YKSE

That's assuming it's a true telecentric, which is not always the case. There're report that ES 3x tele-extender has decreased ER. I would not be surprised there're powermates which have increased/decreased ER too, though milder than a barlow does.

Is this graph sort of you're thinking about?

This is my one case of mismatched eyepieces, which has been stable for some years except the pair of binoviewing zooms 10 different brands

From what I can get from Tammy's measurement, all the Nikon NAV-SW are par-focal within 1mm, and all Pentax XWs from 3.5mm to 40mm are par-focal within 1mm if the 2" to 1.25" adapter doesn't add any length).

Great report and congratulations, it took me many nights under mag 6.1-6.3 sky to see it.

There's this much published picture about CA in achromats According to Telescope Optics by Rutten and van Venrooij, The empirical formula Color Blur (CB)= 735 * aperture (in mm) * secondary spectrum / focal ratio A CB of 1.0 or less is color free A CB of 2.0 or so is a semi-apo A CB of 3.0 or more is an achro Heres's secondary spectrum for some typical glasses: Fluorite 1/16000 FPL-53 is 1/10000 FPL-51 is 1/6000 BK7 is 1/2000 The empirical formula can vary by as much as 10% depending on the mating glass. So the cheap f9 C100ED with FPL-53 would be: CB = 735 * 100 * 1/10000 / 9 = 0.82, i.e. true apo if only CA is considered. An achromat with BK7 will have a focal ratio 735 * 100 * 1/2000 /1=37 to reach CB=1, now that's a 3.7 meter long refractor. With a FPL-51, you can cut down the focal ratio to a third (1/6000 vs 1/2000) for the same CB, considering the relatively low cost of FPL-51, there's not much saving in making a long achromat than a FPL-51 ED for median aperture refractors, IMHO.

This is my first DSLR picture on the Moon with 120ED, clearly a newbie picture, but quite color-free from the scope

As to all round or not, much depending on your sky, 120ED will certainy not go deeper than 159 Mak you had for faint fuzzies.. Maybe you should try with Messiers and some bright NGCs to see how dark is your sky. If it's too bright (say you can't see 1/3 of the Messir), NO aperture will get much better under those skies.

Congratulations to the fabulous scope, Dave, I think you can give the magic word "I'm done" a proper meaning after this beauty.

It's exactly the same eyepiece as the 50mm Revelation, I had GSO one before replacing it with 50mm revelation (TelescopeHouse agree to shortened the exessive long nosepiece ofr me). The measured field stop (both drift-timed and dayime measure) is 40.5mm, AFOV is 47.9°. This EP is just for larger exit pupil in my scopes, not for wider FOV. For the widest 2" FOV, there's Skywatcher 40mm Aero too, in addtion to the ones mentioned by Michael.

I'd suggest that you start using the EPs you have to begin with. 3.5mm Delos will be too high magnification for a f10/f11 SCT 99% of time, you can at most checking SCT collimation or very few doubles under great seeings with it; and for your 90/600 APO, it is quite close to the max magnification you can get under good seeings without much floaters in your eye become distracting. If 3.5mm Delos delivers very often good steady planetary views in your APO, then you might consider a barlow or powermate.

A little update of mine since last year. With 149 new DSO, including 106 galaxies. Two new additions: 50mm Superview helped me seeing the Horse Head nebula with C8 last winter, a well-spend £37 Here's the whole case: And the schematic layout which I've had no trouble to find the right stuff in the dark:

As many others have said, an eye guard of right height may mitigate your issue. There's simple, inexpensive way to make your own eye guard(starman1's post and the link to a picture there), see if it helps: http://www.cloudynights.com/topic/405040-meade-uwa-24mm-2-dialectric-diagonal-159/?p=5212801 I beg to differ If there's one brand plossl which has kidney bean effect, it's most likely to be Televue. Exactly, here's the link to Televue's plossl patent https://www.google.com/patents/US4482217 Here's quote of relevant paragraphs: " BACKGROUND OF THE INVENTION As is well known in the art, a Plossl type eyepiece is a relatively wide field eyepiece comprising two achromatic doublets in which the crown elements usually face each other. Such eyepieces are capable of good performance, i.e., acceptable degrees of aberrations, to about a 50° field. Generally, in order to minimize aberrations at the exit pupil and distortion, all air glass surfaces of the eyepiece are made convex. However, thre have been Plossl type eyepieces used commercially in astronomical instruments in which the external flint surfaces are plano. " GENERAL DESCRIPTION OF THE INVENTION For astronomical viewing, pupil aberrations and geometric distortions are not as important as the correction of coma and astigmatism which control image sharpness at the edge of the field. The red marked sentences mean that TV plossl with its concave eye lens has more aberrations at the exit pupil than other plossls, and the kidney bean effect will show more easilty in day time than night sky. Our eyes are all different, some may cope the kidney bean better, others don't.

I read some optical bench tests, should be interesting to know how it compares to what our eyes see

My wild guess is that you'll be more impressed with 7mm Xcel-LX than 18mm

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".

Just find a correct light path schematic for mirror and prism diagonals. Peter and Stu's understanding was correct, that prism reflects too, the shorter light path in prism is achieved by bending the lights closer to each other than straight reflection in mirror diagonal.

No worry we can have our wishes,just want you not to be too disaapointed when the wishes don't come true. Maybe some more enlightened folk said that, I'm not remotely optical designer, what I understand is therefore a little more aberrations than that, Spherical aberrations is top-priority for correcting, since it affect whole FOV, Coma are second since its unsysmetry affect double-star measurement, and then CA. Astigmatism is more difficult to correct, might need to compromise with FC, SAEP(Spherical Aberration of the Exit Pupil), distortions etc.

Orthoscopic view means no CA, no distortion (straight lines keep straight and same spacing throughout the FOV), which is not compatible with wide FOV. Even with the narrow FOV as plossl, which could be orthoscopic, Al has choosen to have more distortion (and even slight FC) for correcting astigmasm for fast scopes, according to his plossl patent http://www.google.com/patents/US4482217 here's the relevant text in the patent: "For astronomical viewing, pupil aberrations and geometric distortions are not as important as the correction of coma and astigmatism which control image sharpness at the edge of the field." "In carrying out the invention, there is provided a symmetrical eyepiece comprising two achromatic doublets in which the external surfaces of the flint elements are concave. Such a lens configuration provides a significant improvement in the correction of astigmatism and coma at the edge of the field. This results in a sharper image for large field angles with a relatively small undercorrected field curvature."

Nice update Michael, that quality case puts on weight slowly yet steady

I think a shemetic drawing of light path through these diagonals might partly explain the differences: Mirror diagonal: Prism diagonal: For mirror diagonal, light path should be roughly diagonal size (2" or 1.25") plus the thickness of diagonal walls, while it's about 0,71x diagonal size for prism diagonal. As an example, Baader's T2 mirror (2456100) has light path 53mm, while T2 mirror (2456095) has 41mm, that's about 0,77x, roughly right.

To be fair, 6 days are long, HE hasn't managed to create the humans in 6 days.

Thanks for the clarification with emphasis "need buy"

Can we draw the conclusion "never=6 days"?