32 replies to this topic

[slavicek]

With f/10 and f/12 telescopes you should do both - focal reducer and 67 mm converter! The 67 mm converter maximizes NVs FOV so I'd say it's "essential".

[Celestro21]

With such slow and high power scopes as yours, I’d have thought the 67mm would be an essential bit of kit - possibly with a focal reducer too. It’s really useful for large, faint nebula with my scopes, but it really depends what you want to see - objects like the North American nebula are vast, so even with an F/7 85mm refractor, I need the 67mm to frame it well with afocal night vision.

So your saying get both then. Yes I am looking to see .ore nebula fields and a little brighter 

[Highburymark]

So your saying get both then. Yes I am looking to see .ore nebula fields and a little brighter

Yes!

[eblanken]

Hi (aka Celestro21),

 

What I did was to get two 55mm TeleVue Plossls and then adapted one to the 67mm 40 Deg. configuration. I use them if my SCTs which are Meade M8 (203mm x 2,200mm, f/11). The 67mm doesn't get used as much as the 55mm. I also have Pentax XW 40mm and 30mm and Vixen LVW 42mm and Clave' 50mm Plossl too.

 

What Gary (aka GGK) said in his earlier post (#21) matches my experience too.

 

Best,

 

Ed

Edited by eblanken, 20 April 2025 - 03:27 PM.

[Celestro21]

Hi (aka Celestro21),

What I did was to get two 55mm TeleVue Plossls and then adapted one to the 67mm 40 Deg. configuration. I use them if my SCTs which are Meade M8 (203mm x 2,200mm, f/11). The 67mm doesn't get used as much as the 55mm. I also have Pentax XW 40mm and 30mm and Vixen LVW 42mm and Clave' 50mm Plossl too.

What Gary (aka GGK) said in his earlier post (#21) matches my experience too.

Best,

Ed

Okay thanks. I think for now I'm going to get a used reducer and just gor from there. I don't need a super wide field view. Just a little

[Celestro21]

Next question. Televue suggest their 2 inch extender tube with the 67mm converter. Do I need that for my c11 

[eblanken]

Hi Again,

 

Next question. Televue suggest their 2 inch extender tube with the 67mm converter. Do I need that for my c11 

 

What this comes down to is your patience when you are changing eyepieces:

 

The 55mm Plossl is quite different than any other TeleVue eyepiece in that it requires a substantial amount of focus change as compared to any other 2 inch eyepiece (like on the order of moving the focus an inch). The 67mm takes that and makes it even worse of a change in focus. 

 

On the other side the SCT focuses by moving the primary mirror and that change gets "amplified" by the power of the secondary, so the SCT design has a huge range of focus travel. So a 10mm change of the primary gets a 50mm change in focus of a f/10 SCT where the primary is f/2 and the secondary is f/5. 

 

So, if you use a 55mm Plossl and a set of Delos, you already know that focus change between them. Just add a bigger focus change for the 67mm to Delos. Do you want to be cycling focus back and forth by an inch or more ??? Can you use spacers to "make" the focus difference act like "parfocal" eyepieces in your C-11 ???

 

EDIT: Here's the text of the situation in TeleVue's own words:

 

"Equally important as seeing the entire field available from the 2″ eyepiece format, the larger exit pupil produced by the 67mm focal length provides more light to amplify. Additionally, the 55mm Plössl by itself requires more drawtube out travel than most eyepieces and the 67mm attachment moves the focus out by another 1” (25mm) from nominal. Therefore, we recommend the purchase of our 2” Extension Tube (X2C-0008) for your telescope when purchasing this attachment."

 

Best,

 

Ed

Edited by eblanken, 24 April 2025 - 07:55 PM.

[Celestro21]

Hi Again,


What this comes down to is your patience when you are changing eyepieces:

The 55mm Plossl is quite different than any other TeleVue eyepiece in that it requires a substantial amount of focus change as compared to any other 2 inch eyepiece (like on the order of moving the focus an inch). The 67mm takes that and makes it even worse of a change in focus.

On the other side the SCT focuses by moving the primary mirror and that change gets "amplified" by the power of the secondary, so the SCT design has a huge range of focus travel. So a 10mm change of the primary gets a 50mm change in focus of a f/10 SCT where the primary is f/2 and the secondary is f/5.

So, if you use a 55mm Plossl and a set of Delos, you already know that focus change between them. Just add a bigger focus change for the 67mm to Delos. Do you want to be cycling focus back and forth by an inch or more ??? Can you use spacers to "make" the focus difference act like "parfocal" eyepieces in your C-11 ???

Best,

Ed

Thanks Ed

I spoke to someone at Televue today and he stated that I didn't need the 2 inch extender for an sct. He said there would be a lot of play if I went from the 55 to a normal eyepiece. Since I'm pretty much a night vision Observer these days, I guess I don't have much to worry about it. Looking forward in trying it out