Good coatings on SCTs; A long time coming

I figure I've owned more SCTs than just about
anyone I've ever met, around 60 of them. From
4" to 14" and the one thing that has always annoyed
me is that they simply do not have the bright snappy
image of a good Newtonian set of optics. Even though
some produced excellent diffraction images and detailed
views of planets and deepsky objects, they often looked
"veiled" compared to a Newtonian. Even when the diffraction
image was superior to the Newtonian. I put it on the corrector
plates. Meade's coatings prior to the new ones always looked
pretty, but they were not quite doing the job they should have.
Celestron SCTs often appeared to have no coatings because the
coatings they did have were clear, like my last 9.25"
I'm hoping the new coatings will allow SCTs to match up to
good Newtonians in this regard, but there is still the matter of
the glass, neither company using a high grade for it's scopes
right now, except that I believe Meade claims BK(?) for their
12 and 14" SCT corrector plates.
-Rich

"rander3127" wrote in message


I figure I've owned more SCTs than just about
anyone I've ever met, around 60 of them. From
4" to 14" and the one thing that has always annoyed
me is that they simply do not have the bright snappy
image of a good Newtonian set of optics.
snipped

Hi Rich and all. That is indeed a bunch of scopes.
Nice explanation and matches my thoughts and comments on most of the
SCTs I have looked thru. Its ironic that you post this as I have
received dirty looks or comments anytime I have mentioned the same
observation at star parties, especially when noting how dull the image
was of objects such as M13 in an SCT as compared to the same power in a
like-sized newtonian. I have always liked the basic idea of a compact
SCT but could not tolerate the light loss. The manufacturers of
many of the Maks (but not all) seem to have managed better, as well
as providing better focusing stability. But if they have been selling
better optional coatings all this time why has it not helped their
image brightness? What else is missing?
I would also like to see the 2 big SCT makers include a standard
purpose-designed dew shield to their customers so the scopes
could actually be used on typical dewy nights without a hair dryer
or aftermarket heater and shield. It is ridiculous to sell something
with glass on the immediate front open to the air, with no shield
whatsoever, only useful on dry nights. At least Meade seems to have
finally responded with a fix to their age old mirror flop problem.
So, some of the basics ARE catching up with the electronics.
Fair skies,
Bob Cuberly


--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

Bob Cuberly wrote:

I would also like to see the 2 big SCT makers include a standard
purpose-designed dew shield to their customers so the scopes
could actually be used on typical dewy nights without a hair dryer
or aftermarket heater and shield. It is ridiculous to sell something
with glass on the immediate front open to the air, with no shield
whatsoever, only useful on dry nights.

Small potatos, I think. For an 8" SCT, a dew shield is in the $40
range; and I've often used my 5" SCT in the desert for hours without
one. You might also mention the limited quality (and quantity) of the
EPs; my 10" GS Dob came with two EPs, one of which is better than the
single one provided with my 8" SCT

My annoyance is that these scopes, really designed for transportability,
come with no carrying case. So the first, fairly expensive, accessory
we need to buy is a carrying case. I will receive a JMI case for my new
NexStar 8GPS on Monday. In the meantime, I have the shipping box on a
dolly which I pull outside to set up on my patio; and hauling the scope
any distance for use this weekend would be too big a hassle (that box
bulky).

But am I *really* that annoyed by any of this? Not really. I'd rather
have the basic scope at a good price and add my own accessories. Maybe
some owners will never use the scope away from home and not need a case.
Others (including me) already have a range of EPs they prefer. And some
may prefer(or need) a dew prevention heater vs. a dew shield.

I say keep the accessories at a minimum so we can choose our own (e.g.,
I far prefer the JMI case to the Celestron case).

Phil

"Bob Cuberly" wrote in message
news:d96c396cc495449c80e5d0e604c1076b.118439@mygat e.mailgate.org...
"rander3127" wrote in message


I figure I've owned more SCTs than just about
anyone I've ever met, around 60 of them. From
4" to 14" and the one thing that has always annoyed
me is that they simply do not have the bright snappy
image of a good Newtonian set of optics.
snipped

Hi Rich and all. That is indeed a bunch of scopes.
Nice explanation and matches my thoughts and comments on most of the
SCTs I have looked thru. Its ironic that you post this as I have
received dirty looks or comments anytime I have mentioned the same
observation at star parties, especially when noting how dull the image
was of objects such as M13 in an SCT as compared to the same power in a
like-sized newtonian. I have always liked the basic idea of a compact
SCT but could not tolerate the light loss. The manufacturers of
many of the Maks (but not all) seem to have managed better, as well
as providing better focusing stability. But if they have been selling
better optional coatings all this time why has it not helped their
image brightness? What else is missing?

Perspective, maybe. Everything is a trade off, and compactness for light
transmission is one that occurs in the SCT vs. Newt. No big deal. To
increase light throughput, simply bump up the aperture. I think this is the
main motivation behind the design, and its utility. For example, my C5 is
shorter than my 80mm F5 achromat. The C5 aperture wins hands down in light
grasp (and in resolution), and the contrast of these two is theoretically
similar, according to the rule of thumb as it concerns contrast, where
aperture minus obstruction equals unobstructed aperture.

Consider that a 1250mm 5" newtonian would be 50" long. Even a 5" F5 Newt
would be more than twice the length of the C5. Not _as_ portable in a day
pack. Surely someone interested stricty in visual astronomy from a site
within reach of automobile or backdoor, would chose the Newt for the reasons
you give, but for someone who wants the increase in portability, and the
ability to image day and night, as well as having a decent visual
experience, the 5" SCT is a reasonable choice over the 80mm refractor, and
the 5" Newtonian, and you can go ahead and scale that up according to the
contrast rule all you like. The SCT will always be a bit smaller for
transport.

Don't get me wrong, I don't disagree with your experience, I'm just
explaining why SCTs remain popular given there shortcomings.

I would also like to see the 2 big SCT makers include a standard
purpose-designed dew shield to their customers so the scopes
could actually be used on typical dewy nights without a hair dryer
or aftermarket heater and shield. It is ridiculous to sell something
with glass on the immediate front open to the air, with no shield
whatsoever, only useful on dry nights.

Well, I can't speak for the other apertures and manufacturers, but Celestron
does have a hard plastic lens shield for the C8 that costs around $35 IIRC.
(Not that I don't agree with you whole-heartedly that one should be provided
in the basic package).

At least Meade seems to have
finally responded with a fix to their age old mirror flop problem.

Yeah, this one is the most annoying of all the faults in the design, since
it is the most difficult to work around. Aperture increases, better mounts,
and lens shields can't do a thing to stop mirror flop. The other thing in
the design that is annoying, is the inability to get truly wide fields of
view without vignetting. With the Newt, you can tear it down and restructure
the focuser and the secondary to achieve the desired field illumination.

-Stephen

here is and I like it:
http://www.celestron.com/access/photo.htm

Also known as the flowerpot.

but I have always thought that scts should come with retractable dew
shields a la Questar and built-in corrector plate heaters.. probably due to
observing in the middle of the Florida Everglades National swamp

On Thu, 14 Aug 2003 10:18:20 -0400, "Stephen Paul"
wrote:

Well, I can't speak for the other apertures and manufacturers, but Celestron
does have a hard plastic lens shield for the C8 that costs around $35 IIRC.
(Not that I don't agree with you whole-heartedly that one should be provided
in the basic package).

Herm
Astropics http://home.att.net/~hermperez

I've always wondered why the sct coatings couldn't hold a candle to the
american built refractor coatings, those scopes really have a snappy
planetary image.

George

Clear skies and good health
wrote in message
...
I figure I've owned more SCTs than just about
anyone I've ever met, around 60 of them. From
4" to 14" and the one thing that has always annoyed
me is that they simply do not have the bright snappy
image of a good Newtonian set of optics. Even though
some produced excellent diffraction images and detailed
views of planets and deepsky objects, they often looked
"veiled" compared to a Newtonian. Even when the diffraction
image was superior to the Newtonian. I put it on the corrector
plates. Meade's coatings prior to the new ones always looked
pretty, but they were not quite doing the job they should have.
Celestron SCTs often appeared to have no coatings because the
coatings they did have were clear, like my last 9.25"
I'm hoping the new coatings will allow SCTs to match up to
good Newtonians in this regard, but there is still the matter of
the glass, neither company using a high grade for it's scopes
right now, except that I believe Meade claims BK(?) for their
12 and 14" SCT corrector plates.
-Rich

"Stephen Paul" wrote in message ...
[...]
At least Meade seems to have
finally responded with a fix to their age old mirror flop problem.

Yeah, this one is the most annoying of all the faults in the design, since
it is the most difficult to work around. Aperture increases, better mounts,
and lens shields can't do a thing to stop mirror flop.
[...]

Primary mirror flop is NOT inherent in the design of an SCT (or Mak).

The choice to have the mirror cell slide over a layer of grease covering
the baffle tube (instead of a Rolamite or roller bearing approach a la a
Crayford focuser) is one that was made to reduce production costs. Period.

Though it's a Mak, the AP 10" uses (IIRC) roller bearings and has no mirror
flop, and there's nothing preventing Meade, Celestron, et al from doing the
same (except it'd probably add US$100-200 to the final price).

Note also using Rolamite or roller bearings (instead of sliding over goopy
and deformable grease) assures the primary remains concentric with the OTA's
optical axis and provides a commensurate performance improvement, well worth
the approximate US$100-200 end-user price increase. Think about it.

"Thad Floryan" wrote in message
om...

The choice to have the mirror cell slide over a layer of grease covering
the baffle tube (instead of a Rolamite or roller bearing approach a la a
Crayford focuser) is one that was made to reduce production costs.

Though it's a Mak, the AP 10" uses (IIRC) roller bearings and has no
mirror
flop, and there's nothing preventing Meade, Celestron, et al from doing
the
same (except it'd probably add US$100-200 to the final price).

It would be worth the extra. So would reducing the obstruction at the cost
of increased tube length. I'd like to see them get the CO down to 25%.

Rod Mollise wrote:
My annoyance is that these scopes, really designed for transportability,
come with no carrying case. So the first, fairly expensive, accessory
we need to buy is a carrying case. I will receive a JMI case for my new


Hi Phil:

Both Meade and Celestron included cases for years and years. These ranged from
very nice ones, like those used with the Ultima 2000 and Ultima 8, to
footlockers.


I don't recall that. While I am older than you, you are more
"astronomically" old g

As for some of the other comments on this thread. often people's perception of
SCTs presenting "dull" images is a result of comparing an 8 inch f/6 dob and an
f/10 SCT with the same eyepiece--that is, without equalizing magnification.


I figured some of it was more central obscuration. My impression is
they were speaking of equal magnification.


Coatings-wise, I'm all for improvements, but I've been pretty satisfied with
Starbright over the years.


Yes, I do not expect the XLT coatings to make much difference. Still,
best to have.

Phil

I don't recall that. While I am older than you, you are more
"astronomically" old g


Hi:

Wasn't that long ago really. My 1995 Ultima 8 came with a case (thought they'd
eliminated them for the less expensive scopes by this time). Why? Amateurs
insisted on paying the same--or less--they'd been paying for SCTs since the
beginning. Both Meade and Celestron felt the need to cut some of the "frills."
Like you, I consider a good case an integral part of the SCT system.

Central obstruction? I doubt it. The difference there is quite subtle.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
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