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#26 Eddgie

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Posted 05 August 2019 - 02:04 PM

The difference between the two aftermarket filters is as I recall a difference in coating technology. The Beloptik  is a hard coating that should not fail for a very long time

 

The original ITF does not have the edges sealed, and this allows air to get into the layers and causes oxidation.

 

The Maier filter has a black ring around the outside.  This ring is an epoxy (like?) seal that should prevent the air from getting into the filter substrate.  This should be a (relative to the original) long life filter.

 

At least that is the way I understood the differences. 

 

Now, what one looses on the cost difference would likely be made up in re-sale. If I was in the market for a used PST and I did not know how old the ITF was, I would not be interested in buying it unless the seller offered a receipt that showed the ITF was replaced.  If the ITF was replaced with the Beloptik filter, I would probably be inclined to pay an extra $50-$75. If it just had the Maeir, I would give going rate.  If it had no record of ITF replacement, I would want it discounted to the price difference of the Maier filter. 

 

Just me thinking about value..  

 

My bet is that most older PST ITFs have degraded so some degree, but because it is gradual, the seller may not even realize it.

 

This is not a "Usage" issue.   This is oxidation, and oxidation never sleeps.


Edited by Eddgie, 05 August 2019 - 02:09 PM.


#27 Eddgie

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Posted 05 August 2019 - 02:24 PM

As to the tilt.  I too think that some tilt may have been designed into the system.

 

I had straightened the prism in my last PST when I replaced the ITF.  When I looked though the scope, I noticed a slight reflection.

 

Of course I thought it was the filter, so I took it out, made sure it was seated properly, and put it back in.  Still had the reflection.

 

I put a little tilt back in the prism, and the reflection went away. 

 

So, that is why I think that a little tilt might have been designed into the system. 

 

Maybe it was just my sample though, but restoring the tilt made the sky around the disk jet black.  If one straightened their prism though, and did not note any ill effects, then It would of course make sense to leave it in that position.   


Edited by Eddgie, 05 August 2019 - 02:25 PM.


#28 DAVIDG

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Posted 06 August 2019 - 08:23 AM

 If you tilt the prism you introduce astigmatism into the image. That is just an optical fact,  so I highly doubt that it was designed to be made that way. It is same optical issue with the diagonal in a Newtonian when it  is not correctly aligned. 

   Any ghosting is most likely from the ITF filter. You can slightly tilt it with no optical problems to remove any ghosting.  From what I can tell the ITF cell has been machined to have a very slight tilt to it. So you can rotate the filter in the cell to throw  any ghost image out of the field.

 

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#29 DAVIDG

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Posted 06 August 2019 - 08:29 AM

 What Maier's does is fully seal the filter in a metal ring with epoxy but also doesn't not bring the coating out to the very edge of the filter. This gap in coating along with the epoxy makes a hard seal from oxygen and water so the filter last much longer then the standard Meade ITF . Mine are some of the first ones he made and they  are still perfect after at least 5 years now, it might be a few years longer.

 

               - Dave 



#30 Zebra24601

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Posted 06 August 2019 - 11:57 AM

Do the problems discussed here extend to the larger aperture Coronado SolarMax scopes?

 

Don't know how sturdy the PST is but a loctited fixture needs to be heated to facilitate removal.  Sometimes a hair drier might soften up the gunk.

Yes, same issue for the 60mm SolarMax II, at least.  For those, the blocking filters are in the diagonal.  The ITF is the one on the objective side of the diagonal.


Edited by Zebra24601, 06 August 2019 - 06:18 PM.


#31 Jairo

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Posted 18 August 2019 - 02:27 PM

I bought my PST around 2012, and it's working poorly for obvious reasons.

 

I just unscrewed the nozzle of my PST for the first time, and I found out I could unscrew it in the middle, and I found out it has two filters.

I'm confused. Which one should I replace?

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Edited by Jairo, 18 August 2019 - 03:14 PM.


#32 DAVIDG

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Posted 19 August 2019 - 11:31 AM

 The ITF is the larger filter  on the right side of your picture which is located at the very bottom of the assembly and next to the top of the penta prism. It looks fully rusted over. Also check the  alignment of the penta prism by taking the side cover off.

 

                - Dave 



#33 MSWcdavis

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Posted 22 August 2019 - 06:39 PM

I was thinking of selling my double stack but for now have decided against it. Out of fun I have disassembled my pst to examine the ITF. 

 

Can you all provide opinions about its condition?

 

My pst was purchased new from OPT 2 years ago. 

 

Incidentally - I could simply unscrew the “eyepiece element” as if there were no loctite

 

I then used a Neewer threaded spanner to open the lens cell and then to remove the o ring securing the filter. 

 

It was all so easy thanks to cloudy nights members and little to no loctite for my PST (maybe they’ve learned?)

 

 

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Edited by MSWcdavis, 22 August 2019 - 07:02 PM.


#34 MSWcdavis

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Posted 22 August 2019 - 06:44 PM

Also what’s the correct way to put it back in??? - there is one side to the ITF that is nearly parallel to the lip of the retaining ring and another that is maybe a little less than halfway in the retaining ring - I can’t tell which side is clear glass and which side is the red filter part
Here’s another photo of my ITF

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Edited by MSWcdavis, 22 August 2019 - 07:02 PM.


#35 marktownley

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Posted 23 August 2019 - 12:01 AM

Difficult to tell from the pictures but looks fine to me!



#36 Jairo

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Posted 23 August 2019 - 08:22 AM

 The ITF is the larger filter  on the right side of your picture which is located at the very bottom of the assembly and next to the top of the penta prism. It looks fully rusted over. Also check the  alignment of the penta prism by taking the side cover off.

 

                - Dave 

Thank you, Dave. So I should do nothing with the smaller one on the left. What is it anyways?



#37 vincentv

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Posted 23 August 2019 - 08:46 AM

Thank you, Dave. So I should do nothing with the smaller one on the left. What is it anyways?

The little one is actually the expensive one. Its diameter is the difference between the BF5, BF10 and BF15. Its called the trim filter and has a bandpass of ~10 angstron = 1nm. It lets through only one of the transmission peaks of the etalon.



#38 MSWcdavis

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Posted 23 August 2019 - 09:17 AM

Anyone with experience with the correct orientation of the ITF - I cant tell which way to put it back in.

There is a side with little to no indent and a side with quite a bit. Is it like the Maier filter in that one side is clear glass and that side goes in first (I.e. light passes through it first)? I just can’t tell which side would be that.

Edited by MSWcdavis, 23 August 2019 - 09:19 AM.


#39 DAVIDG

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Posted 23 August 2019 - 09:26 AM

 The orientation of the ITF optically doesn't matter in how it transmits light. I would put the more reflective side toward the prism since that would reflect more of the IR and reduce any heating of the filter. Also don't over tighten the retainer ring. The filter needs to move to expand, if not it will stress the two pieces of glass it is made off and cause the seal to fail. 

   - Dave 


Edited by DAVIDG, 23 August 2019 - 09:28 AM.


#40 DAVIDG

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Posted 23 August 2019 - 09:53 AM

Thank you, Dave. So I should do nothing with the smaller one on the left. What is it anyways?

 As explained in  an earlier post the smaller filter is a very narrow band one.  How the filters in PST work is first the Etalon transmits many wavelengths but in very narrow slices. These slices are evenly spaced.  So if you looked at the output with  a spectroscope you would see slices across the whole spectra like a picket fence ie  |||||||||||||||||  . The Etalon is tuned so one of those slices is centered on the H-alpha wavelength of 6562.8 Angstroms. Getting it perfectly centered on that wavelength is what your doing when you turn the tuning ring. Now you need to get read of all the other wavelengths  ie  all the fence slats  So the ITF filter is a  broad band filter  so if  you looked at  output of the  ITF  it would look like this  |||||  The H alpha wavelength would be in the middle but you still see other ones to the left and right  but only a few.  Finally you have the small blocking filter right before the eyepiece. It has  a band pass that  is narrow enough just to let the wavelength of the H alpha line through  and block the other ones.  Two filter are used after the Etalon ( ITF and Blocker) because is easier  and cheaper make two different filters to narrower the wavelength down then one filter to do the job. The ITF has coatings to remove IR and UV wavelengths but the blocking filter doesn't, while blocking filter has coatings to allow it to have the narrower bandpass. 

 

              - Dave 


Edited by DAVIDG, 23 August 2019 - 09:54 AM.

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#41 MSWcdavis

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Posted 23 August 2019 - 12:09 PM

Since my filter is fine - would it still benefit me to get a new ITF from Maier? Would it add to the view?  

 

Also David - do you argue that no additional brightening / increased reflection comes from bending the prism parallel and that one could choose to increase the tilt of the ITF even beyond its in-built tilt to reduce any reflections getting in to the image?



#42 DAVIDG

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Posted 23 August 2019 - 02:00 PM

 The prism needs to be square to optical axis of the objective  and eyepiece. If not you get astigmatism in the image which reduces sharpness and detail.  The more you tilt an interference filter,  the more you shift  it's central wavelength that it transmit. So you don't want to add additional tilt to the minor amount there is to the ITF,  if any. I  would try rotating the ITF and/or the block filter by just slightly unscrewing the eyepiece tube to see if that would thru  any unwanted reflection out of the field of view.  Also double check the tuning of your Etalon. You might have two positions were it comes on band and one might not have any reflection. 

   If you look at the images of a bad ITF  you can see that they  look very hazy when viewed at an angle.  If yours is perfectly clear then there is no need to replace it. 

 

                    - Dave 



#43 Jairo

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Posted 25 August 2019 - 12:44 PM

The little one is actually the expensive one. Its diameter is the difference between the BF5, BF10 and BF15. Its called the trim filter and has a bandpass of ~10 angstron = 1nm. It lets through only one of the transmission peaks of the etalon.

So the most fragile one isn't the most expensive one.

I'm not complaining at all, but... why? Goes against Murphy's law, or something.



#44 Jairo

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Posted 08 September 2019 - 01:30 PM

Hi, I ordered my new ITF. It will take a while before it arrives, so I'd like to ask what are the best tools to substitute it. Do you have any suggestions?

 

Thank you in advance!



#45 Jairo

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Posted 11 January 2020 - 12:18 PM

Hello, I have my new ITF and my old PST here in my desk, ready for the "surgery". Do you have any advice for me before I begin?

I don't really know what to do, except for carefully disassembling the old ITF cell and then doing my best to assemble it again as I've seen it before.



#46 bob71741

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Posted 11 January 2020 - 03:00 PM

I've seen videos on solarchatforum.com that show the dis-assembly/assembly of the PST; look at mark townley's website


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#47 Jairo

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Posted 11 January 2020 - 10:03 PM

I've seen videos on solarchatforum.com that show the dis-assembly/assembly of the PST; look at mark townley's website

Hi, Bob! I'm having trouble registering that forum yet. Which is Mark Townley's website?



#48 bob71741

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Posted 12 January 2020 - 12:33 PM

Jario-  I do not think that you need to register to download the videos off of the site;however, I checked the Solar Reference Library forum and did not find the video that I was talking about. I have his PST etalon disassembly/assembly video, but its not the one you want. It can be found at  https://www.youtube....h?v=llSJy99Om9E

You can look at all his other videos that are on youtube as well.

 

His website is https://brierleyhill...r.blogspot.com/

Look under Imaging Tutorials for his videos, or contact Mark through his blog for help..
 


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#49 Jairo

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Posted 13 January 2020 - 11:38 AM

Jario-  I do not think that you need to register to download the videos off of the site;however, I checked the Solar Reference Library forum and did not find the video that I was talking about. I have his PST etalon disassembly/assembly video, but its not the one you want. It can be found at  https://www.youtube....h?v=llSJy99Om9E

You can look at all his other videos that are on youtube as well.

 

His website is https://brierleyhill...r.blogspot.com/

Look under Imaging Tutorials for his videos, or contact Mark through his blog for help..
 

Thank you, but I really couldn't find anything like that. I'm lost here.



#50 DAVIDG

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Posted 13 January 2020 - 12:39 PM

Hello, I have my new ITF and my old PST here in my desk, ready for the "surgery". Do you have any advice for me before I begin?

I don't really know what to do, except for carefully disassembling the old ITF cell and then doing my best to assemble it again as I've seen it before.

 It is a pretty simple repair. You need to unscrew the top section of the eyepiece barrel from the main body of the PST.  Once that is off you need to unscrew the retainer ring that holds the ITF filter in place. Usually the retainer ring has two slots in it that either a lens spanner wrench is used to loosen it or just a thin strip of metal that spans both slots. With the ring removed the old filter just drops out and you drop in the new one. Replace the retainer ring and just snug it up. don't over tighten it since you want the filter to be able to expand from any heat. Screw the eyepiece barrel back in place and enjoy the much improved views.

 Here is  a link to one of the many threads here that show the process https://www.cloudyni...nado-pst/page-3

 

                  - Dave 


Edited by DAVIDG, 13 January 2020 - 12:41 PM.



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