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Fo_Cuss

Eyepiece Type for 93/1000 refractor?

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The scope spec is firming up to be d93/f1000.
I'll either use flocking material or create a trillion indentations in the surfaces (with peening machine), and use matt barbeque paint.

A laser calibrator is on the aquisition list.
Consequently, everything should ultimately be at 'best possible'.

I will likely need some guidance on the doublet air gap.
Other than that ... with trembling anticipation...

May I ask for views on suitable eyepieces?

I've read a few of the threads on this forum.
It seems that many eyepieces fall short of expectations.
Also, Ive noted that people have talked about the effect of long focal lengths on specific designs.

Either way, I have an initial list, made up from what seems available at reasonable cost.

I intend to carry out both terrestrial, and extra-terrestrial viewing.
Perhaps these different fields require different eyepieces?

***

TMB/Burgess Planetary II

I've read the 'touchy subject' thread.
I understand the issues, but I'm not in any position to take sides.

The design seems to offer a 'free lunch', with remarkable 58 deg viewing.
However, I've read reviews on this forum, talking of ghosting, and scattered light.

My intention is to blacken the scope internals as much as is possible - perhaps this will be enough?

Kson Super Abbe

4 lenses - apparently popular for over a hundred years.

Celestron 1.25" Omni Plossl

4 lenses - apparently popular for a longer period than the Abbe.

In the same Omni range, Celestron offer a 2x Barlow (2 lens).

***

They all purport to be good lenses.
The Celestron range is cheaper, but hey, they might be just fine.

Are any of them good for both day and night viewing, or should I be looking for specific day and night eyepieces?
... and which eyepieces are better suited to a 93/1000 refractor scope?

:icon_scratch:

 

Edited by Fo_Cuss

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Fo_Cuss,

Don't over think these things.....

At f11 any eyepiece will work, and work pretty well.

You probably have to consider the exit pupil and eye relief to suit your needs.

 

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13 minutes ago, Merlin66 said:

Fo_Cuss,

Don't over think these things.....

At f11 any eyepiece will work, and work pretty well.

You probably have to consider the exit pupil and eye relief to suit your needs.

Thanks for those calming words :)
From what you say ... 'going with the long focal length' was the right move

RE the exit pupil and eye relief (is this AKA back focus?)

When testing the scope in its original format (with kellner lenses)...
I found that digital viewing produced the same image that my eye saw, only that it was a far more relaxing experience.

Further; as light fell, the camera software auto adjusted, and produced viewable images that by eye, could not be seen.

The only problem was that the camera lens had to be too close to the exit pupil, slightly cropping the circle.

For this reason, I was less concerned about the small exit pupils of the Abbe and Possl.
However, I do need to research the back focus distance.

If all eyepieces will work fine ... I can select one with a suitable eye relief distance :)

 

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Fo_Cuss, 

The eye relief is the distance from the eyepiece to where the eye should be positioned to accept all the incoming light. This is where the size of the exit pupil should be measured.

Pointed a camera into an eyepiece is always problematic, getting the best position and alignment can be difficult, that's why most imagers work with a lensless camera at the telescope focus without eyepieces.

 

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Merlin66,

Ah!
That explains the SLR adapter contraptions that I fell across, during research.
It is something that I will look into.

My own method was to fabricate an eyepiece diameter tube, permanently fixed to a smart phone spare back; aligned centrally.

I had trialled a 'bought' bracket, but it was useless.
My design was perfect for the job.

It allowed instant changes of the eyepiece - simply push in and mount.
Perfect alignment every time.

Not the most aesthetically pleasing design.
Rather; 'function over form' :)

Great for when with one's son, as both get to see what is going on.
... and of course, images can be recorded by simply clapping one's hands (no vibrations).

My guess is that the moving mirror in an SLR might not be so forgiving.

No doubt this is why there are simple chip based systems.

I saw one, but didn't pursue it.
I need to find it again, and look deeper.

Edited by Fo_Cuss

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I'm not a fan of Omni Plossl's, but find the TMB Planetary II to be quite a nice eyepiece. The original Burgess TMB Planetaries were the best if you can find any. The KSON Super Abbe Orthoscopics, though cheap, are excellent in scopes with long F ratio's. They are not so good at F11 unless you use a barlow. They also make great binoviewer pairs in the longer focal lengths.

Personally I'd keep my eye out for second hand Japanese eyepieces such as the Vixen LV, Ultima 5 element super plossl's or volcanoe top orthoscopics. They are relatively cheap and often unappreciated, but they are top notch optically.

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6 hours ago, mikeDnight said:

I'm not a fan of Omni Plossl's, but find the TMB Planetary II to be quite a nice eyepiece. The original Burgess TMB Planetaries were the best if you can find any. The KSON Super Abbe Orthoscopics, though cheap, are excellent in scopes with long F ratio's. They are not so good at F11 unless you use a barlow. They also make great binoviewer pairs in the longer focal lengths.

Personally I'd keep my eye out for second hand Japanese eyepieces such as the Vixen LV, Ultima 5 element super plossl's or volcanoe top orthoscopics. They are relatively cheap and often unappreciated, but they are top notch optically.

Thanks for that input Mike :)
BTW I like the sig "Many look, few observe".
To that, I think that I would add "and reflect".

RE the long F ratios

I had thought that F11 was towards the long side, but when I calculate 700/60 I see that = F11.6
So by choosing a larger lens (93mm) ... even though the scope will be extended an additional 300mm, the focal ratio actually drops.

Are you aware of the fundamentals (concerning effect on the eyepiece), or was your statement based upon experience?
I'm guessing that the light angle (from horizontal) is too large for the lens design.
Presumably, a longer focal length would bring the light angle into the spec of the KSON Super Abbe Orthoscopics?

The 'blurb' never discusses such matters.
(In every field, the blurb never does)

... it leaves the new entrant with lots of basic questions; like:

My starter scope has a 60mm lens, rated at f700mm.
Yet 700mm measurement is only achieved with the focus tube fully extended.

Is that because it was primarily designed for use with a 45 deg mirror?

Is the focal length derived from perfectly parallel light?

By that, I mean:
An object at 10km is not producing parallel light.
Neither is the moon.
The sun is said to produce parallel light
... but clearly, distant stars more so.

What isn't definitively clear to me, is the focal length at each of the 4 instances for any given lens.

The lens has an ability to bend light.
Logically therefore, for closer objects (the light spreading outwards), the point of focus must be a greater distance than with parallel light.

I saw this, when testing the scope on an object at 10km.
The kellner type lens had to be extended outwards from the housing (focus wound out to the max).
Yet with the 45 deg mirror in place, the focus tube had to be wound in.

It is obviously simple maths, but you know ... it's only simple when you have grasped the fundamentals 🤪

Why bother with the fundamentals?

Well if you have seen my DIY thread...
You'll know that I am modding the starter scope, and extending it by 300mm.

However; maybe I should extend it 330mm
... but maybe that would be good for inverted viewing, but not with the 45 deg mirror.

... and what about fitting a full horizontal & vertical flipper unit
... how might that effect the extension length choice?

These minefields are prepared for us to walk on ... and usually it goes BOOM (without foreknowledge) ⚠️

 

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I don't get too bogged down with the technicalities of eyepiece design but leave that to the experts. My comments are based entirely on personal experience. Eyepieces like the Abbe Orthoscopics were designed for use in telescopes having a long focal length, but today most telescopes are relatively short and so the Abbe Ortho design can struggle to give good correction at the edge of the field of view. More modern designs are much better at correcting the edge of the field, but this means more elements and greater expense. F11 is considered long by comparison to many of the refractors on offer today, but its short compared to the F15, F18 instruments and longer that were the general rule half a century ago. Some modern wide angle eyepieces can cost a small fortune, but for general use at F11 you could find some very nice performers in the Sky watcher range at a fraction of the price of many top brands. 

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Thanks again Mike.
Yes, I see the logic.
Those eyepiece designs were created in a distant era.

All this information helps fill the gaps :)
 

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55 minutes ago, Fo_Cuss said:

....Those eyepiece designs were created in a distant era.


 

So was the design of a long focal length achromat refractor like your 93mm f/10.75.

Well executed plossls and orthoscopics will serve very well in such a scope :icon_biggrin:

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38 minutes ago, John said:

So was the design of a long focal length achromat refractor like your 93mm f/10.75.
Well executed plossls and orthoscopics will serve very well in such a scope .

Thanks John ... yes, that is true.

I think that the point being made by Mike, was that the scopes of that era were much longer than of today.

Having said that, I note your advice that my scope length would fall into the usable category for these type of eyepieces :)
 

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I've used good quality plossls and orthos in my F/5.3 12 inch dobsonian and they have worked very well. Your F/10.75 refractor will pose no challenges to such eyepieces :icon_biggrin:

 

 

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