A Few Beginner's Questions (1 Viewer)

At some point, I'm looking to increase from 60mm, and the obvious
quality/budget choice seems to be a reflector like the C90, yet I don't
see much reference to it. Are there issues with reflectors for the
300yd--3mile terrestrial arena?

Also, zoom is common in spotters but not binoculars.
I'm guessing it's a combination of not-as-much-field-width-to-lose,
comes-in-handy-more-often, and multicoatings-keep-contrast-up-nowadays.
...but I'm not sure. I am striving for fixed oculars, but it seems
almost futile. Maybe I'm staving off something that's not a problem.

For the long range, atmospheric limitations besides, i dont see any problem ... for 300 yards the fov is very narrow due the focal lenght of the maksutovs. The zoom in the c90 is useless, again is a matter of focal lenght, you cant get the typical 20×60 mags range. Go for fixed

Thanks for the insight! That's right, the folded reflectors have the f/ratio of long refractors.
Still might be good for the eagles. Taking two scopes in the trunk is no problem. Someone on another
thread said the refractors had better contrast. Maybe some hooding would help a Mak.

birding with mirror telescopes article

OPTIC_NUT said:

At some point, I'm looking to increase from 60mm, and the obvious
quality/budget choice seems to be a reflector like the C90, yet I don't
see much reference to it. Are there issues with reflectors for the
300yd--3mile terrestrial arena?

Also, zoom is common in spotters but not binoculars.
I'm guessing it's a combination of not-as-much-field-width-to-lose,
comes-in-handy-more-often, and multicoatings-keep-contrast-up-nowadays.
...but I'm not sure. I am striving for fixed oculars, but it seems
almost futile. Maybe I'm staving off something that's not a problem.

Click to expand...

Here is a good article about birding with mirrored telescopes:
http://betterviewdesired.com/Birding-Catidioptric-Scopes.php

You can't beat the portability of these reflecting scopes. But they are not waterproof which could be a huge disadvantage to some people. Something not mention also, best to buy excellent quality eyepieces since these scopes tend to have less contrast than top tier spotting scopes and refractors (I am using a Brandon, an Orthoscopic, and thinking of trying out a Vixen Lanthanum).

Cheers,
B

Yup, I'm already scheming on what that one great eyepiece will be.

Contrast is an issue in comparing reflectors and astro refractors, too.
I've been able to put better darkening in the barrel. Next might be
an iris or two, but that's not an option in a folded reflector.
A nice big slide-forward hood would be, though. Extra-large dew shield.

I'm not really big on weather sealing long-term, since it leads to a lot of
difficulty doing maintenance and tweaks. It's certainly more practical on
regular spotters than on astro or folded.

eyepiece characteristics

OPTIC_NUT said:

Yup, I'm already scheming on what that one great eyepiece will be.

Click to expand...

Eyepieces are warm or cool in tone. The Hutech orthoscopic is a little cool in tone, it needs bright direct sunlight to bring out the colors, visually. I have heard that TeleVue Plossl's are a little warmer in tone, and some birders like them to bring out the colors in the birds (the Vixen Lanthanum's are supposed to be even better). Don't own one yet, so cannot tell.

Yeah, and the contrast thing is important for mirrored telescopes. Owned a Vixen VMC110L once and it was lacking in contrast but did not know how to make it better, so I sold it off for around what I paid for it used. Seems like the cheaper mirrored scopes do not have as much contrast as the more expensive ones. Would like to look through an Orion Mak one day to see what it is like for terrestrial use (with respect to contrast).

I'd be trying to fashion a hood with black emery paper liner if I had a reflector, I think.
It's awkward when they don't make what you need. Easier for a 'smaller' scope, though...like a C90.

Some Mak's and SCT's come with a sliding hood. The Questar does, not sure of others.

goldenarrow said:

Yeah, and the contrast thing is important for mirrored telescopes. Owned a Vixen VMC110L once and it was lacking in contrast but did not know how to make it better, so I sold it off for around what I paid for it used. Seems like the cheaper mirrored scopes do not have as much contrast as the more expensive ones. Would like to look through an Orion Mak one day to see what it is like for terrestrial use (with respect to contrast).

Click to expand...

Scopes with central obstructions above 30% can't have really good contrast performance no matter how much they cost. Even if a scope has perfectly corrected optics a 30% obstruction has about the same effect on contrast as 1/4 wave of spherical aberration. Questar claims at least 1/8 wave P/V or better corrections, but that has to be added to a contrast loss a bit worse than 1/4 wave from the 31.3% secondary. Cheap MAKs and SCTs will be worse than that since they are certain to have higher aberrations and larger obstructions (33.3% in the C-90). Between the compromises inherent in cheaply produced mirrors and the big central obstruction you'll be lucky to get 1/2 wave contrast performance. And, that's before scattered light is even considered. The Questar has sophisticated internal baffling for narrow angle scattered light which cannot be suppressed by any external hood. You won't find that in cheap MAKs, so their contrast falls even lower.

Dang, that's unfortunate.
Is that related to the diffraction signature of the obstruction?

So...you might have to go Newtonian.
It's not quite as tough as it seems..they are big, but they don't have to weigh a lot.
Also, a reflector can have a parabolic grind, and there are no chromatics.
A 4-inch short Newtonian with hood might not be too bad. In dire need of the hood, though.


The contrast, or the noise, might have to be compared to the haze in the air.
Not sure if it's better or worse than on a fairly clear day.

Ah, here it is: yes, the diffraction patterns of obstacles smaller than
the aperature:

http://www.lcsd.gov.hk/ce/Museum/Space/FAQ/others/e_faq_others_2.htm
(with some nice pictures)


You can see how thinner vanes help, how the pattern is formed, and how the
thin-vein compares to a refractor (which still has diffraction issues, of course,
just not as many fringes)..

This dampens my enthusiasm on Newtonians, too.... the glass plate on the
others eliminates the spider diffraction.

The effect dies down a bit (at a given power) as the diameter goes up, though,
and reflectors keep retaining a economic and practical edge (other than size),
so, for example, the Hubble is a reflector and the beastly big eyes in the Andes
and Hawaii are reflectors. The benefits outweigh the troubles as you get bigger.

Maybe one alternative to a reflector, comparing dollars to dollars, is a bigger
reflector.

Here's a 10-inch Dobbie,
http://www.bhphotovideo.com/c/product/735539-REG/Sky_Watcher_S11720_10_Dobsonian_Telescope.html
$700 includes the mount, of course, and two EPs. Would need curtains for daytime use.
Not for hiking! 33 lbs. Fun for eagles at 2 miles, perhaps. But my 70mm or an 80mm refractor is
going up Artist's Bluff, not that thing. Sure would be fun to take it up the Canon Mountain tram.
But...how to keep that thing clean?

How about a 12 inch dobsonian mounted to a tank turret. Make your own trails!

Or just drive a pickup truck to some mountain with a road.
Once you get above ~3000 ft (above surrounding) the atmospherics are much better.
You can watch critters in the next county.

I don't suppose you guys are very serious about trying to use Newtonians for birding scopes. FWIW, there was one semi-serious effort at an erect-image Newtonian appropriate for birding about 20 years ago called the Zuka-Scope.

http://betterviewdesired.com/The-Zuka-Scope.php

Pondering things.
At this stage, I'm willing to ponder anything. Invention is needed.
When they had to, the Brits came up with some crazy inventions
for WW2. Some worked like a charm.

Moving raptor pursuit is outside any common box (short of $10,000 cinematic lenses).
So the Zuka is a 4-inch f/5. Interesting. Is the f/5 hard on an eyepiece (hard at the
field edges over 03-40x) ?

The article says it's stable to roughly twice binocular power (~20x) when hand-held.
If I could be steady at 30-40x with no actual tripod that would be enjoyable for closer raptors.
The tubes can be pretty light, and dropping the tripod is a bonus. I steady my spotter sometimes
at 20x with a 5 foot 2x2. Maybe a stick added to an f/7 barrel.

A 80mm/f10 isn't much more awkward, but the problem of needing to be at the far end is tricky.
I can't figure that one out without a tripod.
Maybe an 70-80mm f/7 ED/APO with a stedi-stick so you can counter balance for a side-view.


I made some headway on improving the tripod for the 70mm-f/10 without getting heavy:
X's made of 6 hacksaw blades taped between the tops of the legs. 1/4 the amplitude,
3x the shake frequency, so it dies down quickly. It adds 2 oz. and 2 minutes.
Otherwise, the tripod would gain ~6 lbs. to be that stable.

Actually, for birding you'd better forget about reflectors. I looked through a variety of reflectors in the field over the years, from Questars to a C8. I even did some experimenting with a Russian f10/1000 Mak for fun.

But compared to what a modern refractor with an 80-100mm objective lens can do, the reflectors, even very good ones, just don't cut it in the field. Problems with contrast, problems with waterproofing (we made a lot of jokes about the guy with the Questar - he only got out in the field when the weather forecast was *just* right, which meant he missed a lot of good birds), problems with narrow fields of view with the Maks, what have you.

Modern refractors runs circles around any reflector for birding.

Hermann

It occured to me one that one reason the diffraction from the obstructions is not as much
trouble for the night sky is, the view is "sparse". That is, just a few bits of light and a lot
of darkness. Darkness by itself has not signature. During the day, however, there are many
illuminated objects at similar brightness to your target, and that produces noise (a haze)
for the reflector. It may not make much difference when the air itself is hazy, though.