Idle curiosity question - a reverse Barlow?

[JOC]

So I've been reading through old threads and many people collecting EP's speak of wanting an EP to work with their existing Barlow, so buy a 10mm and get the apparent magnification of a 5mm with their x2 Barlow.  Not that I'd want one, but I just wondered if there was the concept anywhere of something like a reverse Barlow, where you could buy a 5mm, fit it to a minus x2 converter and get the view of a 10mm - I've never heard of one and then I wondered well why not?  I thought it sounded a wonderful question for theoretical discussion and lots of diagrams 🤪

[johninderby]

0.5x focal reducers are common and cheap and. can be screwed onto an eyepiece.

https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-05x-1-25-focal-reducer.html

Edited July 21, 2020 by johninderby

[Peter Drew]

I think a commonly available focal reducer would provide this effect, a "positive lens rather than a " negative" one.    🙂 

[Peter Drew]

Must learn to type quicker!     🙂

[johninderby]

More than you may want to know about focal reducers. 😁

Good guide though. 👍🏻

https://agenaastro.com/articles/guides/focusers/focal-reducers-guide.html

[Louis D]

9 hours ago, johninderby said:

0.5x focal reducers are common and cheap and. can be screwed onto an eyepiece.

https://www.firstlightoptics.com/astro-essentials-eyepieces/astro-essentials-05x-1-25-focal-reducer.html

In my experience, both the cheap 1.25" and 2" focal reducers add field curvature and some edge aberrations.  They also require a fair amount of in-focus.

Edited July 21, 2020 by Louis D

[andrew s]

I don't think it's a good idea for visual work. Most reducer are either rather crude or designed for imaging.

Better to use longer fl eyepiece and Barlow than the reverse. There are good optical reasons it's not commonly done.

Regards Andrew 

[Louis D]

AFAIK, the only commonly used focal reducers for visual usage are the ones for SCTs.  But, they are more complex than just a simple focal reducer and are matched to the scope's design.  Someday, I want to put one of the Celestron types on the back of a 127 Mak just to see how usable it is since there are no designed Mak focal reducers to my knowledge.  I have a feeling it will vignette 27mm diameter field stop eyepieces at the very least.