Hyperion's

I've nearly got the full set from 3.5mm up to the 21mm. The 21mm is a bit of a funny one as it has less AFOV than the others at 60º instead of 68º so in effect it has the same TFOV as the 17mm so if you already have a 17mm Hyperion its not worth getting the 21mm. However I use mine in 2" mode with the 1.25" barrel and Smyth group lens removed. This turns the 21mm into a servicable 32mm. With my new F4.5 Lightbridge 16" its no longer a servicable EP like it was in my F4.9 Orion XT12i however. Those tuning rings I bought to the rescue though. I'll be looking at the Focal lengths the tuning rings either/or or combined give me. I should be able to change my 21mm into a focal length that might fill in a gap. 15mm or 10mm most probably.

I'll be ordering a paracorr and a much better corrected 31mm Hyperion Aspheric to take over Finder EP duty. I put my Binoview plans on hold....again Jeff (in case he was wondering..."but I thought..." )
Actually can any of the eyepiece guru's tell me which is the longest focal length I can use in a 1800mm FL F4.5 scope? Theres some formula to use to find at what focal length an EP vignettes or something isn't there.

Calibos wrote:

I'll be ordering a paracorr and a much better corrected 31mm Hyperion Aspheric to take over Finder EP duty. I put my Binoview plans on hold....again Jeff (in case he was wondering..."but I thought..." )
Actually can any of the eyepiece guru's tell me which is the longest focal length I can use in a 1800mm FL F4.5 scope? Theres some formula to use to find at what focal length an EP vignettes or something isn't there.

I have a ParaCorr for the 20" f/4 if you want to borrow it before lashing out a lot of money. You may find that you don't actually need it!

The longest FL EP you can use is decided by the maximum exit pupil your eye's iris can accommodate. In a young-ish person the iris can open to a maximum of about 7mm. This also equates to the lowest "useful" magnification for your telescope (and your eye). Any lower power means that your iris is effectively blocking or vignetting some of the light from the scope, giving you a smaller effective telescope aperture!

So, going on what we know about your scope and youthful eyes...

Your 16" scope aperture = 406mm
Your scope's focal length = 1800mm
Your max eye pupil = 7mm (hah!)

406mm / 7mm = 58x minimum useful magnification

1800mm / 58x = 31mm focal length eyepiece.

So a 31mm EP will give you your lowest useful magnification for this scope (assuming a 7mm exit pupil.)

You can monkey around with the numbers and see what you get for smaller iris/exit-pupil diameters!

Phil.

jhoare wrote:

Hi Dave,

The extension rings don't contain any glass. The design takes advantage of the Smythe lens group of the Hyperion, which acts as a focal reducer and is contained within the 1.25" barrel (5mm through 21mm only). When you put the extension ring between the Smythe lens group and the field lens in the main body it reduces the focal length further.

Cool, a pity more eyepiece don't use this technique.

PhilipLardner wrote:

So a 31mm EP will give you your lowest useful magnification for this scope (assuming a 7mm exit pupil.)
Phil.

It is no co-incidence the lowest power nagler is a 31mm, it gives a 6.5mm exit pupil on my scope.

Calibos wrote:

I've nearly got the full set from 3.5mm up to the 21mm. The 21mm is a bit of a funny one as it has less AFOV than the others at 60º instead of 68º so in effect it has the same TFOV as the 17mm so if you already have a 17mm Hyperion its not worth getting the 21mm. However I use mine in 2" mode with the 1.25" barrel and Smyth group lens removed.

I have a full set, 3.5mm through 24mm, and that's how I use the 21mm. A full set of 7 eyepieces may seem like a lot but when one has a collection of scopes with focal lengths from 400mm to 1250mm it allows some flexibility and the really short focal lengths come in useful.

PhilipLardner wrote:

I have a ParaCorr for the 20" f/4 if you want to borrow it before lashing out a lot of money. You may find that you don't actually need it!

The longest FL EP you can use is decided by the maximum exit pupil your eye's iris can accommodate. In a young-ish person the iris can open to a maximum of about 7mm. This also equates to the lowest "useful" magnification for your telescope (and your eye). Any lower power means that your iris is effectively blocking or vignetting some of the light from the scope, giving you a smaller effective telescope aperture!

So, going on what we know about your scope and youthful eyes...

Your 16" scope aperture = 406mm
Your scope's focal length = 1800mm
Your max eye pupil = 7mm (hah!)

406mm / 7mm = 58x minimum useful magnification

1800mm / 58x = 31mm focal length eyepiece.

So a 31mm EP will give you your lowest useful magnification for this scope (assuming a 7mm exit pupil.)

You can monkey around with the numbers and see what you get for smaller iris/exit-pupil diameters!

Phil.

Thanks for the offer Phil but if my short first and second light testing sessions were anything to go by I'll be ordering one for definate. The comet tails on stars radiating out from the centre of the FOV were apparent much further into the centre than my F4.9 scope where it was really only apparent around the outer 10%.

I'll still take you up on the offer of borrowing that Ronchi eyepiece though....although not so much borrow it as borrow you and it for a few minutes the next time we are at an observing session Not much use in my borrowing it but not having the first clue how to interpret it!

You'll also be pleased to note that I have decided not to build a new base for my lightbridge after all, so I won't need to take you up on your very kind offer of helping me build a totally new one but I was wondering does the offer still stand to cut me a few pieces of your marine ply offcuts to use as anti flexure bracing. 5 pieces for different parts of my lightbridge and 3 for my 12in Orion. I want to get the Orion as perfect as possible before I sell it so the new owner knows that everything that should and can be done to improve the scope has already been done. They can be satisfied that the scope is performing as well as a chinese dob can and not have to put up with any of the niggles I did. Of course these niggles are the price we pay for scopes so cheap but with decent quality mass produced mirrors but it would be nice to be able to sell the scope as a plug and play solution.

Calibos wrote:

I'll still take you up on the offer of borrowing that Ronchi eyepiece though....although not so much borrow it as borrow you and it for a few minutes the next time we are at an observing session Not much use in my borrowing it but not having the first clue how to interpret it!

Thought it might involve sending a 'technician' along with the Ronci grating! Just remind me to bring it next time we're out observing.

...but I was wondering does the offer still stand to cut me a few pieces of your marine ply offcuts to use as anti flexure bracing.

No worries - just let me have the dimensions and I'll knock them out for you.

Phil.