Louis D

Is this similar to your scope?  If so, it's an f5 parabolic mirror and that's probably where your coma is coming from, although it seems a bit excessive from your description.  If you were in the US, I'd recommend you watch for a second hand GSO coma corrector to clean up most of this for not a lot of money (usually under $90).  However, I've yet to find anyone selling it in Europe under any brand.  Maybe someone more familiar with European retailers could chime in here.

Is this similar to your scope?  If so, it's an f5 parabolic mirror and that's probably where your coma is coming from, although it seems a bit excessive from your description.  If you were in the US, I'd recommend you watch for a second hand GSO coma corrector to clean up most of this for not a lot of money (usually under $90).  However, I've yet to find anyone selling it in Europe under any brand.  Maybe someone more familiar with European retailers could chime in here.

14 hours ago, steviebee said:

Thank you Peter, I will have a look now. I have been looking at various posts on here. Do you think it could be Chromatic Aberration? If so is it possible to reduce it. On another post here it was described perfectly as a star having a comets tail.

Were you using the 45 degree Amici prism diagonal?  They can cause a diffraction spike when a star crosses the center line.

Is the comet tail the same color as the star or is it more of a rainbow?  If the former, it's coma.  If the latter, it's chromatic aberration.

Does it grow longer the farther the star is from the center?  Does it always point back to the center as the star is moved around the FOV?  How does it look on either side of best focus?  Which way does it point when the star is dead center?  Have you swapped eyepieces to make sure it's the eyepiece and not miscollimated or pinched optics?

5 hours ago, YKSE said:

I'm no optical expert, but I think your observation is correct.

My thought: shortening the light path with a prism is more pictural description, because with prism diagonal, we gain backfocus compard to a mirror. What prism does is actually push the focus position more rearwards, therefore then gain. My 2" prism diagonal and mirror diagonal is of the same physical size, focusing on short distance (some 20-25meter), focuser needs to rack out clearly more with prism than mirror.

It is the same with a barlow or GPC in binoviewer, barlow or GPC, with their diverged beam(therefore actually longer light travel to focus) push the focus plan much more rearwards, so we gain backfocus, therefore the descriptive word "shorten the light path".

I think you're absolutely correct.  Everyone is always saying they're shortening the path with a prism to increase available backfocus, when in fact they're lengthening the light path in glass to increase backfocus.  You want to shorten the air part of the path as much as possible with stubbier connectors, but you want to increase the glass part as much as possible.

I'm no optical expert, but that diagram seems to show the exact opposite of what is common knowledge, that prism diagonals shorten the path length over a mirror diagonal.  I'm still working out what is going on.  The angles of refraction seem correct.  The converging beam is actually made to diverge somewhat within the prism, lengthening its path while in the glass.  I always thought just the opposite was happening and accounted for the shortened path length.

On 9/9/2016 at 04:19, steviebee said:

The only way I could lose this was to move the position of my eye on the lens.

Have you seen this same ghosting using other eyepieces?  How about the same eyepiece in your Skywatcher 200P?  Based on the above sentence, it is possible you're seeing a reflection of the moon's image reflected off of your cornea back into the eyepiece.  Try to narrow down which element is introducing the reflection.

1 hour ago, Stu said:

I was assuming a 127mm Mak, not the 102mm which has a 20.7mm baffle tube which I guess matches the BCO closely at 45 degree afov. 

When I mentioned vignetting with my binoviewers, it was with respect to my AT72ED which has a true 2" focuser with no aperture restrictions and with 32mm GSO plossls which have true 27mm field stops.  So the 20mm to 22mm clear aperture of the binoviewers (not exactly sure of the number) is quite easy to see with this combination.

2 minutes ago, F15Rules said:

Your 32mm plossls will likely vignette a little (show a darker edge to the field of view) but should still give a nice view.

Oh, they'll vignette more than a little.  The outer 25% drops off to blackness before the fieldstop in my Arcturus binos.  I still like the views of nebulae in them because it really brightens them up by compressing all the light into a smaller image than with higher powered eyepieces.  That, and the eye relief with glasses is super comfortable.  Enjoy using them.  They're a lot of fun once you get a good setup.

11 hours ago, Stu said:

The speed of light is constant in a vacuum but it does change (slow) in other media such as glass. I think astronomers only normally consider the speed in a vacuum because that is what it is for the vast majority of its journey to us from the stars.

The shorter light path on a prism however has nothing to do with the speed of light, it is simply due to the geometry of the prism vs the mirror i.e. The prism has a physically shorter light path. I'm not totally sure why, but putting my Baader Zeiss T2 prism next to the BBHS T2 mirror, the prism is clearly a smaller package.

Well, EdZ on CN stated the following back in 2012:

The optical path of a prism of same size is shortened (by physics) by the slowing of light traveling thru glass. The convergence of the rays is slowed and we can determine the optical path length by taking the physical length divided by the refractive index. So prisms (of same size) will ALWAYS have a much shorter optical path than mirrors.

So I'm going to go with EdZ because he always seems to have a pretty good grasp of optical physics.

7 hours ago, Knighty2112 said:

Don't quite get how the light path would be shorter than less expensive options here?

Prism diagonals have a shorter light path than mirror ones because the speed of light traveling through glass is slower than through air.  At least, that's what I've heard.  I'm no physicist or optician.

5 hours ago, Knighty2112 said:

Got a Altair 2" dielectric mirror diagonal.  Got 80mm focus on my dual sped focuser, so hoping that should be enough also to bring the binoviewer into focus OK.

I couldn't reach focus with my OPT 2" dielectric diagonal my AT-72ED, so I ended up getting a 1.25" WO CF dielectric.  It works great, no flexing, no induced aberrations, and it weighs less.

Before giving up on the scope's altitude bearings, try locating a sheet of pebbly, gloss formica.  In the past, Wilsonart Ebony Star Formica was the go to choice, but I think it's discontinued, so you'll have to locate something similar.  Ask around on the ATM forums what's best these days.  Cut the sheet into strips and glue them to the curved bearing surfaces attached to the mirror box.  Next, locate some blocks of Teflon (PTFE) and countersink screw them to the rocker box, one toward the front and one toward the back of the bearing surface.  Check that Obsession's bearings, I think you'll find a setup similar to this to guide you.  Nylon can be substituted for Teflon if the motion has too little sticktion for your liking.

4 hours ago, Peter Drew said:

It seems better to use stainless steel ballrace bearings and stainless steel strips on the side bearings and a stainless steel disc running on bearings for the azimuth axis.

I seem to recall seeing a goto truss design similar to this.  The bearings reduce friction to a manageable level for the slewing/tracking motors.  Still, there's no grease outside of the bearing races.

4 hours ago, FLO said:

Mert noticed it should be greased

I've owned several truss dobs, and not one uses grease on the side bearings.  Virgin Teflon blocks riding on Ebony Star Formica (or equivalent) should need no additional lubrication.  Nylon blocks can be substituted for Teflon to add a little more sticktion.

You might try mounting fans to blow across the front of the mirror.  I've heard good things about this approach removing the boundary layer of air just above the mirror.  Kind of counter-intuitive.

If the bearings had been teflon on pebbly Formica, no lubrication should have been needed at all.  In fact, the motion can be too smooth with too little sticktion to hold heavier eyepieces.

Reminds me of the Stabilite mirrors that Zambuto abandoned over a decade ago because it was too expensive to deal with print-through issues during figuring.  Has Sky Watcher found a way to deal with this?  Both were/are radial rib designs.