Does F10 Matter in CaK?

Use this section to discuss "standard" Baader/Coronado/ Lunt SolarView/ Daystar, etc… filters, cameras and scopes. No mods, just questions/ answers and reviews.
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Re: Does F10 Matter in CaK?

Post by gbealnz »

Yep, agreed Mark, a very valuable post. I read it, and read it again.
Thank you for al this information.
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Re: Does F10 Matter in CaK?

Post by sullij1 »

I hope somebody refers Pedro to Marks post, I know he has had serious issues with his Cak Modules corroding. it MaaaaY help him.


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Re: Does F10 Matter in CaK?

Post by marktownley »

The 5" Meade AR5 i'm talking about is not really bad for Cak, just the worst performer of the 3 refractors that I own...

http://brierleyhillsolar.blogspot.co.uk ... ngths.html


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Re: Does F10 Matter in CaK?

Post by solarchat »

the bigger the aperture the worse the CaK image? Id have to disagree on that one....:)

or did you mean the worse the Chromatic Abberation? Oh, sorry. nevermind. acronym confusion on my part...


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Re: Does F10 Matter in CaK?

Post by markthais »

Thank you for your comments.
From the bandwidth view F/10 doesn't matter. The bandwidth doesn't change that much from F/10-F/25 with broad filters.
From a deep of field view, then the longer the focal length the more room you have to be in focus.
Larger scopes,
The real disadvantage with larger apertures is that you are heating two different types of glass with different index of expansion. It may take a while to come back to 1/4 wave.
I had this question from a guy that has a 12" F/12 refactor.He wanted a telecentric so he could use it full aperture. I was concerned with what he wanted to do. I call an optical shop that I knew and asked them. They told me the problem with the different expansion rates and really doubted that it would ever stabilize. They suggested to stop the scope down and not take the change of damaging the objective. This was for Ha. But most of us don't have 12" refractors.
I can't see any problems with 6" scopes. We have used them for years but for K-line just to keep the heat down, I would add some BG25 or a Baader K-line filter or a bandpass per-filter before your K-line filter.

I wanted to add what I forgot on my other post.
You need first to be able to see K-line. The older you get your eye start to yellow and will filter out the blue. This will make it so you will be unable even to see K-line.

Mark W.


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Hi Mark,

Most of us here are more concerned with what our cameras will see and how the camera sees from an image gathering standpoint. I would venture to say that 80 to 90 percent of us that can afford a CaK filter are past the age that visually seeing Cak is even a consideration. :) Just speculation on my part. :) Our concern is imaging.


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Re: Does F10 Matter in CaK?

Post by astrodanco »

Yup, almost no one uses CaK modules for viewing! However, now that you've conveniently brought the subject up, I might as well report my rough unscientific findings on that subject.

For ASAE I put a Lunt CaK module on a scope for viewing. I called it the “Calcium Challenge.” People in their age 20s and 30s reported seeing quite a bit of detail including filaments, sunspots and plage in sharp focus. None of them reported seeing any proms though. The age 40s, 50s could generally only see sunspots and plage in soft focus. The age 60s could see only a featureless disk with limb darkening. The age 70s and up could not see anything at all, not even a purple blob.


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Re: Does F10 Matter in CaK?

Post by markthais »

That is so true, your challenge is right on. Then you get your cataracts taken out, and you can see K-line again.
Since where more concerned with cameras. Has anyone every uses one of those Sony UV video camera they sell? I've only seen them in adds.
Mark W.


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Ok class let’s review: :D

First let me start by saying this is not the conclusion to this experiment, you all saw Astrodanco’s generous offer and my theory on the diagonal mirror causing the weird reflection. Further we have only evaluated a few scopes and more can be done. With that in the open:

For anyone considering purchase of a CaK unit like the Lunt diagonal, Lunt straight through or Coronado mod’s like Mark Townley’s here are a few things to consider:

Markthis pointed out
1) The color of the sky makes a difference. The bluer the better.
2) 2) AR’s (anti reflection), if the scope has a broad band AR (BBAR) or the eyepieces do, this could drop the contrast.
3) The type of scope. Stop down faster scopes to F10 up to F15. There was a reason that the old scopes where F/15. They have higher contrast and can be corrected easier. “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.”
4) The simpler the better. Air spaced doublets are your best for K-line.
5) The F ratio, the main problem is the concentration of heat. Fast scopes create more heat on filters be they heat rejection filters or the primary. So F6 very hot. F10 or more, are much cooler. Easier on the filter components in general.
6) It was noted “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 if there is KG in the train and you start having fogging problems this could be the culprit.
7) If you go over F15 consider a prefilter prior to the primary filter like a K-line or BG25, no bigger than 80mm to prevent chromatic aberration (CA)
8) 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).
9) “Using a filter at F/25, 12 times a year 2 hours. This is not going to do any damage to the filter.”
10) “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.”
11) Disadvantage with larger apertures is that you are heating two different types of glass with different index of expansion. It may take a while to come back to 1/4 wave.
12) Try and keep to the manufactures aperture specification but it is said “I can't see any problems with 6" scopes. We have used them for years but for K-line just to keep the heat down, I would add some BG25 or a Baader K-line filter or a bandpass per-filter before your K-line filter.” So if you don’t want to stop down that 6” F10+ use prefilters on the diagonal/filter end.
13) The scope we tested that delivered the best result was the Celestron 102GT F10 (or its clones under skywatcher and othe brands).
14) Others report that theTal100R @ f10 gives really nice contrasty images, Celestron 80 ED, Borg 76 F/10 apo.
15) The Explore 102 APO, the Meade 127 and other fast APOs work but lack contrast, place a load on the filter and require more processing.

Bottom line-F10 + will work better for you giving more contrast and increasing the life of the filter components.

This is a summary so far, If I messed something up – say so, I have pretty thick skin. :) Please continue to expand the image comparisons, APPLE to APPLE and report scopes that are delivering well in the K-line.

Hope this thread helps those in the solar community considering a K-line filter figure out where to put their money.


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Re: Does F10 Matter in CaK?

Post by swisswalter »

Thank you Joe

for that sum up


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Re: Does F10 Matter in CaK?

Post by etatsolarchat »

So for many reasons some scopes work better than others, just trying out a Cak filter for the first time so I figured I'd add these pics to the thread..

AR102 f/6.5 Air-Spaced Doublet does not seem good for cak,



Celestron 102GT F9.8 with a .5x reducer a lot better image, for the price this scope is awesome.



Celestron hd150 stopped down to 110mm so F10.9 with .5x reducer


Couple days later
Celestron hd150 F8 with .5x reducer


Lunt 152 F6 with .5x reducer


Couple days later..
Lunt 70ED


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Re: Does F10 Matter in CaK?

Post by astrodanco »

Hmm, the LS152 looks best to me.

It's funny though how good that C102GT is when you consider that the whole thing, computerized mount and all is (or was) less than $200 at Costco. Such an amazing deal.


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Looks to me like the Celestron 150 stopped down and the Lunt 152 are equals. The C102 still shines. What CaK filter was used?


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Re: Does F10 Matter in CaK?

Post by MapleRidge »

I agree the C150 with the aperture mask looks pretty good.

I have used my old Celestron C102 f9.8 scope for WL, CaK, and Ha and have found it to be a good performer. I picked up a used CR-150 in the fall, and have not had time ot use it much, but I just picked up the focuser assembly for my machinist today (had to modity the focuser to allow more in-travel...just a bit short of focus with the B1200CaK and Lunt soalr wedge) and hope to try it on WL and CaK the next clear day. I may even try it with the PST mod.

Thanks for posting...looks promising ;)

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Re: Does F10 Matter in CaK?

Post by etatsolarchat »

I was using the B1200CaK also.

Honestly what I got was not the best to compare since it was various days, but I kinda agree the stopped down hd150 got the best image but can't tell if stopping it down really changed anything since it was a different day, the lunt152 got similar results, with the 102GT vary close to them. The ar102 and lunt70ed were disappointing..especially the 70ed as I wanted to use it for cak so I'm going to try it again to make sure..

I'd be interested seeing if any of your results compare.


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Re: Does F10 Matter in CaK?

Post by swisswalter »

Hi Eric

interesting comparison. At my place I found it almost impossible to make a fair shoot out if I made it on different days because of the seeing conditions beeing so different


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Re: Does F10 Matter in CaK?

Post by marktownley »

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).

...

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.


I found this transmission curve and thought it relavent to what Mark is talking about:




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Re: Does F10 Matter in CaK?

Post by Pedro »









I bought second hand recently two iris that can make things easy as far as aperture reduction is concerned.

The Baader solar iris (10 to 113mm) is ideal for reducing the aperture when observing conditions are worse (I will install it on the TV101):

http://www.365astronomy.com/solar-diaph ... p-710.html

The other bigger iris will be installed on the LUNT152 for WL and H-alpha imaging (60 to 150mm).

Baader iris


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Re: Does F10 Matter in CaK?

Post by Derek Klepp »

Pedro will be interesting to see the CaK results here.


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Re: Does F10 Matter in CaK?

Post by Pedro »


Hi Derek:

I have not yet received the two iris. Maybe next week. I bought these second hand (cheap price).

I now always image the CaK sun at F/10 with the TV101. I use a 54mm aperture reduction with excellent results.

best regards


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Re: Does F10 Matter in CaK?

Post by marktownley »

I have written an article on my website regarding Nyquists Sampling Theorem, undersampling, oversampling and the implications on solar astronomy. I believe the content is very pertinent here and moay well go some considerable way in explaining why people have been getting the results they have at CaK wavelengths.

http://brierleyhillsolar.blogspot.co.uk ... pling.html

I would welcome peoples thoughts on this.

Thanks, Mark.


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Re: Does F10 Matter in CaK?

Post by etatsolarchat »

Thanks Mark for spending the time to post that info good stuff.


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Just to complete this thread, AstroDan did acquire and let me use his B1800 straight through. Imagery was the same except for not having the image reversal from the diagonal. Unfortunately so was the reflection that I noted earlier. As AstroDan noted. It is the reflection coming from the module ERF bouncing off the front objective. No Big still, it can be controlled in post processing.


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Re: Does F10 Matter in CaK?

Post by marktownley »

As AstroDan noted. It is the reflection coming from the module ERF bouncing off the front objective. No Big still, it can be controlled in post processing.

You can remove that real easy. Look on the barrel on the CaK module, on the side there are allen bolts recessed in, these are the retaining screws for the ERFs (there's 2 of them in series), if you loosen the one nearest the objective end of the module you can alter the tilt of it, and in turn throw the reflection out of the fov when imaging. Seriously, this is real easy to do...


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Mark, thanks, perhaps a person may be able to achieve a minimal amount of tilt using that method but the set screws are for spacing and retaining the filter in the barrel. If you remember C Schure and Stephen R both disassembled their filters so we could peek.

If a person were inexperienced, I would not recommend this as the filter could slip and drop against the bottom filter and break.

I may attempt it with my filter when it arrives, if the same problem exists.


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Here are a couple of pics as a reminder:





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Re: Does F10 Matter in CaK?

Post by brianb11213 »

Mark, thanks, perhaps a person may be able to achieve a minimal amount of tilt using that method but the set screws are for spacing and retaining the filter in the barrel.
Minimal is the word, mine is quite a tight fit.

The actual filter element in the objective facing front element in my B1200 is tilted very slightly in its mount ... so when pointed accurately at the sun, the bright patch on the objective illuminated by the mirrored surface is off centre (but wholly within the free aperture of the objective). I suspect it's better to have the reflected heat passing through the glass rather than heating one side of the tube / objective cell which would be likely to cause tube currents and/or distortion of the objective by differential expansion.

If there is a contrast reduction caused by reflection off the mirror surface & back off the objective, this will probably depend critically on the curvature of the elements in the objective as well as the efficiency of the anti-reflective coatings on the objective at 393 nm. The only thing I can say here is that, from practical experience, my B1200 CaK module seems to deliver slightly more contrast with my f/7 FLT 110 than it does with the (supposedly CaK optimised) Lunt 60mm f/8 scope it came with. There's not much in it but I'm convinced the effect is real.


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Re: Does F10 Matter in CaK?

Post by marktownley »

Looking at the pictures looks like you could also alter the tilt of the filters in the cells they are mounted by pulling off the 'gobs' of silicone seal that seem to locate them in place and resealing them in place again, maybe... You don't need much tilt, the sun is only half a degree across and you only want to throw the reflection by slightly more than this.


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Honestly, when doing full disc and close-up in active regions etc. It is not much of an issue. The only time it is a hassle is when trying to grab CaK proms. Then it becomes frustrating trying to keep the reflection out of the close-up prom image.


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Re: Does F10 Matter in CaK?

Post by Pedro »

I agree, capturing CaK proms is not easy. I've tried lots of times without success. I guess that the DMKs are not good cameras for this kind of work (low blue sensitivity). I've seen great prom images with PGR cameras though.

Pedro


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Re: Does F10 Matter in CaK?

Post by marktownley »

I guess that the DMKs are not good cameras for this kind of work (low blue sensitivity). I've seen great prom images with PGR cameras though.

Pedro

I've never thought about it this way before Pedro, but I think is a very good point...


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Re: Does F10 Matter in CaK?

Post by Derek Klepp »

This tilt issue is prominent when it comes to prom capture in CaK I find that my success is dependant on where the prom is in relation to the module set up and that I had best sucess using a 2.4ext on my camera. I am still experimenting here.


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Re: Does F10 Matter in CaK?

Post by swisswalter »

This tilt issue is prominent when it comes to prom capture in CaK I find that my success is dependant on where the prom is in relation to the module set up and that I had best sucess using a 2.4ext on my camera. I am still experimenting here.


hi Derek

very interesting addition


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Re: Does F10 Matter in CaK?

Post by sullij1 »

Hi Derek, It just dawned on me after a month, of what you were trying to say about te extension helping with the proms. Can you show a pic of how you set up the extension? I am interested in getting better CaK prom in my images.


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Re: Does F10 Matter in CaK?

Post by sullij1 »

:woohoo: >2000 views! Guess there was some interest here. Ya think! :lol:


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