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newbie: trouble with Coronado PST

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#1 dmcnally

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Posted 17 March 2016 - 11:42 AM

Hi,

I've been doing night time observing and imaging for a few years.  A few months ago I mentioned some interest in observing and imaging the sun.  Yesterday a friend loaned me his Coronado PST.  It was given to him a long time ago and he never had the original eyepiece.  He used the PST with a DMK camera for imaging and never really did much observing.

 

He gave me the PST with a 12.5mm no-name/markings eyepiece in the PST.  I tried to get the scope to focus a few times but I was never able to get the scope to focus.  This morning I tried again.  I could get the scope to focus by sliding the eyepiece completely out of the holder (a 1-1/4" extension was way to far).  After that I went and grabbed a 25mm Meade (Kellner??) eyepiece.  I had to slide it out of the holder about 1/4"-3/8" to get this eyepiece to focus with the scope.

 

Do the PST's have a long backfocus (sorry, nomenclature isn't my strong point) and need eyepieces with long barrels, or is there something wrong with the scope, or am I just doing something dumb?

 

thanks,

Dave

 



#2 skyward_eyes

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Posted 17 March 2016 - 12:10 PM

You shouldnt have to back the eyepiece out to focus on a PST. They generally come with focus fairly easily with a 25 Plossl. 



#3 Jeffrey C.

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Posted 17 March 2016 - 12:38 PM

Hmm, I have the same issue with my Lunt LS35tha. Isn't it normal either?



#4 DAVIDG

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Posted 17 March 2016 - 12:40 PM

  The unit focuses by moving a penta prism so it is has a fairly large amount  of the focus range. Does the image look like it is getting close to focusing as you turn the knob ? If you see no change, the prism assembly may have come loose from the threaded rod. You'll need to take the side cover off to examine what is going on.

   Has the unit been dropped ? Another possible problem is that the Etalon has become decontacted. One of the signs that has happened is as you turn the tuning ring, you get multiple images of the Sun.

 

               - Dave 


Edited by DAVIDG, 17 March 2016 - 05:08 PM.

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#5 Great Attractor

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Posted 17 March 2016 - 01:19 PM

Hmm, I have the same issue with my Lunt LS35tha. Isn't it normal either?

 

Note that the nosepiece of the diagonal is quite long - that's so you can slide it out a bit from the clamping ring at optical tube's back (instead of sliding the eyepiece out of the focuser).



#6 dmcnally

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Posted 17 March 2016 - 02:45 PM

  The unit focuses by moving a penta prism so it is has a fairly amount  of the focus range. Does the image look like it is getting close to focusing as you turn the knob ? If you see no change, the prism assembly may have come loose from the threaded rod. You'll need to take the side cover off to examine what is going on.

   Has the unit been dropped ? Another possible problem is that the Etalon has become decontacted. One of the signs that has happened is as you turn the tuning ring, you get multiple images of the Sun.

 

               - Dave 

Thanks for reply.  The focus barely changes over the entire range with the focus knob, that's why I decided to try sliding the eyepiece out.

 

Changing the front ring (etalon) does change the image (red disk to granulation??) and I don't get double images.

 

I'll take the scope apart and see what's going on.

 

Thanks again,

Dave



#7 DAVIDG

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Posted 17 March 2016 - 05:07 PM

The two 90 degree faces of the prism should be parallel to the inside top surfaces of the body of the scope. What very commonly happens is the glue allows the prism to twist out of alignment. If you move the prism to the very top of the enclosure with the focusing knobs you can easily tell if the faces are parallel to the inside of the enclosure or not. Many times they are many degrees out of alignment.

    Here what the prism would  look when it is out of alignment  and what it looks like when aligned correctly

 

                     - Dave 

   misalignedprism.jpg

 

alignedprism.jpg


Edited by DAVIDG, 17 March 2016 - 05:09 PM.

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#8 dmcnally

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Posted 17 March 2016 - 07:01 PM

The two 90 degree faces of the prism should be parallel to the inside top surfaces of the body of the scope. What very commonly happens is the glue allows the prism to twist out of alignment. If you move the prism to the very top of the enclosure with the focusing knobs you can easily tell if the faces are parallel to the inside of the enclosure or not. Many times they are many degrees out of alignment.

    Here what the prism would  look when it is out of alignment  and what it looks like when aligned correctly

 

                     - Dave 

 

Hi Dave,

Thanks for the instructions and pictures.  It's always nice to know what to expect before you crack the cases on something.

 

Dave



#9 Cometeer

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Posted 17 March 2016 - 07:33 PM

Btw, the 12.5mm eyepiece is the original that came with PST's.



#10 dmcnally

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Posted 17 March 2016 - 08:13 PM

I took the PST apart and it wasn't a pretty sight.

 

Dave

 

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#11 DAVIDG

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Posted 17 March 2016 - 08:48 PM

Paint the  chips flat black. Glue the prism back on and align it. The size of the light cone is smaller then the prism face so the chips shouldn't hurt .They just don't look  pretty. 

 

             -Dave


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#12 dmcnally

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Posted 17 March 2016 - 09:10 PM

Paint the  chips flat black. Glue the prism back on and align it. The size of the light cone is smaller then the prism face so the chips shouldn't hurt .They just don't look  pretty. 

 

             -Dave

 

Thanks for the advice.  I don't have any black paint and I got impatient.  I've already glued the prism onto the slider.  If the performance isn't acceptable I'll take it out (and maybe apart again) and paint the chips.

 

Dave



#13 DAVIDG

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Posted 18 March 2016 - 09:02 AM

 While you got the scope apart, check the ITF filter at the very bottom of the eyepiece tube. It is very common for it to be "rusty".

 

               - Dave 



#14 dmcnally

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Posted 18 March 2016 - 01:27 PM

Hi Dave,

Thanks.  I took the cover off and then took out the prism.  The eyepiece side looks like a mirror.  The prism side looks a little rusty.

 

Dave

 

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#15 DAVIDG

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Posted 18 March 2016 - 04:26 PM

Yep, that is a  rusty ITF. I wouldn't say it's a little rusty but fully rusted over !   As I said Meade didn't fix the problem they just moved it  so it's harder to see. Google "ITF H-alpha 656nm" and  you'll find replacements. You just unscrew the retainer ring and it comes right out and a new one drops in. You get a brighter, sharper and higher contrast image with a new one installed.

 

                   - Dave 


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#16 dmcnally

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Posted 18 March 2016 - 05:51 PM

Hi Dave,

 

Thanks and thanks for the search terms. 

 

I changed the search around a little and found your CN thread from Dec 2013.  How's your Maier filter holding up?  I'm just wondering if this is going to turn into a scheduled maintenance task every 2, 3, or 4, years.

 

Dave



#17 DAVIDG

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Posted 18 March 2016 - 07:49 PM

I just checked both my PST's with Maiers ITF filters and they show no signs of rust.

 

      -Dave


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#18 germana1

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Posted 18 March 2016 - 08:12 PM

I can also say the Meier filters work very well as I've changed 2 in my blocking filters, and also a great deal with fast shipping.

Pete


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#19 dmcnally

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Posted 18 March 2016 - 08:50 PM

I just checked both my PST's with Maiers ITF filters and they show no signs of rust.

 

 

I can also say the Meier filters work very well as I've changed 2 in my blocking filters

 

Dave and Pete,

Thanks.  It's good to know that the Maiers filters will last for a while.

Dave



#20 M A S

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Posted 16 May 2016 - 06:37 AM

Hi Just caught this topic, I also have a problem with a rusty filter in my PST, have  just taken delivery of a filter from above firm in the States.

 

It appears to have a Red side and a green side , when I replace which side faces the Sun. 

thanks in advance.



#21 JoseBorrero

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Posted 18 May 2016 - 11:03 AM

I think there's post here to subscribe the serial number of your scope and report the state of your scope  :question:



#22 JoseBorrero

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Posted 19 May 2016 - 09:18 PM

Have try to polish it?  I dont know which filter type is compared to a Lunt 80 who got similar problem. It was solved by polishing compund made by meguiars, like a car lens cleaner.

 

I believe the lunt 80 has a blue green glass with out any coatings



#23 DAVIDG

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Posted 20 May 2016 - 08:11 AM

Hi Just caught this topic, I also have a problem with a rusty filter in my PST, have  just taken delivery of a filter from above firm in the States.

 

It appears to have a Red side and a green side , when I replace which side faces the Sun. 

thanks in advance.

 You want the red side to face toward the eyepiece.. 

 

               - Dave 


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#24 DAVIDG

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Posted 20 May 2016 - 08:16 AM

Have try to polish it?  I dont know which filter type is compared to a Lunt 80 who got similar problem. It was solved by polishing compund made by meguiars, like a car lens cleaner.

 

I believe the lunt 80 has a blue green glass with out any coatings

 

 

 ITF filter that goes bad in the PST is not the same in function or design as the one in the Lunt that goes bad. ITF is an interference filter with the coatings applied between  two glass plates. These coating are moisture and air sensitive so the edge of the filter is sealed. When the seal fails the coatings deteriorate starting from the edge and working inward.  The only way to fix the problem is to replace the filter which is not difficult. 

 

                 - Dave 



#25 Brian Risley

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Posted 22 May 2016 - 04:08 PM

Not to hijack this, but my issue could be similar.  (109837) If I turn the tuning ring, I see a dark shadow move over the image, kinda like there is a wide hair in the system.  The overall image is darker when compared side by side with another older PST.  It shows excellent surface detail when this area is over the image.  (It used to, and the other PST does, have a smooth transition when turning the tuning ring.)

What is the Meade warranty on a PST, website only lists the standard Meade 1 year warranty?

 

(Just found my invoice/paperwork, I see it is 60 Months and this is just over 3 years old, so I will be calling them!)

Thanks, Brian




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