YKSE

8 minutes ago, Louis D said:

Telecentric magnifiers work by returning the light bundle to the same cone angle it was before the diverging element by adding a converging element.  The effective focal length on that chart would be very, very long indeed.  The effect is such that there is little to no extension of the exit pupil or vignetting of the outer regions of the FOV.

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.

2 hours ago, John said:

The effect on the eye relief depends on the focal length of the barlow lens as well as that of the eyepiece. The "shorty" types of barlows increase eye relief more than longer designs. So a short barlow used with a longer focal length eyepiece gives the maximum eye relief change.

I wish I had a neat "sliding scale" type diagram to illustrate this, but I don't

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

25 minutes ago, John said:

not all ranges are par-focal thoughout by any means.

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.

On ‎2017‎-‎09‎-‎05 at 11:42, spaceboy said:

I was just thinking, as you do. Is there a highly praised achromatic refractor out there ? With ED and APO refractors so far out the reach of the modest earning folk I would have thought there must be one high spec achromatic refractor that gives as good as it gets ?

Or does the result of all that refinement to make an achromat well corrected and tolerant of CA end up with a cheap ED refractor like Celestron's C100ED. I like the look of the Altair 152 but at £800+ again your getting very close to Chinese ED budget. I appreciate your not going to get the same aperture in an ED as you can a well corrected achromat but even so good glass does seem to compete very well often giving equal if not better planetary views than a larger achro.

From what I gather acromats can be just as good as ED refractors but at the expense of rather long focal lengths? I'm sure even if you could pick up a long frak at a fair price it would still end up fringing on Chinese ED due to mounting issues occurring from wielding such long refractors ?? 

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.

4 minutes ago, BinocularSky said:

...and even less charitably: most of the time they can actually see what another is using; most of the time, we can't.

But also, for most astronomers, binoculars are not the primary instrument and the main lucre goes into a scope, mount and decent camera and .....

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

2 hours ago, Ruud said:

Kidney beaning, however, should not happen with TeleVue or any other Plössls as they tend to have well behaved exit pupils.

I beg to differ If there's one brand plossl which has kidney bean effect, it's most likely to be Televue.

2 hours ago, Paul73 said:

Could be that the world has moved on a bit from the Plossl design? Sure, the transmition is splended. But, there are limitations.

ie. Egg shaped sun in the outer 1/3 inthe 25mm TV.

But the main question must be around black outs linked to head positioning. 

 Why didn't TV sort this out.?

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.

50 minutes ago, F15Rules said:

That's interesting...have you read reports to that effect, or used them yourself?

Dave

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

1 hour ago, F15Rules said:

Will be interesting to see how the 60 degree Xcel LX's stand up against my Pentax (65deg) and Morpheus (76 deg) units...I've read good things about their bang per buck performance:-).

Dave

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

45 minutes ago, Louis D said:

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.

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.

9 hours ago, Highburymark said:

Not meant as in any way feasible suggestion - but as ever with SGL I've learnt something from the expert responses

No worry  we can have our wishes,just want you not to be too disaapointed when the wishes don't come true.

11 hours ago, rockystar said:

Isn't there something in optics design that says: here are the three aberrations - CA, Coma, Astigmatism - I can fix two of them, choose which two?

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.

9 hours ago, Highburymark said:

Maybe Al Nagler could design a range of orthos with 20mm eye relief? Case complete!

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.

8 minutes ago, DRT said:

To be  fair, I said I would never need to buy another eyepiece, not that I would never buy another eyepiece

 

Thanks for the clarification with emphasis "need buy"

On ‎2016‎-‎05‎-‎28 at 16:23, DRT said:

So here it is, the very last picture I will ever have to post in this thread as I will never need to buy another eyepiece. This is the same as last time but with the edition of a Pan 15mm sourced from a nice man in Texas...

I also have two TV 32mm Plossls which live in my solar case as I only use them for Bino-viewing through my Quark. 

That's it. The end. Absolutely done. Almost definitely. 

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

Congratulations John.

15 hours ago, Moonshane said:

For your sake I hope it's sharper than the 120Ed and for my sake, I hope not by much!

Either way, you cannot lose!

I'm quite sure the Tak will be sharper than 120ED, by simple reasons as

1. It's floruite, better than FPL53

2. it's 4", not 4.7",. easier for better colour correction.

3. it's f9, better than f7.5, also easier for better colour correction

4. and it's a Tak.