blue filter?) prior to the bandpass filter cut much of the heat. Still it is fair to assume a "very small" percentage of heat is still being past to the primary band pass filter.

If you reduce the focal length to 500mm this makes the solar disk approximately 5mm in size. My mind wants to think this smaller disk size is carrying more energy in the form of more heat in the smaller size. (burning ants) Again it is reduced by the Hot mirror and (Blue filter? we still don't know. :)) but is still carrying more energy/heat in a smaller area.

So as much as I want to align myself with brianb what Markthis "seems to be saying" still makes sense. The heat load delivered by the shorter focal LENGTH is higher.

Where is my admittedly layman thinking flawed?

This is all regarding filter life more than image. (and the question that confuses me most)

By and large I am convinced the longer f ratio for the Cak is better image wise.

So now we are getting a good image, the next question is what f ratio is easiest on the filter thus prolonging its working life? Lets face it these suckers are expensive!
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Re: Does F10 Matter in CaK?

Post by sullij1 » Sat Nov 03, 2012 9:19 pm

Don't ya always hate the guy in the back of the class who is always raising their hand asking questions? :D
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Re: Does F10 Matter in CaK?

Post by markmanner » Sat Nov 03, 2012 9:24 pm

[quote][quote]
...If you reduce the focal ratio to 500mm ...quote]

I know you probably realize it, but you are referring to focal length here, not focal ratio. For purposes of some of the things discussed in this thread, the difference matters.
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Re: Does F10 Matter in CaK?

Post by astrodanco » Sat Nov 03, 2012 9:57 pm

Please note that I have personally confirmed the Starlight Instruments A20-292 adapter is a perfect fit for the Celestron NexStar 102 GT OTA when used with the Starlight Instruments Feather Touch FTF2025BCR focuser.

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Re: Does F10 Matter in CaK?

Post by sullij1 » Sat Nov 03, 2012 10:07 pm

Mark,

Thank you, yes, you are so right! I DO mean FOCAL LENGTH! Post edited.

(We applaud you for beating on the guy in the back :D) 1 karma point delivered!

Sorry all.
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Re: Does F10 Matter in CaK?

Post by sullij1 » Sat Nov 03, 2012 10:47 pm

A very interesting thread Joe! Thanks for creating it :)

I think, the biggest variable here is the scope being used; people appear to get good and bad results in CaK with a variety of scopes at a variety of f-ratios. My unbranded 70mm f6 achromat performs very well at f6, my Tal100R @ f10 gives really nice contrasty images, however my Meade 127mm AR5 @ f9.7 gives the worst results of the 3 scopes. I think it's down to the objective lenses themselves...

Just my 2c... :whistle:

Mark,

Thanks for saying. I was considering an AR5 for this and other purposes. You just saved me and perhaps others some money.
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Re: Does F10 Matter in CaK?

Post by patholland » Sun Nov 04, 2012 5:09 am

Please note that I have personally confirmed the Starlight Instruments A20-292 adapter is a perfect fit for the Celestron NexStar 102 GT OTA when used with the Starlight Instruments Feather Touch FTF2025BCR focuser.

Thanks for that. I will have to REALLY, REALLY like the scope to invest that kind of money. What did you think of the stock focuser? BTW, my scope did not come in yet, it's stuck in UPS pergatory (also known as Earth City, Missouri).

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Re: Does F10 Matter in CaK?

Post by sullij1 » Sun Nov 04, 2012 7:40 pm

Personally I like the stock focuser. I have yet to have a problem hitting focus with it. I was actually going to buy one of the focusers that come on the Celestron unit for a PST mod. You don’t have the 10:1 control but it has yet to matter. I admit that I am a cheapskate, not by choice but by necessity.
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Re: Does F10 Matter in CaK?

Post by patholland » Mon Nov 05, 2012 3:06 am

Hey Joe. That makes me feel better knowing you have bad seeing as well and the standard course focuser works for you. I will try the standard focuser and report back when I get it in and have a chance to use it.

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Re: Does F10 Matter in CaK?

Post by sullij1 » Tue Nov 13, 2012 5:43 pm

Thanks pat, based on our other conversations at the sissy site (:)as Stephen says)I am now sure the reflections we saw in my images are the reflections in the diagnol B1800 I was using. You don't see them cuz you are using the straight through Cak Module. So the straight through is a better decision for imagers.
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Re: Does F10 Matter in CaK?

Post by astrodanco » Tue Nov 13, 2012 7:17 pm

I've noticed that the reflection from the hot mirror on the front end of the CaK filter out the front of the objective is an extremely bright beam of light. There must be a corresponding bright reflection from the rear of the objective back to the CaK filter and that's what we see in images. How annoying.

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Re: Does F10 Matter in CaK?

Post by sullij1 » Tue Nov 13, 2012 7:26 pm

A bit annoying for sure but as you saw it can be dealt with on surface features fairly well. It is on the limb trying to image proms that it becomes a bugger. To be more clear: I am cerian the reflection is from the mirror in the diagnol. With a straight through the mirror is ommited from the image train so no reflection (as in Pat's shots). Although the straight through is a better choice for imagers, both products are superior to any at the price point.

I would highly recommend either with the reflection caveat to the imager.
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Re: Does F10 Matter in CaK?

Post by sullij1 » Wed Nov 14, 2012 2:35 am

Dan, thanks again for allowing the comparison and the use of the filter. It was a pleasure.
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Re: Does F10 Matter in CaK?

Post by astrodanco » Wed Nov 14, 2012 11:54 pm

If you're up for it, next up we can compare straight through to diagonal. I'll place the order.

Okay, one order placed for an LS18CaKMDs2. Rikki says three months. Not holding my breath on that time frame though.

EDIT: It ended up taking five months from initial order to final shipping.

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Re: Does F10 Matter in CaK?

Post by markthais » Thu Nov 15, 2012 1:57 am

HI Sullij,
OK
You had asked if I build K-line filters. I have make filters from K-line to CA2 at 8542Ang.
Before I go into the different designs, there are a few things about viewing K-line (or H-line) that should be pointed out.
1) The color of the sky makes a difference. The bluer the better. Take the same filter at sea level then take it to 8000 ft., the higher elevation will show much more contrast. Where I live you get a lot of haze or the sky looks white. If you can block the sun with your hand and the sky around the sun is not blue then the view will not be as good as it could be.

2) AR’s, if the scope has a broad band AR (BBAR) or the eyepieces do, this could drop the contrast. In the BBAR design, they are designed to keep the reflection down from about 400nm to 700nm. The problem is that the TIO2 that is commonly used as the high index layers absorbs really well below 410nm. The more surfaces the less contrast. A simple ¼ wave MGF2 will do better (unless the AR is designed for the shorter wavelengths).

3) The type of scope. The most common scopes today are the F/7-8. If you are doing a ray trace for their designs you will find that they are probably not ¼ wave at K-line or even close. With these scopes you will want to keep the magnification low to keep the image sharp. To improve your image the best thing would be to stop them down in half. There was a reason that the old scopes where F/15. They have higher contrast and can be corrected easier. It’s true the point where the blue light and the red light come to focus is not the same place but the image of both wavelengths will be better.

4) The simpler the better. Air spaced doublets are your best for K-line (unless you can afford an Aspherical objective). Also keep away from cemented objective lens they will fail over time when used as solar scopes.

5) Resolution is a function of wavelength. So a 50mm K-line scope will have the same resolution as an 85mm scope at Ha.

6) The difference in design of the two. The Daystar has two different K-line housing. The standard tiltable K-line is nothing more than two 10Ang blockers and an ITF. So it went AR, 10ang, BG25, ITF, 10ang, BG25, AR. Del would call this a 5ang filter but it was really not. But it does work, the plage was there and you could see prominence sometimes.
The etalon based filter was the other design. At K-line it takes much more temperature to move the bandpass. So unlike Ha the temperature control is less critical. The other advantage is the bandpass are broader so tilting can be use easier. I did put some etalon filters in the tiltable housing but added a strip heater and temperature controller to have more control of the bandpass.

Ok, the design for the etalon based filter would be AR, 10ang,(sometime BG25 and UV Polaroid )etalon, ITF, BG25,AR. You would need to use the UV Polaroid when the thickness of the mica was not a ½ wave plate or the next peak was not outside the blocker. This is because the mica is birefringent and will have two peaks. One for the different indexes on the different axis.
It should be noted that the UV Polaroid material we used is no longer made. So the ones I make need to be ½ wave plates to keep the HW as narrow as possible.

7) The F ratio, the main problem is the concentration of heat. The standard Daystar oven uses a Delrin plate on both ends against o-rings to hold the optics under pressure. I have seen where a 100mm F/15 melt the Delrin in the front of the filter. Whereas F/20 did not hurt the Delrin. This was the limiting focal ratio that we would use.

8) The Lunt design looked like it uses a single cavity bandpass. But you could use an etalon also. The specs wouldn’t have to be very tight to make a 2.5Ang peak. Since I have not taken one apart I will go with what has been posted. So you have your bandpass , the blue glass will block anything to the short side. Now the long side, a bandpass at 393.4nm is going to have another bandpass somewhere in the green. If there is no ITF then you would need to block this next peak. It was posted that it looked like there is some KG glass. KG glass is good for absorbing the IR. One problem is that it is soda glass. This type of glass absorbs water. So over time it would fog out this may or may not be a problem. So they are blocking till the KG glass can take over. The yellow glass is there to make the filter dimmer and help with what the blue has blocked. It does not look like this filter design is blocked as well as the Blocker/ITF design but is probably good enough. I could be wrong, they may have an ITF, am sure someone will point it out if I am.

9) LIABILITY: With my company and the old Daystar we ever said that K-line was a visual filter. To be safe it was always a filter for a camera. And that is how they were marketed. We used them at F/20-F/30 which keeps them dim. I’m not saying that it will hurt your eyes at F/6 you just need to keep the transmission low just to be safe. Unlike Ha where it can be brighter I think you would want to keep the blue end of the spectrum dimmer just to be safe.

OK,
Going through your questions, some may have been answered already.
I would disagree with Daystar about using F/15 unless I was using a Baader K-line filter as a per-filter.
With K-line you want to keep the heat down.We would not use any ERF and only use an 80mm CA.max. You would be hard pressed to find a blue glass that would work as an ERF larger than this. By using Baaders K-line filter as an ERF just in front of the filter is not a bad idea. Your main problem would be the heat building up inside the scope as your lost of resolution of the image.

Lunt uses smaller blockers then Daystar style. It is much cheaper to use a 15mm blocker over a 38mm one. But if you need to replace the Lunt filter changing blocker in the Daystar style will cost less. Now with this design, you are fitting the suns image through the smaller blocker to save on cost. But like a have said before if you have 100mm of light and you compress it to .2” compared to 1.08” which do you think is going to be more intense. It’s the same amount of energy just spread out farther at the longer F/#. See how long you could hold your hand in the 100mm F/6 beam; this is what the Lunt filter is working with. A standard blocker is speced at 50C, I have seen them run at 70C and last year’s when being used a couple times a month.

So the Lunt design is to run them hard and expect the filter to be used once an awhile.
The IR blocking in front of the filter is not that important. There is far more energy in the blue end of the spectrum then the IR when it comes to heating the filter. So I’m not to sure why Daystar says to have an IR blocker or solar film in front of the filter. (This goes back to blue filters move less with temperature)

The next question was what type of scope is best. If you where making a custom scope, you would make it a doublet with both elements made of BK-7 and air spaced and design it for K-line.

Since most of us don’t have this, keep to F/10 and up and keep it air spaced. Most scopes are corrected for somewhere near Na. and try to cut out anything shorter then 400nm.

You had a question about exposure. You can always make the filter dimmer by adding light yellow glass before the camera.

Internal reflections, The Lunt design uses tilting to try to keep the reflections down. This does not always work.There should not be any double images visibly. If you are imaging and get the exposure longer for a prominence then you may start to get problems. The other place they can come from is if an element does not have an AR on it. I am not a big fan of tilting elements. Working with the blue end of the spectrum does have the advantage in which you can cut down the internal reflections with dark blue glass. (BG25). This does not work at Ha, putting a red glass between two mirrors does not do much for the internal reflections.



Your other question was about using a filter at F/25, 12 times a year 2 hours.
This is not going to do any damage to the filter. First, K-line blockers tend to last longer than Ha blockers. I think it is because the layers are thinner. I have seen K-line filters last 10 years but none shorter than 5 years.
What is the best K-line filter? Well a hard coated 1 ang bandpass would be everything an amateur would need. The problem is the cost would be high.
Well, the Mica etalon is limited to around .8Ang -1 ang. but 2 ang is a nice bandwidth to work for. This is cause by the coating material. At 410nm you can get down to .25Ang but below this the high index absorbs too much. So to keep the transmission up you need to drop the reflection on the mirrors.
The Lunt design, I would keep to about 50mm and use the longest focal length that still fits the full disk through the filter. If you plan to use a 100mm F/6 expect the filter to be replaced in a couple years. If they have a 5 year warranty make sure you send them back for repair.
Sorry if this was a too long but I’m sure there will be comments.
Mark

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Re: Does F10 Matter in CaK?

Post by sullij1 » Thu Nov 15, 2012 3:57 am

Mark,

WOW, oo00oo, WOW! I am near speechless, I know I have probably not been your favorite poster in the past and I am truly grateful and humbled by you taking the time and going to such great effort to take the lot of us to SKOOL!

I know I feel schooled in a good way. :D

Your experience and knowledge in this area and you willingness to share is appreciated by many at this point, I am certain! It is by me I assure you! I have saved your comments to my knowledge base folder as truly I feel I will not find a better answer to my questions in the future. I am certain I will refer to your comments often.

You were not long winded, what you were is very precise in you answer and I am certain it is going to help me and many others in this community as we work to better our CaK instruments and help us get the most life/use out of our equipment.

I for one am in your debt! Thank you Mark! :)

Kindest regards,

Joe
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Re: Does F10 Matter in CaK?

Post by sullij1 » Thu Nov 15, 2012 4:07 am

Finally, the engineer guy who could really shine some light (no puns intended) appeared.
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Re: Does F10 Matter in CaK?

Post by sullij1 » Thu Nov 15, 2012 4:19 am

If you're up for it, next up we can compare straight through to diagonal. I'll place the order.

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Re: Does F10 Matter in CaK?

Post by swisswalter » Thu Nov 15, 2012 5:40 am

Hi Mark

thank you so much for that wealth of information
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Re: Does F10 Matter in CaK?

Post by marktownley » Thu Nov 15, 2012 7:10 am

Some fantastic info there Mark, many thanks 8) Alot for me to think about and play around with :)
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