Binoviewers

[redhillastro]

Just purchased a Tak Mewlon 250 that will be used primarily for visual work. I have read that binoviewers work really well but the budget won't stretch to the really expensive ones. Can anyone recommend something that will work well and won't degrade the optical train if used with good eyepieces (Tak or similar)?

[Kunama]

Michael, the main difference in binoviewers is the clear aperture which "restricts" the eyepieces that can be used without vignetting. The actual image quality of the cheaper binoviewers is not noticeably different to the top tier stuff.
Mechanically the build quality of the Denks, Baaders and Televues is better than the William Optics etc. but the images are very good in any of them.

The Denk, Baader and Televue (especially the Baader MkV) have larger prism and clear apertures and suit the eyepieces with larger field stops.

With the Mewlon M250 you have a focal length of 3000mm and you may need to add a Glass Path Compensator to reach focus so you will likely be using eyepieces in the 20mm to 32mm focal lengths most. The Mewlon needs to be allowed to cool properly to give its best.

EDIT: The Mewlon should have 210mm back focus so should be fine without a GPC if the rest of the components are kept 'short'.

The Tak Abbe 32 or the Tak LE30 & LE24 would be an excellent starter pairs giving a magnification of 100X-125X, other nice ones (and cheap) are the TV plossls in 25 or 20mm. I use the Nagler T6 in 13mm and find them excellent.

The main thing to remember is that you need to keep the light path as short as possible, I use a Baader Zeiss T2 Prism diagonal coupled to the Binovue using the Baader Quick Connect system. There will not be room for a 2" diagonal due their long light path.

[dannat]

i think with a scope like the mewlon you are not ever going to get wide with a binoviewer -purely buy them for lunar/planetary, you won't need the larger prisms.
id be grabbing the binoviewer from matt at telescopes-astronomy in SA -they are an inexpensive but solid unit. as mat suggests you should be able to reach focus, keep in mind the moving of the mirror shifts the focal length of the system a bit longer longer, with a binoviewer in place youll prob be operating at something like 3200mm

[redhillastro]

Thanks to both for your help. I have a Tak 1.25" diagonal so that should work well with a binoviewer, maybe with a couple of 20mm TV Plossls as a start? I also have a problem with "floaties" in my visual field, so I am hoping that a binoviewer will help with that as well.

[dannat]

the 20's should be a good start for planets -I always find a bit lower mag in binoviewer on planets compared with mono view, but I reckon youll ant some 25 or 32's also

[bratislav]

If they are collimated. Which cheaper units often aren't (and some more expensive ones too). But big difference is when you need to collimate - my Denk II is very easy to do, and holds it well afterwards more or less forever. WO (and clones) are nearly impossible to collimate.

Disclaimer - I often use very short focal eyepieces in bino which makes colimation hypercritical. Also, simple sidescrew eyepiece lock (as on cheaper units) will throw off the collimation again. Proper collet type holder is a must IMHO in a binoviewer.

[Kunama]

I have had two pairs of William Optics as well as Denk II, Baader Maxbright and my current TV Binovue. Only time collimating was required was recently when the Binovue finally succumbed to a knock it had taken sometime in the last 9 years....

[bratislav]

What is the shortest eyepiece you are using with a binoviewer? My WO was reasonably collimated (but still not perfectly, around 10 arcmin which is bearable but not good enough in my book). But what made it impossible to use with anything shorter than about 20mm is that set screws would push the eyepiece sideways enough to cause headache (literally!).

BTW, the quickest way to check collimation is to handhold the binoviewer and point it towards the Moon or a bright distant light and look from telescope side (that is eyepiece holes towards sky), naked eye. If you see one Moon/Jupiter or light you are fine. If you see double image (I bet you will with most units) Moon serves as an excellent gauge on how much beams diverge. Moon = 30 arc min, so 1/3 of a moon displacement == sides are off by 10 arcmin. If you use a small finder to magnify you can measure misalignment in arc seconds! I aim for 3 arc minutes or less.
But again all this works only if eyepieces are kept centered (which setscrews can't do).

[redhillastro]

Thanks everyone! I will be sure to check collimation in whatever binoviewer I get. Currently thinking the Baader Maxbright might be a good compromise that will perform well with the Mewlon while not blowing the budget too much initially.

[bratislav]

On a budget, you can't really get anything better (for the money) than this

http://www.teleskop-express.de/shop/product_info.php/info/p1855_TS-Optics-Astro-Binoviewer-for-telescopes--with-compression-ring.html

This would be my budget choice.
Good size prism, compression ring for eyepieces, dedicated glasspath correctors available, short optical length at a price that can't be beat.

[Kunama]

That would be my choice as well.
The Baader Maxbright has been discontinued with a new version due sometime this year.:shrug:

[thebonz]

The baader maxibright with baader t2 prism/diagonal works in the M250 without the glasspath compensator. This is a great combination that is the shortest light path and keeps the native focal length. It is also a very strong combination. The beauty of the Baader T2 system is that it can be used on larger prism binos.
The maxibright will vignette any eyepieces longer than 26mm.
If you have the corrected mewlon, the glass path compensator is required as this has a lot less backfocus.
Cheers John

[Hoges]

If you do have any collimation issues, check the eyepieces - I recently bought a pair of 21mm and 10mm Hyperions and the found the 10mm's are a little 'wonky' - one in particular. I had to collimate by rotating the eyepiece untill I found the best spot and then put a small reference mark on the eyepiece. The other was better, but still not quite 'true'. I then checked the 21mm more closely and found that with a really critical eye, they weren't spot on either. Not something I'd ever previously had a problem with but just goes to show not all eyepieces are perfectly lined up I guess. When collimating, I tend to center something obvious. like a shed or distant tree and back off from the eyepieces 6 inches or so till I can just see the object in the exit pupil - this shows up any mis-collimation pretty quick. If you can't get 'em perfect, the object can be slightly closer to each other with respect to the center of the binoviewer (Horizontal convergence IIRC) but they can't be further away from each other or mis-aligned vertically (Horizontal/Vertical divergence) - that will give you headache real quick.