Posts Made By: clive gibbons

Recently, in this forum and another, it has been stated by a knowledgable source (Vic Maris), that the SV102EDT used "short flint" SF3 glass in it's lens design.

When the SV102EDT was being produced, there was controversy regarding it's color correction and lens design.
To hopefully clear up a lingering question about this scope's optics, I'd like to know if the EDT used ED (Extra-low Dispersion) glass.
If so, what was the ED glass?

Vic??

Thanks.

News Flash!....

http://skyandtelescope.com/aboutsky/pressreleases/article_1677_1.asp

The subject line is a question that comes up every now and then when considering apo refractor performance. I first heard about it in relation to the Zeiss APQ series back in the '90s.
To achieve the very best correction, those scopes require the use of a prism-type star diagonal.
Some apochromatic refractors work best with a mirror diagonal. Others benefit from the refractive properties of a prism. Rarely does the vendor indicate which works best with what.
I've often used a prism with my apo doublet refractors. The dearly departed Tak FS78 was always coupled to a Tak prism 1.25" diagonal and displayed flawless images.
More recently, I tried my Sky-Watcher ED120 with both mirror and prism diagonals and observed that the mirror-type showed slightly more chromatic aberration in the image. Thus, a '70s vintage Celestron 1.25" glass diagonal (original equipment with C8's of the day) has been used with that scope since the testing was done.
This past weekend, I was trying out a used TeleVue 2.5x Barlow with my Megrez 110ED, to see how it compared with a 2.5x Powermate. The Powermate wasn't showing it's best with the Megrez's f/5.95 focal ratio and FPL-51 doublet optics. Sure enough, the old TV 2.5x Barlow (actually 2.8x as measured) worked better than the Powermate. No stopping down to 101mm (f/6.5) was required to sharpen up
the diffraction image.
Just before deciding to bring the scope in for the night, a thought popped into my head ... why not try the
old Celestron 1.25" diagonal with the Megrez, in place of the William Optics 2" dielectric unit? It would be much lighter than the WO diagonal, but how would using a prism work in such a fast ED refractor??
I suspected it wouldn't be pretty.
The Megrez was aimed at Delta Cygni. Using a 12.5mm Ortho and the TV 2.5x Barlow (147x) showed the companion fairly plainly even in the so-so seeing. Not bad. But, SURPRISE-- switching to the Celestron diagonal revealed Delta's faint companion with slightly less difficulty and it looked like there was reduced red defocus in the primary's image. Markedly less!
The view was so promising, it tempted me to up the power to 183x and swing over to Epsilon Bootes (Izar) to compare the two diagonals once again.
With the dielectric, the main star's yellowy color showed it's typical ruddy excess at best visual focus. Swapping
to the Celestron diagonal reduced the unfocused red quite a bit. There was a real improvement.
It was like the FPL51 doublet had been upgraded to FPL52. How best to quantify this?
I'd say the 110ED at f/5.95 using the prism diagonal, showed better correction than the same scope stopped
down to 101mm (f/6.5) using the WO dielectric diagonal. In fact, the Megrez stopped down further to 94mm f/7
displayed about as much c.a. as at full 110mm aperture with the prism. Very surprising.

The upshot of this improvement is the WO 2" dielectric is now packed away in it's box.
It’s stylish shape and carbon fiber trim sure is impressive looking on the back of the Megrez and delivers a wonderfully bright image. A great piece of kit.
BUT, the smaller, lighter Celestron unit results in significantly reduced spurious color at best focus.

All I can suggest from this observation, is that if you currently own a relatively fast ED doublet refractor, it's
high magnification performance (ie. above 150x) could benefit from switching to a prism-type star diagonal.
But, you won't find out until you try.
If you have the chance to pick up a good quality right-angle prism diagonal for a reasonable price, it's fun
to experiment and possibly discover for yourself how to optimize your scope's image quality.

Just wondering if the optical gurus (paging Rolando! )
could offer some insight/opinions/commentary about the info shown here--
http://astro.uni-tuebingen.de/~grzy/APO115auswertungneu.jpg

Specifically, the range of focus between C and F (70 microns).
e= 0
F= -50
C= +20 (microns)

How good an APO are we looking at, here? :S

Eddie Casey said:

Just been reading a few first light reports on the Meade RCX, not for buying info, curiosity mainly. Particular interest is a review by a respected contributor to the mapug list. Seems this scope is streets ahead of the LX, visually at least. Another guy had the 10"RCX setup next to his C11 and described the differences as huge on observing jupiter. Anyone here had a nosey down the ep of one yet? Forgive my cynisicm but, all things being equal, I wouldn't have expected such an appreciable difference between a SCT and the RCX for planetary viewing. Seems Meade's advo is being answered anyway!
Ed

Hi Ed.
The 10" RCX is probably a very good instrument. About as good as a 10" with a near 40% obstruction can be. Should it outperform a similarly fine 10" SCT for visual use? Not really.
If the scopes are both cooled and well collimated and of equal optical quality, you won't see more with an RCX. For on-axis sharpness, the RCX design has no inherent advantage over an SCT.
It's certainly a neat-o scope with lots of gadgetry. Some folks report it's actually fun to play with the focus/collimation feature. I was brought up to believe that if you have too much "fun" with something, you're likely to end up breaking it.
One very weighty thing to consider about the RCX scopes... they're heavy! The 10" tips the scales at around 160 pounds, as set up on the tripod. This is without a wedge. Much heavier than equivalent size SCTs. The manuals even contain a health warning about this. I don't think Meade has advertised the weight issue. Probably because it might scare off potential buyers.
In any event, we'll have a clearer idea of how good these scopes are after the first year of use in the field.

Clear skies,

Clive.

dan hilts said:

Nobody cried foul when Vixen introduced their NA series scopes. A neo-achromat? now what the heck is that? Just another round of marketing hype, a new name for an old concept.

So how come every one gets excited over the latest hype from China? It is what it is and it will fail or flourish based on what it delivers, not what it promises.

Now since these new Chinese scopes are Petzval designs or an variation of, could it be they actually use ED in the secondary element? Sort of a semi-ED scope since only half the objective has ED? Does anyone really know?

clear skies,
dan

Hi Dan.
Vixen's introduction of the name "neo-achromat" wasn't a big deal, since it was a new iteration (for them) of an achromatic design. Neo = new. Achromat = achromat.
Sure, it's a Petzval. Televue called theirs a Genesis. I'm shocked that Christians didn't march on Pearl River, back then!


According to the vendor, there's no ED used in the "semi-ED" Antares refractors. So why call them that? It's a very dubious way of marketing telescopes.

Take care,



Clive.

Doug Peterson said:

Well that is what I get with my new 8" F6 TMB achromat and a Chromacor. Anyone else tried this?

Hi Doug.
Technically speaking, what you have is an 8" achromat, with a Chromacor at the back.
Not a Petzval design.

I once owned a Tak FS-78 and still own the ED100.

One night, did a direct "side-by-side" on the Moon, some doubles and deepsky.
Both scopes were excellent, but the ED100 equalled the Tak's color correction and provided brighter, higher resolution images.
The Tak wasn't much smaller or lighter, either, so it went up for sale shortly thereafter.

No doubt about it, The Tak is a great 3" APO... but it's not as good as a fine 4".

There's very few SV105's in owners hands so the optical performance of that scope is not firmly established. It's only been on the market for a short time.

The Takahashi TSA-102 has been available for quite a while longer and has earned an impeccable reputation. Last year, an experienced amateur astronomer tested a large number of refractors in that size range and the TSA-102 (along with a TMB 4") came out on top.
http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/2624353
(The latest version of SV105 wasn't available at that time, so wasn't included in the comparison.)

In any event, the article makes for some interesting reading and might help with your decision.

Based on experiences with a 5" f/9 ED doublet, that scope is getting too long and cumbersome for my taste.
A larger aperture, slower f-ratio ED doublet is tempting... but not for this user.