Until recently, I have been using my 100mm PST Tal based stage 2 mod happily in mono and getting excellent detail. Over the past few months I have been working towards using binoviewers and initially was very impressed with the views as they increased the level of contrast and viewing was so much more comfortable. I was though getting slightly frustrated as the views were getting much darker than I was used to in mono even at lower powers.
At the DIY Peak Star Party at the weekend, I had a slight epiphany when I looked through Stu’s similar 102mm Vixen based PST mod. The views were really quite phenomenal compared with mine. Even at really high magnification (perhaps around 150x-200x) the views were bright and detailed. Mine were dim and with less detail in the proms but possibly dimmer but more detail in the surface detail, almost like when thin cloud passes across the sun while observing.
We tried all combinations, and the views did not change. That was until we tried Stu’s whole stage one assembly in my focuser. This then provided comparable views at last! This to our eyes meant that there were fewer ‘culprits’ causing the differences: the blocking filter, the etalon or the internal workings of the PST box. We considered the latter to be unlikely so the etalon or the blocking filter were thought the more likely two.
Along with another SGLer, I consider fellow member Peter Drew a genuine expert of all matters solar and he built Stu’s version so I sent Peter a message and described the symptoms. Peter highlighted a more likely cause and one that matched my symptoms well. His prognosis was that my ITF (the little reflective filter at the base of the PST eyepiece holder) was the problem. I could not really see any issues in situ but on removal (it was pretty firmly held in its cell with loctite) and holding against a light box I could immediately see the apparent signs of rusting/heat damage. I suspect this has always been there (since I bought my PST mod parts anyway) but as I have only used it in mono (and those views have not changed) the brightness levels have been high enough for lower power views anyway. I attach pics below which show slight milkiness in all but a clear ring about a third out from centre.
As I use a Baader DERF, the ITF is not actually necessary (but the blocking filter definitely is necessary) as all the heat and UV are removed from the path before even going through the etalon. Therefore I have removed the ITF and now hope that the BV view will be more like those I had through Stu’s at least at higher powers. Lower power viewing might result in an image too bright that will wash out the contrast on the full disk so I may need to add a further Baader Ha filter (I have one) into the train, possibly between the etalon and blocking filter or if it works on the nose of the PST mod scope side.
I’ll report back in due course regarding results but thought I’d start this post to encourage others to check their ITF if their views are not as expected or if they have deteriorated.
Big thanks to Stu and Peter for helping me with this – I hope!