Parfocal rings

[Knighty2112]

As I'm off work and it was a slow day today, I thought I'd get down to the job of fitting the parfocal rings I'd bought in November last year. I was using my ED80 DS-Pro with my Baader 1.25" diagonal to do this, so after focusing on a distant chimney stack, I found that my 5mm Starglider EP needed the most in focus to achieve correct focus, so I set about fitting the parfocal rings to four of my most used EPs. However, I found that even though I have a very mixed bag of 1.25" EPs (Celestron Omni & X-Cel, Skywatcher, Baader, Televue, Starglider, William Optics, TMB II, Explore Scientific), the only one that I could actually fit a ring to was my 10mm Tele View Delos EP. As can be seen from the photo attached it is about 7.5mm difference in focus from my 5mm EP. All the other 1.25" EPs I own were already not far from been parfocal anyway, so the difference was to small to allow a ring to be fitted. I double checked that this was not just a quirk of the ED80 scope, and tested them in my Celestron 4SE Maksutov and my ST120 refractor (I assume the same for my other scopes too). So from the pack of 4 parfocal rings I got I only needed one after all. All the other EPs needed the focus changing either fractionally, or by less than the thickness of the 5mm parfocal ring itself. Seems like I was winding focus in or out more than that when observing, but obviously wasn't  changing too much after all! ?

Did anyone else find that with such a different range of brands of EPs that they own gave a similar result if you tried to use parfocal rings at all? Just curious!  

 

[John]

They can be useful things !

The Delos up to and including the 12mm fall into TV's par focal group "B" which have their focal plane around 12.5mm below the chrome "shoulder" of the eyepiece. Many TV's fall into this group but it is quite a few mm further out than many other non-TV eyepieces. My Delos 17.3mm (not a group B eyepiece) is not far off par-focal with the Pentax XW's that I use it with but the Panoptic 24mm and Nagler 2-4mm zoom are group B which can be mildly annoying. I could use parfocalisation rings with them but I've not bothered to fit them as yet.  I've given up trying the par-focalise the Ethos eyepieces - they are all over the place ! 

 

[ronin]

I looked them then decided they would be of little advantage to me. The reason simply being that whenever I change eyepieces immaterial of how good the immediate result is I will start adjusting the focuser. In effect I move it even it appears good. Suppose in the hope that it will magically get better. So adding a par-focal ring to still adjust the focus just didn't appear much use.

On an SCT or Mak I can see the advantage as they often have the need for the observer to make multiple turns of the focus adjuster in order to get the good image back, but owing to the relative ease of a refractor or newtonian type focuser the time involvement seems much the same with or without the use of par-focal rings.

Will say the adjustment aspect is case of "what if???" or "can I get better?" but usually adjustment even small is made and therefore to my thinking the time involved is similar, except as I suggest on an SCT or Mak.

Just me but seeing the Delos with the ring on I would rather seat it well down and make some adjustment then have what appears only 2/3 of the lower section in the focuser.

[Knighty2112]

18 minutes ago, ronin said:

Just me but seeing the Delos with the ring on I would rather seat it well down and make some adjustment then have what appears only 2/3 of the lower section in the focuser.

Good point. I hadn't also taken into consideration that I will sometimes put a filter in the EP too, so that would move the barrel of the focuser to sit even less in the diagonal etc. As it is only one EP that I can utilise the ring then I may take it off if it threatens the stability of it as it sits in the focuser or diagonal. 

[Louis D]

I parfocalized a 12mm Nagler T4 with about five 4mm thick 2" O-rings.  I had to then add a 20mm extension ring to the bottom of the 2" skirt to have enough to clamp it into the focuser.  I did this mainly to get better correction in my GSO coma corrector since it is non-adjustable.  The 12mm NT4 focuses about 3/4" below the shoulder.  It seats plenty firmly in every focuser despite all the rubber between the top of the focuser and the shoulder of the eyepiece.