Low power views in long f/l scope

Well this is a first for me. There's something vaguely intimidating about this forum format that's had me muzzled until now, but I've got an issue that I need a bit of advice on - so here goes...

I'm about to order a Tak Mewlon 250, which has a focal length of 3000mm. Naturally I've already checked that it will work with my Zeiss bv's (otherwise I wouldn't get it), and I've been told that the combo will even reach focus without the 'glaspath compensator'. With my Tak LE30's, this should give me reasonably low power views of 100x. Two questions:
a) do the Baader 'glaspath compensators' actually do anything apart from act as a barlow that would give me a problem if I didn't use them for views at this sort of power? I've always understood that they were supposed to correct aberations caused by the binoviewer, but how much of an issue would this be?
b) are there any premium 1.25" oculars out there in the 35mm+ range that don't suffer from lack of flat field, narrow AFOV, and excessive eye relief? I think I know the answer, but it's worth checking...

I haven't mentioned the option of using Baader's Alan Gee Telecompressor (focal reducer for Baader bv's) because I've already decided that I'm going to try that one when the Mewlon arrives (c/o Alpine Astro) - and it's very possible that this will resolve the 'low power issue' in one stroke. However, it never hurts to have more than one solution prepared, does it?

OK - debut over. Maybe make the message posting window a tad bigger, chaps?

Best

Martin Hopewell

Hi Martin:

The compensators such as the glaspath and the 1X corrector from Tele Vue will correct for any prism aberrations introduced by the binoviewer prisms. However, that appears to only be an issue on faster scopes. Yours is starting at F/12, and will be even greater with the extra optical path of the binoviewer. So, you should not need to use a compensator at all. Here's the reference from the Tele Vue web site. The reference to Focal Ratios is down near the bottom of the page.

Tele Vue Bino Vue

There aren't any 35mm eyepieces with wide flat fields. The apparent field of the 35mm Ultima is 49 degrees. That's the max because of the limit of the 1.25" barrel size of the eyepieces. TV says that the 24mm Panoptic is about the widest field they can do in a 1.25" eyepiece, using the same field stop size as their 32mm and 40mm Plossls (27mm). The Ultima has a 29mm field stop, accomplished by pulling the lower element up into the body of the eypeiece where it is wider than 1.25".

Clear skies,
Jim

Thanks Jim. Getting the hang of this format now!

I've just read the Televue info, and I think that it lays the glaspath compensator query to rest. As far as the long f/l ocular issue is concerned, I'd settle for an AFOV of around 50° as my Tak LE30's are 52°, and I love 'em - but I understand that the Ultimate 35 also suffers from high eye relief making it hard to hold the view (and that the 42mm is even worse).

I think that what I'm looking for probably doesn't exist (approx 52° AFOV, > 20mm eye relief, flat field) but 'if you don't ask you don't get'!

Thanks again,

Martin

Hi Martin:

If you don't wear glasses while viewing, the 35mm Ultima is pretty long on eye relief, although easier to hold the view with a binoviewer than in single eye mode. The 30mm Ultimas and 32mm Tele Vue Plossls are easier, but a little higher power.

People who wear glasses while viewing seem to like the 35mm Ultimas a lot.

Once you add the Alan Gee Mark II telecompressor, you're going to get pretty low powered views anyway. Since it takes an F/10 scope down to F/5.9, your F/12 should come down to about F/7.

Clear skies,
Jim

Hi Martin,
Have you looked at the Richey-Chretien design as an alternative to the Dall-Kirkham?Harry Sieberts latest
eyepiece designs are very well corrected for field flatness
and feature a sliding eyecup that can be locked into
position for exactly the right eyerelief whether you wear
eyeglasses or not.It appears that the Alan Gee Telecompressor is a design similar to the Star Sweeper
and Galaxy Viewer.Focus travel requirements should
probably be checked with the Tak. Did Alpine Astronomical
say that this combination of binoviewer and telescope would
reach focus with the Telecompressor.It does sound like
slick solution to low power viewing.Have fun with your new scope.I hope you dodge the new equipement weather curse.
Clear Skies,Scotty

Hi Martin,
Have you looked at the Richey-Chretien design as an alternative to the Dall-Kirkham?
It appears that the Alan Gee Telecompressor is a design similar to the Star Sweeper and Galaxy Viewer.Focus travel requirements should
probably be checked with the Tak. Did Alpine Astronomical
say that this combination of binoviewer and telescope would
reach focus with the Telecompressor.It does sound like
slick solution to low power viewing.Have fun with your new scope.I hope you dodge the new equipement weather curse.
Clear Skies,Scotty

Hello Scotty,

Must admit that I haven't checked the Richey-Chretien route. To be honest it's taken me about 3 months of solid research to get where I am with the Mewlon, and I'm not sure I could face doing it all over again! I'm also a real Tak fan - and already have lots of their gear to interchange between 'scopes with - and the Mewlon 250 has been very highly recommended.

As far as the Alan Gee Telecompressor is concerned, Bob at Alpine Astro was very helpful, but couldn't guarantee that I wouldn't have problems reaching focus. He also pointed out that the unit was designed to be used with bv's, but with SCT's in mind, and differences in field curvature from the Dall Kirkham may have some visible impact. There's also a question over exactly what reduction on focal ratio would be accomplished. The Alan Gee was apparently designed to provide f/5.9 in a standard C8 SCT but the result with the f/12 Mewlon would perhaps be in the f/7 - 9 range. Maybe Bob can add further info if people are interested (or is that not allowed this week.. ;^] ). Actually it would also be really nice to hear from Russ @ Denkmeier what he thinks the chances would be with the StarSweeper with the Mewlon and Zeiss bv's...

That's about all I can say on the whole thing, 'cos that's about all I know. I've posted a picture of the Alan Gee unit below (I hope..) and here's the link to Alpine Astro's site with all the blurb:
http://www.alpineast...accessories.htm

All the best,

Martin

Hi Martin,
As long as you can return the telecompressor if its'
performance is unsatisfactory you should be in the clear.When I last talked to Harry about how these things
worked he said there was always a tradeoff made between
how low a focal ratio could be achieved vs field distortion vs focus travel requirements.His Galaxy viewer is fairly conservative in design and has minor field correction
errors at the edge of the field.He traded a minor reduction in the true field of view for less focus travel requirements and better edge illumination of longer FL eyepieces.A 30mm eyepiece can be used without vignetting.
Of course this is not a complete replacement for the simpler optical train possible with shorter FL telescope.
Please report your experiences with the Telecompressor.
As far as the RC vs the Dall-Kirkham designs go, here are some links I found to info on the subject. http://www.rfroyce.com/cassegrains.htm http://www.rcoptical...com/10inch.html http://www.earlham.e...all-Kirkham.htm
The R.F.Royce material was particularly interesting.
Don't forget the Lunar Eclipse tonight,
Scotty

Hi Martin,

finally someone with an Alan Gee - we're dying to know how it will work, we've wondered ever since we started discussing using focal reducers with a binoviewer, so please make sure you tell us what you observe when you get to use it!

The Alan Gee has been on the market much longer than the binoview-specific focal reducer designs from last fall, yet we could never find anyone who had actually used it in combination with a binoviewer. Probably partially because it supposedly only worked with a Zeiss BV and diagonal (so I was told when I researched it last year), but the Alpine site says it works with many binoviewers - maybe it's only the Zeiss diagonal that's needed, or maybe it's the T2 ring - partially because it apparently was never specifically promoted for use with a binoviewer as far as we could tell - the Alpine Astro link you posted is the first time I see that specifically addressed outside a peripheral mention on the Baader web site in German. Must be the increased popularity

Some of the language on the Alpine Astro site actually sounds pretty similar to what we discussed before in the binoviewer group, and some of it actually doesn't sound quite right - I would guess that with the Alan Gee positioned in the baffle tube, no scope will reach focus with a binoviewer as that is the position for imaging, but maybe, who knows The link to the proper use it there, though: http://www.alpineast...bino_config.htm

As far as reaching focus goes - as with other focal reducers, it's try-and-see - as long as you have enough intravel on the primary mirror, it should work. Since it turns a f10 SCT into a f6, it seems the focal lenght of the Alan Gee is similar to the Starsweeper or a stock SCT f6.3 reducer, so reaching focus should be OK given "normal" primary mirror travel in an SCT.

Anyways, you may be the first one in this group to get to review this! Good luck!

Hinrich

'Anyways, you may be the first one in this group to get to review this! Good luck!'

Hinrich: Well I'm not sure that I'm worthy of the honour - but I'll give it a go. My main area of astronomical expertise seems to be shopping...

"As long as you can return the telecompressor if its'
performance is unsatisfactory you should be in the clear"

Scotty: Yes - Bob was kind enough to offer that safety net.

Thanks for the cassegrain info links - it looks like I'll have plenty of time to read them as it's 3:40am here in the UK, and the eclipse is currently taking place behind 5000' of cloud...

Best

Martin

Hi Martin,
I know what you mean.I don't know how many metor showers
I've missed due to overcast conditions,certainly all of the
display of a century or lifetime ones so far.At least I can see this lunar display.Clear Skies, Scotty