Louis D

My 15x70s are excellent for large open clusters such as the Pleiades, Hyades, and Collinder 70.  They're also handy for picking out comets near the horizon.  In short, they're great for larger objects.

Since you're operating at ~f/6, you'll need eyepieces that do well in moderately fast telescopes.  If your budget is tight, I would start with the 12mm, 8mm, and 5mm BST Starguiders.  If you only have a 1.25" diagonal, get a 32mm Plossl for lowest power viewing.  If you have a 2" diagonal, I'd recommend the 35mm Aero ED SWA for widest field viewing.  You might want to add something around 17mm to 22mm in the future, but I tend to skip past that power in scopes near this focal length (900mm).

If violet fringing bugs you, you might want to look into a minus violet filter of some sort.  The Baader Contrast Booster seems to be well regarded in this capacity.  Here's a comparison of the effect various filters have on the view through an achromat:

10 hours ago, John88 said:

Didn't know if there was any difference build quality wise between the Celestron and the opticron. 

Probably none if both were recently made in China.  I have Chinese made Galileo 15x70s I picked up over a decade ago that look identical.  I love the wide field and long eye relief of the eyepieces on them.  These binos are heavy and long, so I grab them by the objective ends and wedge the other end into my deep eye sockets while sitting to observe.

They were also great for watching the Balloon Fiesta launch field in Albuquerque a couple of years ago from a nearby field over a mile away.  I just supported them on the top of the steering wheel while I stayed warm in my car awaiting the morning warm-up.  The view was very similar to this telephoto image I shot:

I have some eyepieces with out top caps.  I just keep disks of closed cell foam from eyepiece bolt cases over them in the Pelican-style case when stored to avoid the possibility of the case foam from disintegrating onto the eye lens or just plain sticking to it.

Keep in mind that budget wide fields are not known for their excellence when viewing planets, which is what you're after.

On 26/08/2020 at 20:19, Pryce said:

Won't that cause too much vibration to be sustainable?

Quite the opposite for many types of rubber.  It will actually act like a shock absorber and dampen vibrations faster than if the feet were directly on a rigid surface.  The best dampening material is probably still Sorbothane from the 80s.  I use pads of it under each foot of my tripod to dampen vibrations from 3 seconds to 1/2 second.

2 hours ago, Jm1973 said:

So do you think upgrading the tripod to the 1.75" stainless steel, version, which will take 30kg, will not help that much? Is it the mount itself that is the weak link?

Generally, yes.  Mounts that are overloaded tend to flex a lot in the breeze and take forever to settle down whenever they're touched in the slightest.  That tripod won't be too bad if you don't extend the legs, hang a weight under it in the center, and put anti-vibration pads under each foot.  Don't over-tighten the leg locks.  On many of these, the bands are made of plastic and will rupture under the strain.  I've also heard of people disassembling the legs so they can be filled with packed sand to make them more resistant to vibration.

You might check in on Cloudy Nights as well.  There are far more Americans on there than here who might be able to help you out.

If you can spring for a 150mm APO triplet with FPL-53 glass on a very heavy duty EQ mount, it would be very nearly ideal for both imaging and viewing.  However, that approach can be cost prohibitive (well over $10,000).  Most folks go with a largish reflector of some sort on an alt-az mount of some sort (under $1500 total) for visual work and an 80mm to 100mm ED or APO refractor for imaging on a heavy duty EQ mount (under $2000 total).

Most likely Universal thread M42 mount.  Sigma did not make any T-mount lenses to my knowledge during that era (mid-70s by the look of it).  They were all dedicated to a particular mount.  Get an EOS to M42 adapter.  There's plenty of film to flange distance for it to work.  I've got several M42 lenses that I've used on my EOS cameras to good effect.

5 hours ago, alex_stars said:

Now regarding the suggested 7.2-21.5 Zoom, I seem to find a confusing variation of specs on different sellers homepages:

• Skywatcher Hyperflex 7E1: 60-40 FOV, 7 lenses - 4 groups, weight 190 g
• Auriga version: 60-40 FOV, 7 lenses - 4 groups
• Orion version: 60-40 FOV, 7lenses - 4 groups, weight 170 g
• Omegon "premium" version: 53-40 FOV, 7 lenses - 4 groups
• TS-Optics "premium" version: 53-40 FOV, 7 lenses - 4 groups, weight 170 g
• Lunt Solar Systems version: 53-40 FOV, 7 lenses - 4 groups

My version from Surplus Shed has between 9mm and 11mm of measured, usable eye relief, a measured AFOV of 33 to 49 degrees, and weighs 180g.

4 hours ago, Nair al Saif said:

Ok, and the exit pupil is generous in the 8mm as well, I think on perfect nights I might Barlow it to get 340x magnification.. What about low magnification? Should I get the 18mm, I read somewhere that a 1mm exit pupil and an 3mm exit pupil ep is a must have?

Did you mean does the 8mm have generous eye relief?  If so, it has about 12mm of usable eye relief which is very comfortable when not wearing eyeglasses.

The 18mm is a decent performer compared to simpler designs such as Konigs, Erfles, Kellners, and low cost SWAs.  It isn't as good as the 5mm and 8mm BSTs, however; so temper your expectations.

Both lines reportedly work well at ~f/6.  I can personally vouch for the 5mm and 8mm BST Starguiders.  They work very well at that f-ratio across the field.  Are they as good as my Delos, XWs, and Morpheus, not quite; but they're not far behind.  The biggest difference is the quality of the lens polish and stray light suppression which slightly reduces their contrast and ultimate resolving power.  However, for ~$60, they are hard to beat.

When star testing, astigmatism of the eyepiece will appear as a tangential line on one side of best focus and a radial line on the other side.  At best focus, it will look like an enlarged star with the possibility of small spikes in both directions.

You may also be dealing with field curvature.  Focus on a star in the center and then move it to the edge.  Does the tightness of the star image improve by refocusing?  If so, you've got field curvature in either or both the telescope and eyepiece.

Without a coma corrector, you could also be seeing coma from the mirror near the edge, though it isn't commonly seen in the narrow field of view eyepieces commonly packaged with telescopes.  Generally, the eyepieces have enough aberrations of their own to overwhelm any mirror coma.

What eyepieces came packaged with the scope?  Are you wearing your new eyeglasses at the scope?  To eliminate it being eye astigmatism, rotate your whole head around the eyepiece to see if the aberrations rotate with your eye.  If they don't, they're in the eyepiece.  If they do rotate, they're in your eye.

The 11mm DeLite would satisfy most of your requirements except for price.  It's reportedly just a slight step down contrast wise from dedicated planetary eyepieces like the ZAOs and Pentax SMCs.

53 minutes ago, Don Pensack said:

I found it just simpler to use them as 1.25" eyepieces and reduce the necessary range of the focuser.

IIRC, in 1.25" mode, the 12mm NT4 required about 0.25" of IN focus relative to the shoulder, which was a pain in it's own right.

55 minutes ago, Don Pensack said:

Of course, using a Paracorr, they are all parfocal anyway.

That's the main reason I parfocalized it the way I did.  When I use the GSO CC, everything needs to focus within about 5mm of the shoulder with the spacing I have to get 95%+ coma correction.  The 12mm NT4 was so far off that uncorrected coma was plainly obvious without having to go searching for it.  With parfocalization, coma is no longer noticeable.  With the correction level I'm at with the GSO CC, eyepiece field curvature and residual edge astigmatism once again dominate the view, so I have never felt the need to get a Paracorr.

23 minutes ago, Don Pensack said:

6mm and 8mm Ethos (used as 2" eyepieces with barrel extenders added): 0.7" OUT from the focal plane.

So pretty similar to the 12mm NT4 at 0.78" OUT.  I had to add five 4mm thick O rings and a 20mm 2" extension ring to the 2" skirt to make it roughly parfocal with my other eyepieces that focus near the shoulder (reference surface).  It was practically unusable otherwise because it required so many turns of the focuser to reach focus.

Try both metric and non-metric hex keys.  I usually have to try both sets to find one that fits best.

The Celeston Luminos 10mm might also be worth a look as well.

Get a lower priced pair to start with to see if you even like using them.  Some folks just don't get on well with binoviewers no matter the type or brand.

Stacking filters can lead to reflections bouncing back and forth between them causing ghost images of bright objects.

Glad it all worked out for you.

And, the smaller binoviewers have a shorter optical path due to using smaller prisms.  Thus, there will be less vignetting if you shorten your truss poles to reach focus.

If you go the route of the barlow to reach focus, I use the optical nosepiece from a 90s vintage Meade 140 2x barlow to good effect.  It works out to a 3x boost.  At that focal ratio, the 23mm Vite/Svbony aspheric 62 degree eyepieces work very well in them for maximum field viewing.

On 25/07/2020 at 17:14, Franklin said:

I think the narrowband O-III and others might be lost on it.

It's really exit pupil dependent, but even at smaller exit pupils, it's quite helpful on many emission nebula.  Just start with large exit pupils and move your way up in power to see the effectiveness or lack thereof at smaller exit pupils.

For galaxies and comets, there's no substitute for dark skies.

Globular clusters need lots of magnification to resolve them.

Many planetary nebula do fairly well at high power without filtration.

Open clusters are of such high contrast that they are fine without filtration as well.

The human eye is pretty tolerant of edge vignetting.  Even 50% is difficult to detect while looking in the center of a wide field eyepiece.  Complete vignetting generally happens when the clear aperture of the binoviewer is significantly less than the field stop of the eyepieces because they're optically very close together.

2 hours ago, BS269 said:

Is there a way of calculating how much vignetting there will be?

Try this calculator.