Friday, October 4, 2013

Mikes PST Mods , 150mm Istar , !00mm Skywatcher and Original Celestron 102 XLT

                        150mm Istar / PST Build

i became interested in the Istar objectives after seeing solar images taken by "JP" from France with his 230mm Istar / PST . A 2300mm long scope would be more that I would want to handle so I opted for the 150mm / 1500mm F/L combination .
  I had 2 scopes left over from previous solar experiments that I could combine into 1 long OTA . The scope I used for the objective end was an old Apogee 127mm acro . This one has a dew shield that was slightly larger than the 150mm I needed to mount the Istar objective . For the back end of the scope I decided to use the tube from my original Celestron XLT 102 mod , this one already had an adapter made for the back end to accept the PST etalon and focuser assembly .
  Next I lay'd everything out on the floor and simulated the light cone using 2 tape measures . From this I could verify that the light cone would clear the steps in the various diameter tubes and also determine the location for the internal 100mm Lunt ERF

Next I had to source some aluminium to make a ring to mount the objective to the dew shield . I was lucky to find a place willing to slice me a couple of rings from a heavy wall aluminium pipe .

                            Here is the finished ring with the objective mounted to the dew shield

Now I needed a way to mount the Celestron 102 tube squarely into the Apogee 127 tube . For this I made a PVC ring nut to thread on to the tube where the 102 objective was once mounted , this ring fits the 127 tube and keeps the two tubes aligned and also acts as a baffle . To lock this into position I made a flange with 3 mounting screws and an additional 3 set screws to allow me to adjust the overall length of the OTA

The lens cell for the old Apogee was just plastic and the dew shield just slid on . This was nothing that I would want to trust but would be sufficient for testing the scope . This is the scope loaded on my Losmandy GM8 for the initial test .

The initial testing was very positive as far as the scope was concerned but also suggested that my mount was struggling with this load . One thing always seems to lead to another in this solar game so the solution was to look for a bigger mount . As you can see from the pic there was also an issue of the tripod legs interfering with the scope . I was able to find a nice gently used EQ6 Pro mount on a column type base and solved the mount issue . This left 2 more things to do to complete the scope . First I replaced the plastic lens cell that was joining the dew shield to the 127 tube with another machined aluminium ring so now all the tubes are firmly held together and aligned . Last but not least was a paint job to give it a more professional look and get away from the cobble scope look .
 Here is the completed scope on the new mount , I am very pleased with how this project turned out and have been posting the initial images from it on the blog .

There will be one last thing I will add to this scope and that will be a remote drive for the etalon . The focuser I use on this scope is a Moonlight DC motorized focuser . The hand control has 2 channels so I have ordered another motor that I will adapt to adjust the etalon , this will put my focus as well as etalon adjustment on the same hand control .

Any questions or comments are always welcome .

                SkyWatcher 100 Pro Fluorite ED APO / PST Mod

This is my second mod and for this one I used a higher quality donor . I picked up a second used PST a while back and decided to use the motorized Moonlight focuser from my SM 60 II for this mod . For the scope I removed the focuser and made an adapter for the end that could lock the "gold tube" into.

Next I removed the aperture from the gold tube and made another adaptor to mount the etalon to the Moonlight .focuser . When I am sure of the best etalon position I will make a proper length tube to replace the gold one .

The last thing to do was to make another ERF cell to fit this scope , similar to what I made for the XLT mod below only bored to fit this scope .

I have a SM 90 II etalon on order for this scope so stay tuned for part 2 of this new adventure !
First Light Images from this scope can be found in the March 17 2014 blog post

                            First 102 XLT / PST Mod

These mods are very similar and I have followed the people who have gone before me with this . There are 2 key elements that need to be followed  ,
 1) the "donor" scope must be as close to F 10 as possible to match the optics in the PST etalon
 2) the etalon needs to be placed 200mm into the optical path .

The required pieces are a PST , an F 10 donor scope , an Energy Rejection Filter. (ERF) and a larger blocking filter for a stage 2 Mod .

I did try the stage 1 mod which uses the PST black box and blocking filter but found the 5mm BF in the PST to be too restrictive and will not recommend it .. Everything here relates to a stage 2 PST mod .

                                                           ERF CELL

 I prefer the way Bill mounted his ERF cell directly to his objective lens cell because there is no chance of forgetting to mount the ERF before using the scope . When I did mine I made a separate cell to mount on the end of my XLT . This is how I did mine but would strongly recommend mounting it in a  more permanent fashion as Bill has done , I will be changing mine soon .

 I used an off the shelf PVC drain pipe fitting . The glass ERF fit perfectly so I just machined the fitting to fit the end of my scope and trimmed off the excess material , here is how I made the ERF cell .

                                                          The Fitting

                                                   After machining ready for assembly

                                                 ERF assembly mounted to XLT

Next the PST will have to be disassembled , loctite is often used but can be overcome . I used rubber and hose clamps to grip the pieces without marking them . All that is used is the PST etalon .

What is required next is a way to adapt this etalon into the donor scope . Antares makes a 2" twist lock extension that can work well for this . The 2" socket end of this extension is a direct fit to the etalon . The nose piece of this extension is too small to fit the other end of the etalon . For Bill I made an adaptor ring to match the 2 threads . For my etalon I used the nosepiece from a cheap 2" eyepiece I was not using

Now the etalon needs to be positioned 200 mm into the optical path . This scope is 1000mm FL so converting this it works out to close to 31 1/2" from the objective . As you can see from this picture the supplied focuser is too long . My 2 options were to either cut the tube and use the supplied focuser or use a low profile focuser . I all ready had a low profile focuser so I adapted it to the XLT and my etalon placement is correct . And the BF 15 works well with this combination .

Although the above did work we found out that it is much better to leave the etalon positioned at or close to 200mm into the optical path and do all focusing adjustments behind the etalon . To achieve this I shortened the XLT tube by 3" and reinstalled  the original focuser to use to position the etalon .
  The other change I made was to mount a Moonlight focuser to the etalon . This allows me to focus now without disturbing the etalon position .


Bill Griffith and Mike Bridges said...

Just a wonder piece of work Mike. :-)

John Hicks said...


You can use a Lunt variable focuser easier than that bulky Moonlight focuser with the PST Etalon. Makes a nice trim apparatus. I would like you to explain how you adapted the Antares 2" twist lock extension tube to your etalon - the M50 thread on the PST front Etalon is a "lost" pitch in the USA and Canada, and you have to employ a good machinist to cut a "crafted" thread to fit it. The second thread in holds the collimator lens and its a regular 42 mm fine thread but if you use it you will find its to short and does not grip the nosepiece satisfactory. Is that the thread you found on the Antares 2" twist lock extension tube?. I had to machine the entire nosepiece to fit the PST Etalon, and I machined a 42mm fine thread in front of that for the new Collimator position. It is placed as you know -200 mm up the focus tube.
I use a Meade 102ED f/9 refractor - perfect for the PST MODS as the 2" nosepiece goes in about 1-1/2" up the focuser. I use a Lunt 4" ERF over the objective. I find the PST MOD outflanks both my Lunt 50 mm and my Coronado 60mm solar H-alpha filters, but curiously if a place either in front of the objective (without the Lunt ERF)
and add the PST MOD at the tailpiece end, I get fantastic results (a doubler). No
matter what you do, the PST Etalon has its shortcomings in off-band information which you can sometimes tune-out but most often only gives you partial on-band results. I prefer to image more than just flare detail, and activity areas are really all the PST MODS are good for. Ken Harrison is placing his Lunt 50 mm in
the PST MOD position to use a higher quality etalon with greater diameter which would field more on-band H-alpha wavelength, but at the expense of requiring a Baader telecentric TZ2 Barlow and adding a ton of weight on the tailpiece end which will stress your drive. Have you any better ideas?> I'm on the verge of buying a Daystar Quark to use with my Lunt 50 or Coronado 60 (front-mounted). It looks like that system is really the best option for high resolution. The PST MOD is exciting to monkey with but you can burn up a lot of bucks trying to fit it and still get
terrible discontinuaties in resolution in the field of view.
John Hicks, Operator., New Forest Solar Observatory, Ontario, Canada

Bill Griffith and Mike Bridges said...

Hello John

This is Mike , I was the one who made the adapter for Bill's PST etalon and you are correct about the thread . The Antares adapter thread was enough oversize that it allowed me to cut a new thread the correct pitch on top of it . Antares have changed their adapters now and the new ones will not work , I bought a couple of the old style adapters for this purpose . I am a retired machinist and have a small lathe so this was easy for me to do .
As we have read PST etalons can vary and some are better than others as far as the sweet spot goes . I have two that I use for my mods and one is considerably better than the other . I agree with you that the mods are better for the close up imaging and find the sweet spot more manageable when there is a barlow in the stream .
The best modded scope I have for non barlowed shots is my Skywatcher 100 , I recently found a Coronado 90 mm external etalon which I used to double stack this scope . By tuning and rotating the etalons I am able to spread the on band details very well . I know that a SM 90 etalon is an expensive option but I was very pleased with the results and have no regrets buying it .
My Quark has not been a user friendly piece for imaging . I have tried many combinations and scopes with it and have found it to be very dependant on viewing conditions . It performs ok for visual but the 4x barlow is usually too much magnification for any decent imaging unless you have pristine conditions . I have also tried to use the Quark with my Coronado 60mm external etalon and again had poor results . The Quark chromosphere is already down to double stack range and using it with an external etalon darkened the view and slowed down the capture rate to a point where it was very difficult to capture crisp detailed frames . Of all of the gear that I have experimented with I have found the Quark to be something I could have done without .
As far as focusers goes I do like my Moonlights , I bought the first one to replace the sub standard helical focuser that comes on the SM 60 . The ones that I use on my modded scopes are the motorized version and I have been spoiled with these , very handy for focusing without taking my eyes off of the screen . I have also added a remote drive for the internal PST etalon on my Skywatcher mod , now I can adjust focus as well as both etalons without taking my eyes off of the screen .
I hope this answers your questions and if we can be of any other assistance please do not hesitate to ask .
A quick question for you now , where is New Forest Solar Observatory located ? I am in the country in the Brantford area

Best regards