Eyepiece for the finderscope

[bambuko]

My RACI finderscope has an illuminated 23mm eyepiece giving it approximately  x10 magnification.

I feel I would prefer x7 magnification for my visual only astronomy, which would require 32mm eyepiece.

Can I get 32mm 1.25" illuminated eyepiece? or is this a DIY project?

and... what would be the best 32mm eyepiece for this very fast (f/3.75) little scope?

Obviously I want it as wide as possible as well.

BTW - am I correct in thinking that lower magnification would also be easier on the distortion?

[crashtestdummy]

Don't worry about the f3.75 bit as you will be using such low mag that it won't matter too much

Have you tried googling 32mm 1.25" illuminated eyepiece?

[bambuko]

Don't worry about the f3.75 bit as you will be using such low mag that it won't matter too much

yes, I thought this would be the case, just wanted confirmation - thanks

Have you tried googling 32mm 1.25" illuminated eyepiece?

yeah, I have done the obvious (ie google it up dummy ) - with no result, hence the question here.

[crashtestdummy]

I've seen a 32mm 2" but not a 1.25"

[bambuko]

I did a bit more searching and found: this, but it is 2" barrel

[crashtestdummy]

same one I found.i think you are either going to have to settle for something under 30mm or get a 2" finder

[bambuko]

or modify 1.25" 32mm eyepiece with graticule and  illumination...

need to hunt for a cheap 32mm to experiment with

[bambuko]

The closest eyepiece to what I want seems to be Antares 1.25" Illuminated Reticle Eyepiece - 27mm but it costs whopping amounts of money

It looks like it will have to be simple 32mm eyepiece if I am to get what I want (ie 7x60 finderscope)

[L8-Nite]

The closest eyepiece to what I want seems to be Antares 1.25" Illuminated Reticle Eyepiece - 27mm but it costs whopping amounts of money

It looks like it will have to be simple 32mm eyepiece if I am to get what I want (ie 7x60 finderscope)

Not alluminated, but works for me, is a Russian "Tal" reticle insert which screws into a 1.25" eyepiece like a filter does.

Sorry, but I'm not sure of a current source or dealer which supplies them.

[bambuko]

Thanks for that L8-Nite

I have TAL cross-hair (but it is the old one with no thread - just pressure fit) so could esily insert it - question is: where exactly should it go? Will have to experiment.

It should also be realtively easy? to add illumination.

[michael.h.f.wilkinson]

I got a cheap 25mm Antares 70 deg EP, and it was quite simple to add cross hairs:

http://stargazerslounge.com/topic/185558-modding-my-diy-16x70-raci-finder-to-a-14x70-raci-finder-with-diy-crosshair/

Works a treat, and I do not miss the illumination.

[bambuko]

I have received my 32mm (50 deg) eyepiece today and it has totally transformed the finderscope for visual use.

OK it hasn't got the reticule (not to mention illuminated one), but that's something I will have to experiment with later.

For now I am absolutely convinced that changing from 23mm to 32mm eyepiece was the way to go.

I guess original 23mm eyepiece is meant for people who do guiding (needing higher maginification to centre object more precisely).

For my visual needs 32mm is the best I can get for 1.25" (I guess).

[bambuko]

For anyone reading it - a word of warning.

My initial excitement with 32mm eyepiece has cooled off a bit.

Not because I didn't like low magnification and wide field, but because in actual use it turned out a bit more difficult.

It was perfect when aimed at stationary view, but when I started using it as actual finderscope I had problems with correct positioning of my eye, resulting in massive blackout blobs blocking the view, unless my head was aimed perfectly.

I believe it is known in the trade as kidney bean effect?
Generous eye relief (around 20mm), combined with short eyepiece rubber (eye guard) made centering of the eye at correct distance quite difficult...

Things are possibly not helped by exit pupil being rather large for this combination (32mm/3.75=8.5mm), although this is not a problem for a refractor without central obstruction.

Perhaps another eyepiece with better/longer eye guard to guide my eye?

Perhaps "pupil guide" accessory (as offered by Tele Vue - link)?

Perhaps bit more practice?

or... ultimately another eyepiece?

[michael.h.f.wilkinson]

What you describe is a kidney bean effect. I had a 36mm Plossl that had a similar problem (at 3.6mm exit pupil, so quite reasonable size). I would try to get some cheap 25mm Erfle design or similar. The Antares is perfectly OK at F/5 in my 70mm finder. These can still be found new from TS

[bambuko]

Thanks,

I already have 23mm eyepiece for this finderscope which works fine with no kidney bean effect at all, but this only give me x10 magnification.

I tried to achieve x7 magnification to improve the finderscope for visual only (no guiding) use, which requires 32mm eyepiece.

It seems (from googling the subject) that Plossis are particularly prone to kidney beaning? so maybe another type of eyepiece as you suggest might help indeed?

[bambuko]

Perhaps someone (more knowledgeable than I am) can help me with comparing of the two eyepieces:

On one hand I have 32mm Plossi with 50 degree AFOV, giving me x7 magnification.

It's great when I can avoid kidney bean effect and I have an idea how to extend eye guard to help with it, but...

I also have 27mm BST Flat eyepiece with 53 degree AFOV, giving me x8,3 magnification and easy viewing with great extending/un-screwing eye guard.

So if you were me, would you faff around with eye guard on 32mm Plossi or would you use 27mm BST Flat?

Obviously I want to see as much sky as possible through my finderscope, but would greater AFOV (on BST) make up for slightly higher magnification?

[bambuko]

I have entered the numbers for all three eyepieces (original 23mm illuminated, 27mm BST Flat and 32mm Plossi) into Stellarium ocular plugin and compared the view:

both 27mm (x8.3) and 32mm (x7.0) are better than the original 23mm (x10.0), but there is little difference (at least according to Stellarium) between 27mm and 32mm.

[PurpleFringe]

Nice plugin - I like Stellarium better and better.

You could try get the best of both worlds with a 24mm ~68 degree 1.25" eyepiece. Televue, ES and Baader all make something like that, though all are more expensive and bulky than a 32mm plossl. Like the 32mm plossl they should also show the maximum possible field of view from a 1.25" eyepiece.

[bambuko]

Yes, that would indeed be another option and I would definitely explore it if funds were no object

You can have 32mm Plossi for about £30-£40 (or cheaper if second-hand)

TV 24mm Panoptic is about £240-£265 

Baader Hyperion 24mm is more like £100

Definitely not my budget

And yes - I love Stellarium as well - it's such a great tool!

[bambuko]

anti kidney bean modifications to 32mm Plossl

As described previously, when I tried to use 32mm eyepiece I was disappointed with kidney bean effect.

To avoid it, my eye had to be precisely centred over the eyepiece and kept hoovering at precise distance from the eye guard.

It proved impossible (at least for me) when used in the finderscope, so I have decided to modify eyepiece by adding extension which would bring eye guard out to support my eye and add "pupil guide" à la Tele Vue accessory described earlier.

Starting with original eyepiece:

Eye guard removed:

Eyepiece with extension:

Front view, showing pupil guide in the extension:

Completed job:

Whilst the proper verdict has to wait for a better weather (as usual in Devon - it is piddling today), the initial test seem very positive.

I still need to blacken the inside and secure the extension to the eyepiece (plan to glue it), but that will wait until I am 100% sure it works OK.

[bambuko]

I have used it "in anger" now and it works great!

Kidney bean effect is totally gone and the eypiece is very comfortable to use.

Pupil guide is very effective and extending eye guard helps as well.

On top of it the mod is very easy - I thoroughly recommend it!!