Louis D

Since the rubber grip on the focuser shaft of my used 127 Mak split and immediately fell off of it anyway (this was not disclosed by the seller, grrrr), I wrapped it in racquet grip tape and then shoved a cut down bicycle foam grip over that.  Both were sourced from ebay for $1 each.  With so much foam between my finger and the shaft, I don't get much shake during focusing.  There is still a gap next to the 2" visual back as seen in the photos below.  I also keep Sorbothane pads under each tripod foot to cut settling time from 3 seconds to 1/2 second.

The Barlow will likely cleanup the edges of the 16mm Nirvana.

The Barlow will likely require refocusing unless you get a telecentric magnifier like a TV Powermate.  There may also be vignetting, but you won't know until you try them together.  A telecentric magnifier will not vignette.

A 2x Barlow may not really be 2x, and certainly won't be for all eyepieces.  Even those that are bang-on 2x when the eyepiece focal plane is right at the reference plane (eyepiece shoulder) will be some other power if the eyepiece focal plane is above or below the reference plane.  Thus, it's not a foregone conclusion that you will get exactly an 8mm equivalent with the combination.

Swapping in a Barlow and refocusing is a pain in the dark.  I prefer to leave the Barlow in all night if I am using one and work at elevated powers across all eyepieces.

A 40mm Plossl in a 1.25" barrel yields a fairly narrow apparent field of view with very long eye relief but nets you a slightly larger exit pupil in the process.

Does the image alignment vary as the diopter adjustment is spun on either eyepiece holder relative to the other?  My Arcturus BV is only usable with both adjusters screwed all the way down.

I'm not the familiar with the WO 20mm eyepieces, but if they have undercuts, there's a strong possibility that they will tilt during tightening of the holder collet.

Upon looking at the WO BV, it appears to have a single thumbscrew that pushes against a compression ring rather than using a proper collet to maintain centering.  This could be pushing the eyepiece to the side, and if the thumbscrews on both holders are not in the same radial position, they could be pushing the eyepieces off center in different directions.

When I was regularly using my 15" Dob, I would quite often use 350x thanks to our often steady Texas skies.  At an exit pupil of 1mm, it was quite usable.

For your 12", that would equate to 300x.  Thus, I would say 300x for sure.  430x?  Probably not very commonly considering the 0.7mm exit pupil which is right at what I consider the usable lower limit for exit pupil size.  If your skies are steady, though, it should be doable.  I have a 3.5mm Pentax XW that I rarely use in my f/6 scopes to produce a similar exit pupil.  Such an eyepiece would probably be my last eyepiece purchase while building a set because it gets used the least, at least by me.  I only have it because I snagged mine for $212 off of Amazon a few years back during a flash sale.  I couldn't resist. 😁

Unless it's something you're passionate about, you'll probably regret splashing the cash on such a specialty eyepiece.  I rarely find that I can see any more detail at a 0.6/0.7mm exit pupil than at a 1mm exit pupil.

I'd splash for a binoviewer if you want to tease out detail at high powers.  I always find two eyes are better than one for such purposes.

1 hour ago, Adam J said:

Don't see how that would help, can you show me what you mean? 

There's no central obstruction when using a parabolic mirror off axis via a circular mask.  Here's a discussion of the concept on CN.  Here's a technical description of the optics.

There are still aberrations as discussed in the latter treatise.

Here's how to get a 14.5" unobstructed telescope using a 36" primary mirror:

DGM Optics used to make and sell the OA-4.0 Off Axis Newtonian, among other sizes, using off axis circular cores from larger mirrors, IIRC.

1 hour ago, Adam J said:

In theory the mirror could be tested as perfect. In reality the in use value is lower due to diffraction from the central obstruction and so it would never reach 0.98 in use. 

What about with an off axis aperture mask between the secondary spider vanes?

4 hours ago, Zermelo said:

One of the big advantages of the Mak's long focal length is precisely that you don't need to use (expensive if they're good) short eyepieces to get decent magnifications.

But on the flipside, it's long focal ratio makes getting to large exit pupils for narrowband nebula filter usage difficult.  If you want a 6mm exit pupil at f/12, you need a 6*12=72mm eyepiece!  Good luck finding a 2", let alone 1.25", example of one.  Surplus Shed, Russell Optics and Siebert Optics (in 2.5" to 4.3" barrels) all sell some home-brew examples on this side of the pond, but that's about it to my knowledge.

1 hour ago, Stu1smartcookie said:

Really informative ( not just for the OP ) Vlaiv . I like the look of the ES 82 degree EP's .. and i totally get the max power of an 8mm EP ... giving 187.5 x mag . on a 127 mak . There will be times when the magnification can be pushed of course and theoretically this scope can take in excess of 200 x  , but as we know , UK skies are very unpredictable . 

It's not just seeing.  The exit pupil becomes so small at very high powers that floaters in your eye's vitreous humor start to noticeably block the tiny pencil beam of light before it gets to your retina, just like clouds in the sky.  I flick my eye to the side to get a few fractions of a second better view before they drift back to the center and block the view again.

I would put in some downward-firing solar powered LED landscaping lights along whatever is supposed to be lit up by the floodlight.  They are much less blinding due to their low intensity and remain on all night.

My neighbor has some, and they're pretty innocuous for astronomy.

5 hours ago, oymd said:

Always wondered who makes their scopes?

I believe Long Perng.

3 hours ago, Cosmic Geoff said:

There does not seem to be enough focal depth to fit any sort of diagonal.

Exactly.  I don't know why they don't offer a visual version with a shorter main tube to accommodate a diagonal.  They could use a longer focuser drawtube to support straight through usage as well.

2 hours ago, Second Time Around said:

Ease of reacquiring an object is yet another advantage of zoom eye pieces.

I almost pointed that out as one serious advantage of zooms at high power in my post above.  It's one of the main reasons I often use a pair of 8-24mm zooms in my BV.  Operating at 3x with the Meade 140 Barlow nosepiece, I'm getting a 2.7-8mm zoom range without changing eyepieces.  This is very useful for high power BV observing.

It's also why I like photographing wildlife with a long telephoto zoom on my DSLR.  I can acquire the target at low power and then zoom in as needed to properly frame the subject without losing it while swapping lenses since I can't always use a tripod.  High resolution sensors are also nice because I can crop in post while still having plenty of resolution for printing or web usage.

It's simply not possible for me to swap two eyepieces in a BV on an undriven scope at high power without losing the object, so a pair of zooms it is.  Thanks for bringing up zooms in this context.

For me, it's a pain trying to reacquire an object in narrow-field eyepieces at high power using undriven mounts during eyepiece swaps.  Even with widefields, I have to position the object at the very edge or just off the edge of field, swap eyepieces, and then quickly find the object again before it drifts out of field.  This is made especially difficult if the alt-az mount swings upward slightly while swapping out heavy eyepieces or if the eyepiece snags in the focuser taking the scope upward with it.

That aside, I much prefer my 9mm Morpheus to my 9mm Vixen LV despite the views and eye relief being very similar.  The view is more immersive, less claustrophobic, and offers better context in the Morpheus.

I have a 60mm finderscope made from the front end of a 60mm pair of binos.  The rear end is a machined Delrin adapter for 1.25" diagonals.  I think it was made by Russell Optics in the 90s.  It's mounted in 50mm finderscope rings toward the back end.  I use a Celestron 90 degree erect image prism diagonal focused by slipping it forward and back.  It pairs quite nicely with a 127 Mak to bring in the widefield views impossible with the Mak.  It's a bit heavy, so it's  CG has to be mounted inline with the altitude axis pivot point to prevent the alt-az mount from flopping about at differing altitudes.  I use a 24mm APM UFF in it to cope with the ~f/4 speed of the objective.

I also modified a 70x300 cheapie telescope from ebay to take a 2" diagonal and eyepieces to really max out the possible widefield.  So far, it doesn't work as well as the 60mm bino conversion, so it's a WIP.  It needs flocked and the PVC bits need to be centered better.  So far, I've got less than $30 in it, so I'm not complaining.  It would be different if I didn't have so many spare parts from previous projects laying about.

4 hours ago, Xgaze said:

I think it's a little unfair and surprising that people still think stuff made in China is poor quality, they have been producing excellent products for many years which are great value.

Based on reviews from that company you despise, watch out for the zipper breaking.  China's use of pot metal instead of steel is pervasive and troublesome.  One of the screws in my GSO coma corrector sheared off in my fingertips.  I can't get the part stuck in the hole out, so I replaced the other screw with a cap head steel screw and now make do tightening only one screw.  Also, every alignment screw on my 2000-era ST80's 6x30 finder's two rings sheared off the same way making it useless.  A well known Chinese alt-az mount also uses molded pot metal instead of machined steel, leading to the possibility of an entire side simply shearing off under load.  It is cost cutting measure such as these that have led to the poor reputation of Chinese made goods, not some shadowy conspiracy trying to besmirch their good name.

13 hours ago, Concordia000 said:

That's quite a "small" refractor, being a grown person's height...

I guess different folks have different ideas of what qualifies as small.

Of topic a bit, I also never realized how big "small" fighter jets had gotten until I toured the production line of McDonnell Douglas in the 80s and realized how huge the F-15 really was while looking up at it.  It's about the same size as the B-24 from WWII and can carry almost 5 times as much payload at over twice the cruising speed and range!  That's crazy for a "fighter" plane.  It's no wonder they created the F-15E Strike Eagle version based on these specs.

Compared to a modern 24" Dob (bomber), I guess a fast 6" APO (fighter) could be considered "small". 🤔  It's all relative.

As long as you don't need to wear eyeglasses at the eyepiece, the TV Pan 27mm is a fine eyepieces.  When I used my 15" f/5 Dob regularly in the past, it was my main observing eyepiece providing 71x which was just about perfect for most OCs and nebula.  However, I did scratch up an eyeglass lens on the eye lens's retaining ring trying to push in to see the entire FOV once.  I never did that again. 🙄

I've since replaced it in my A-team eyepiece case with the APM UFF 30mm.  It now resides in the B-team case with the 12mm and 17mm NT4s that were replaced with the 12mm and 17mm ES-92s.  I'll probably pass the B-team along to my grown daughter as "starter" eyepieces for her newfound interest in astronomy. 😄

The 10" and 12" Dobs you mention are quite fast at f/5, and even the mighty Leica ASPH will suffer at the edges from astigmatism.  If you put your Dob on an EQ platform or get a goto version, this may not matter all that much to you.

The Tele Vue Ethos line quickly became very popular among the fast Dob crowd because they are very well corrected even at low f-ratios and have a very wide field allowing for long dwell times on objects in nontracking scopes.  In other words, as the object drifts from edge to edge, it remains quite sharp and observable for a longer time than in a narrower, more poorly corrected eyepiece.  No zoom eyepiece comes close to this capability.

I like zoom eyepieces best in binoviewers because changing out a pair of eyepieces and making sure they're correctly seated so they'll merge views is a royal pain.  Also, because I use a 2x Barlow nosepiece operating at 3x to reach focus, the zooms are insulated from the fast f-ratio of the scope and perform perfectly from edge to edge.

The Altair 30mm Ultraflat is a bit above that budget (but right at it excluding VAT, which is how Americans would price compare).  Being the same optically as the 30mm APM UFF, it would make an excellent low power eyepiece.  It's basically a no compromise eyepiece with excellent eye relief.

I don't know if it or the APM is in stock in the UK or not.  There's also the newer Meade UHD and Celestron Ultima Edge versions which might be in stock.

Thanks to your Mak's f/12.7 focal ratio, you're going to max-out on usable focal lengths at around 0.6*12.7=7mm.  This is assuming you can tolerate a 0.6mm exit pupil.  I cannot.  I have too many floaters in my observing eye.  I can barely tolerate a 0.75mm exit pupil which would be a 0.75*12.7=9.5mm eyepiece in your scope.  Splitting the difference, I wouldn't go below an 8mm eyepiece.  The BST Starguiders have an 8mm for a good price.  I typically don't go below 9mm in my 127 Mak (f/12.1).

2 hours ago, banjaxed said:

I have used the red line SvBony 20mm eyepieces in my binoviewers and IMO they are very clear and impressive, easily as good as my WO eyepieces.

Agreed, see my post about them here in a BV.  With the right 2x (or 1.5x perhaps) Barlow ahead of the BV, it should be possible to get them to operate at about the 10mm level.

This is purported to be the Swarovski version of the Meade 7.7-23.1mm eyepiece:

I guess I never heard about it having gotten into the hobby in 1997.  It was apparently an early 1990s offering.

3 hours ago, jacko61 said:

I saw a filter advertised yesterday that cost over £1000!!!!  How do they justify that for a slice of glass?

If it has 40, 50, or more dielectric coatings applied to create very specific bandpasses for one shot emission line imaging.  The Radian Triad Ultra Narrowband Filter 2" comes to mind.  If it was simple to duplicate, I'm sure Svbony would have come out with their version by now for $20.  I'm curious how well it would work visually.  The human eye is just not very responsive to far red.

28 minutes ago, DhamR said:

Missed this earlier, is it only the edges that suffer at f/5 or across the range?

From my reading of Ernest's tests coupled with user reports, it's quite sharp in the inner 70%, and then dropping off in the outer 30% at f/4 due to astigmatism and field curvature.  It would be slightly better at f/5.  If you keep an object centered and concentrate your attention on axis, you might never notice the aberrated edges.

Did you mean the Nirvana 16mm UWA?  If so, my understanding is that it is decent at f/5, but not outstanding.  Remember, these were selling here in the US under some brandings for about $65 6 or 7 years back.  Don't expect 16mm Nagler T5 performance in your scope.  Also, remember it only has about 10mm of usable eye relief; so you won't be able to use it with eyeglasses.

If you can pick up a used one for roughly 65% of new, I'd say it's a pretty good deal.