Need eypiece advice for Celestron M2 Regal 100ED (1 Viewer)

Need eyepiece advice for Celestron M2 Regal 100ED

I'm hoping I can get some advice on selecting an eyepiece for my M2 Regal 100ED spotting scope. Basically my goal is to get the lowest magnification, widest FOV (and AFOV), largest exit pupil, highest quality fixed power eyepiece, for under $200. I'm finding out though that it seems it's more difficult to pick an eyepiece for this scope vs a traditional refractor, as I see reports of certain stipulations using certain eyepieces in spotting scopes like these.

I have been looking at a Baader Hyperion 24mm eyepiece, it seems to be my first choice. However, I would prefer to go to an even lower magnification eyepiece, if possible, but again, with this spotting scope I'm not sure what kind of focal length eyepieces I can use.

I'm really wanting a minimum of an AFOV of 55 degrees, the bigger the AFOV the better. And I'm very concerned about the optical quality, and am willing to pay the full $200 even for small increases in quality.. Just things to keep in mind while giving suggestions. Any advice would be greatly appreciated.

EDIT: Forgot to add I don't wear glasses so eye relief is a non issue.

Hi Eric,

first of all, welcome to birdforum.

I would not recommend a Baader Hyperion 24mm since this series is best used at f8 or above. In faster instruments their performance is degrading - at f5.4 in your scope it won't be so great.

The best solution for widest field in your scope if money is no issue would be a 24mm Televue Panoptic - neatly pulverizing the $200 limit, probably even used.

Thus my recommendation is the 24mm Explore Scientific 68 deg, aka the poor man's pano. It is actually cheaper than the Hyperion and is only a little bit behind the Pano optically. The crux with all the ES EPs is that they're usually a bit larger and heavier than their Televue counterpart - with the 24mm 68 it's not too bad at 330g...

Getting a head like the 500AH which allows to change the position to balance the scope is recommended anyways.

Jo

Hello Eric and another welcome to B.F.

I've been along the path you seem to be following - wanting a wide field view in a 100 mm scope. But in my case it was a Pentax PF100ED rather than a Celestron M2 Regal 100ED. I found the limitation on field is the 1.25" eyepiece barrel. To maximize the field with that constraint, it is possible with the Pentax to hold a 2" barrel diameter eyepiece - without its 2" lower barrel - over the 1.25" port. The absence of the constricting 1.25" barrel gives the maximum possible illuminated field stop for the scope. It just matches the field lens of a 70˚ AFOV 26mm SWA eyepiece, giving the maximum possible field with a relatively low cost eyepiece. Any longer FL eyepiece would give the same TFOV, but just lower magnification.
See this old thread for the solutions I found for Pentax scopes:

http://www.birdforum.net/showthread.php?t=216528

I have not worked with the Celestron 100 ED, but perhaps you can do a similar modification to give you what you desire.

I agree with jring's post that the 24 Panoptic is a nice eyepiece, but expensive. It also has the problem of many Televue eyepieces in that it is optimized for astronomical use. It has very little edge-of-field aberration (AKA "fuzziness") but LOTS of edge-of-field distortion. This pincushion distortion is inconsequential for astronomical use, but distracting for some when viewing terrestrially. Straight lines like trees and telephone poles bend inward as their images approach the edge of the field. Some are not bothered by this, but many are. I have a 24 Pan but only use it for astronomy. I have not tried the Explore Scientific 24mm 68˚ eyepiece, but their 25 mm SWA functions very much like the 26mm SWA's from other vendors like Agena. I find the slightly increased field edge aberration not too bothersome at the low spotting scope power. The 26mm SWA in my Pentax 100ED gives 24x with a 2.9˚ TFOV.

The 26 (or 25) SWA's available have more aberration but less distortion at the field edge than the Pan. They are also much cheaper, so if you use your scope for outreach, as I do, you won't be so concerned when a little kid puts his thumb on the eye lens. Ha!

Sorry I don't have specific information for your Celestron scope, but hope this may help a bit. -Bill

For visual i think larger afovs are overrated. The eye relief is tight, the quality towards the edge is getting poor if its not a expensive ep, and you need to roll your eyes and head to cover the view.

Look for another feature as flat field if ita visual. 66 degrees with a sharp flat afov is better than 82 with distorsions.

jring said:

Hi Eric,

first of all, welcome to birdforum.

I would not recommend a Baader Hyperion 24mm since this series is best used at f8 or above. In faster instruments their performance is degrading - at f5.4 in your scope it won't be so great.

The best solution for widest field in your scope if money is no issue would be a 24mm Televue Panoptic - neatly pulverizing the $200 limit, probably even used.

Thus my recommendation is the 24mm Explore Scientific 68 deg, aka the poor man's pano. It is actually cheaper than the Hyperion and is only a little bit behind the Pano optically. The crux with all the ES EPs is that they're usually a bit larger and heavier than their Televue counterpart - with the 24mm 68 it's not too bad at 330g...

Getting a head like the 500AH which allows to change the position to balance the scope is recommended anyways.

Jo

Click to expand...

Thank you for the great advice. I have been looking at the ES 68 and it looks like its the best choice. Besides it, Are there any longer focal length eyepieces, maybe 25mm 26mm, or longer, that are in the mix at this price range?

What's the difference between a ep that more suitable in a fast scope to some that are not?
I have a Pentax PF80 and a couple of BST ep's, they seem to be the same as the Opticron lenses I use.

Hi,

longer focal length EPs with an 1,25" barrel trade lower magnification for smaller afov. The Pan 24 or 24 ES 68 deg have the largest field stop possible with a 1,25" barrel.

For very fast scopes one needs increasingly complex EP designs to work well. The Pan is good to around f5 or so, faster than that you want Naglers or even Ethos...

Joachim

Thanks for that, still a bit confused though. Are you saying if you use a 24mm ES82 deg in a spotting scope. you won't get the full 82 deg FoV?

Just checked the 24mm Es is a 2in, so it only really matters when you want low mag and a wide fov.
Is that correct?

Ron

Spotting scopes are restricted by the quite small inside diameter of the eyepiece barrel. So wide fields are generally only at higher magnification.

My 123mm aperture f/5 astro refractor has a 3 inch eyepiece barrel.
So I can use a Kodak 76 degree eyepiece of 38mm focal length, giving 16x and about 4.7 degree field.
It also comfortably goes to 145x, and 210x but here a slight misalignment was noticed.

Large refractors had 4 inch or larger eyepiece barrels.

In 2inch fit I have good 32mm Erfle and 50mm and possibly 60mm Kelners.

Normally star diagonals are 1.25inch and 2inch but larger can be made quite easily with good quality flats.

Just to throw another suggestion out there, take a look at the Celestron X-cel 25 mm. I have used one with an 82 mm scope (with a slight modification) for several years. It has always impressed me with its CA correction, neutral color representation and reasonably wide field of view (I think the apparent field was 60 degrees IIRC). Plus plenty of eye relief and a large "sweet spot". Edge performance is quite good for my preferences.

Just a thought and it is relatively inexpensive at around $70 US.

Thank you everyone for your advice and suggestions. I did see the Celestron X-cel line but I assumed that, due to the lower price, it may not be optically as good as some of the other choices I had been seeing.

The Explore scientific 62 degree 26mm eyepiece has been brought to my attention, and it looks like it could be the one I'm looking for. I do see there is more talk about the 68 degree series though, not sure if that's because the 62 degree series is newer or what. Decisions decisions.

Hi,

with 2" EPs one can go wider and lower mag of course, but with spotters it's a moot point since I don't know any to take 2" EPs.

Regarding the 26mm ES 62, there is indeed not a lot to be found since they're new. BF User Gwen Cole (gcole) has reviewed one in her Pentax 65ED spotter and liked it. So if you need 20mm ER or want to save 100g of weight, you might try it. I think the 24mm ES 68 deg is the better deal since the price is not a lot more expensive and the performance is well known.

http://www.birdforum.net/showthread.php?t=343081

Joachim

I received my 24mm 68 degree ES eyepiece today. It doesn't work. Instead of a circle image the bottom of the image is cutoff. Very very disappointing. What's even more disappointing is now I'm left having no clue what eyepiece I can get that won't cause me the same issue. The factory zoom eyepiece is 24mm and there is no cutoff so I'm left wondering if maybe the large AFOV has something to do with it.

Hi,

ouch, the dreaded small prism problem... it's not the afov but the tfov or rather the size of the field stop (which governs the tfov) which is too much for the spotter.

Can you take a picture of the vignetting - one could then fit a concentric circle in there so that it barely touches the cutoff tangent and with the known field stop diameter of 27.2mm of the 24mm ES 68 make a pretty good guess of the max field stop for your scope.

I hope you can return the EP...

Joachim

jring said:

Hi,

ouch, the dreaded small prism problem... it's not the afov but the tfov or rather the size of the field stop (which governs the tfov) which is too much for the spotter.

Can you take a picture of the vignetting - one could then fit a concentric circle in there so that it barely touches the cutoff tangent and with the known field stop diameter of 27.2mm of the 24mm ES 68 make a pretty good guess of the max field stop for your scope.

I hope you can return the EP...

Joachim

Click to expand...

Hello. Thank you for all your help so far. I'm not able to take a picture as the eyepiece is already back on its way for a refund. I can say though that the cutoff was BARELY there, but just enough to be noticeable. At this point I'm thinking I may have to get the 20mm ES 68. Its field stop diameter is smaller so I'm hoping it will work.

Hi,

this will most probably work although the true field goes down by half a deg from 3.0 to 2.5 deg.

Joachim