Image of the day

From the
ATWB Customer Gallery

Alpine Wolf

My Account

New to Astromart?

Register an account...

Need Help?

Need MILD "Barlow"

Started by Thomas Dey, 12/14/2015 10:39AM
Posted 12/14/2015 10:39AM Opening Post
Hoping someone there can point me toward a source and/or provide technical advice. Here's my Moral Dilemma, Dr. Laura: I have an f/4.5 DOB system and can't quite reach focus with my favorite eyepiece. (Yeah, it's the NV Micro IIE Image Intensifier eyepiece) I think I need 6-9mm of additional "in-travel" and have considered most of the usual suspects --- move PM forward, move focuser inward, machine the "eyepiece" so it slides farther in. All three of those has significant difficulties, so I am considering a 4th alternative that I have successfully used before. To add a MILD Barlow upstream from the eyepiece. There is a perfect location for it at the bottom of the focuser tube. If you're into Optics, I come up with a two-inch diam lens (1.75 actually used) that transforms an f/4.5 feed to f/5.2-ish. My rough computations show that a thin BK7 singlet negative meniscus with a power of -1.0 Diopter would work fine. Ideally, the meniscus would be "bent" to share the ray-bending twixt the two surfaces (to minimize spherical and chromatic).

I surfed lens houses and am perplexed that they have POSITIVE lenses in that size and focal length (1000mm of course) and even meniscus .. .but no NEGATIVE lenses except for small or short focal length ones. I know, my application is somewhat unusual, so no surprise. So far I have looked at Edmund Optics, Andromeda, B&H, Sunny Precision Optics, Lens Factory, Thor Labs, Westech Optical, Anchor Optics (= Edmunds old random surplus offerings), HIS Engineering, Altechna --- and the places that supply ophthalmic Test Kit lenses. GOD, I miss the (pre-retirement) days when I was an optical engineer at B&L, Kodak, ITT etc. Walk into the production areas and lenses being made far as the eye can see… Always doing developmental work so these wizards could fab ANY lens and Corp ENCOURAGED work-like hobby projects!

OK, Ok, ok … so I either need to Find a source, Commission custom ($$$?!) or [dread] actually get my hands dirty and make it (two, actually) Myself!


29-inch Dob in a dome
36-inch upgrade soon
LUNT 80/80 solar scope
FLI 6803 cam
APM 100mm APO Binos
JMI RB-16 Night Vision Binos
Zeiss 20x60 IS binos
Posted 12/14/2015 10:21PM #1
Any way you could get this by using the collimation screws to change the mirror height?

[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue grin
Amateur Imager
Posted 12/15/2015 05:42AM #2
Use a Paracorr (or Paracorrs)?

Another idea: what focusers are installed on your RB-16? Could you change them for lower profile focusers?

I am compelled to poke: you have 16" of aperture and are using an image intensifier? Goodness, are you trying to see Kuiper belt objects?