Parfocal rings

[sorrimen]

Hi all

Right slightly niche question here. Just installed a skywatcher auto focuser (not actually an auto focuser, just a motor). Haven’t had the chance to use it on a night observing yet but the little use I have had makes me think I’ll probably want to make my eyepieces parfocal or at least as close as I can get with parfocal rings, just to cut out the waiting between swapping out eyepieces. My question is regarding my barlow; I use my barlow for my higher powers of 200x and 300x and I use it somewhat frequently. In your opinion, would it be worth getting an extension tube so that my eyepieces are parfocal to their barlowed equivalent? Also, are extension tubes normally the same size as barlows (I have a bst 2x barlow if that helps). Of course, if all barlows and extension tubes are of different lengths then they won’t be useful for parfocalising.

Apologies if this is a bit of a ‘depends on how much it bothers you’ question, but would be interested to hear opinions and experience with parfocal rings.

Thanks 

Ross

[Don Pensack]

Extension tubes have no lenses, so do not alter magnification or eyepiece focal length.

As for Barlows, they extend the focal plane of the scope backwards.

But not all Barlows extend the focal plane to the opening of the Barlow, i.e. not all Barlow will be parfocal with the eyepiece when the eyepiece is used alone.

Some Barlows' focal planes fall short of the opening in the Barlow.  These require in-focusing.

Some Barlows' focal planes extend the focal plane outward by MORE than the length of their upper barrels.  These require out-focusing.  You could make these parfocal with the eyepieces with 

barrel extenders and parfocalizing rings on the eyepieces (not extension tubes--they pull the eyepieces out WAY too far).

Can you know in advance?  Check the on-line reviews for the Barlow in question.

 

Parfocalizing your eyepieces is a good idea.  It is important if using a Barlow, and it is especially important if using a GSO coma corrector in a dob.

It makes setting up the coma corrector easy, and it makes acquisition of the object easy using a Barlow in case it's drifted in the field.

 

[sorrimen]

19 hours ago, Don Pensack said:

Extension tubes have no lenses, so do not alter magnification or eyepiece focal length.

As for Barlows, they extend the focal plane of the scope backwards.

But not all Barlows extend the focal plane to the opening of the Barlow, i.e. not all Barlow will be parfocal with the eyepiece when the eyepiece is used alone.

Some Barlows' focal planes fall short of the opening in the Barlow.  These require in-focusing.

Some Barlows' focal planes extend the focal plane outward by MORE than the length of their upper barrels.  These require out-focusing.  You could make these parfocal with the eyepieces with 

barrel extenders and parfocalizing rings on the eyepieces (not extension tubes--they pull the eyepieces out WAY too far).

Can you know in advance?  Check the on-line reviews for the Barlow in question.

 

Parfocalizing your eyepieces is a good idea.  It is important if using a Barlow, and it is especially important if using a GSO coma corrector in a dob.

It makes setting up the coma corrector easy, and it makes acquisition of the object easy using a Barlow in case it's drifted in the field.

 

Oh of course, very airheaded of me. Annoyingly my barlow requires in focus. Think parfocalising my eyepieces to each other and forgetting barlow for now is what I’ll do for simplicity. Thanks for the help as always, Don.

[jjohnson3803]

This is not quite your situation, but I have a small finder that I use as an RFT.  The diagonal was designed for use with an illuminated EP, so regular EPs don't focus unless they're backed out of the diagonal a bit.  To solve the problem, I bought several very inexpensive 1.25-in rubber O-rings which I slip on the EP barrels.  Provides just enough spacing to come to focus.

O-rings might be useful (and cheap) to experiment with to see if parfocal rings would work for you.  YMMV.

 

[Louis D]

I prefer rubber O-rings to parfocal rings because they won't create dimples in the insertion barrel and don't have issues with tightening on the edge of undercuts.  This is the case with my 12mm NT4 which needed 20mm worth of 4mm thick O-rings.  The last one is riding on the lower slope of the undercut.  Check ebay for O-rings.