Posts Made By: Hinrich Eylers

See below 11" SCT aperture mask consisting of three 120 degree "triangles," resulting in area equal to a 7" scope.

There are people who claim to have read that this pattern eliminates the effect of the central obstuction. I don't really believe that, but can't convince myself either way as my physics studies were way too long ago, and I don't have a program to calculate this anyways. Who can calculate a pattern like this and is up to the challenge?

We'll test this on real planets, but what does the MTF math say?

Can we create a 7" scope with the resolution of an 11" and APO performance this way ?

Due to recent discussions in this very forum, to finally bring the truth and nothing but the truth to light, because I feel controversial today, because it comes up all the time anyways, because it needs to be said, because I just found an excellent flame suit ....
Conventional wisdom (CW) in astronomy:

(1) CW: A larger telescope is more affected by bad seeing
Truth: A larger telescope is affected the same by seeing given same magnification, but it may not live up to its higher resolution potential except on good seeing days (or never, if it's a 24" ). See various threads below

(2) CW: Quality APO refractors are the best planetary scopes
Truth: Quality APOs do perform as well or better than other telescopes of same aperture and quality. They are also the by far most expensive scopes per inch of aperture/photon collected/arc sec resolution, bar a few handmade and rarely sold other types.

(3) CW: The central obstruction in SCTs (or most other reflectors, for that matter) greatly reduces contrast
Truth: The central obstruction in SCTs slightly reduces contrast. Consider that practically all professional telescopes built in the last 100 years (ground or space-based) have a central obstruction, and if they are Ritchey-Chretiens, which many are, a rather large central obstruction at that!

(4) CW: Binocular viewers are only good for bright objects because of the light loss
Truth: Quality binocular viewers are good for all objects. The brain makes up for (almost all/all) of the light loss so that only on very dim objects at the edge of visibility there may be a difference in what can be seen.

(5) CW: Beginners need to learn star hopping first
Truth: Depends on if you like the journey or the destination! Seriously, not finding objects is the largest reasons for beginners to ditch the hobby. Not having to star hop does not preclude anyone from "learning the sky" if they so desire. And, most beginners observe mostly from their light polluted back yard, and star hopping to objects never seen before using a few dozen visible stars is very challenging event for experienced observers to say the least!

(6) CW: Aperture rules
Truth: Almost true. Aperture rules as long as the scope isn't to large/heavy to be used. Resolution increases linearly with aperture, light gathering with the square of the aperture, and weight almost with the square of aperture. But: Aperture = power, not magnification = power (as anyone who ever owned a 50mm Tasco refractor knows quite well)

Woohoo! Let's get this show on the road.
And - it's OK if someone else but the three APO musketeers (it's all in good fun, right?) posts/flames/(or dare I say) agrees!

H

OK, my guess:

none: 6.5 or more
low: 5-6.5
medium: 4-5 (typical big-city suburbia; I think S&T assumes 4.5 for their reporting)
high: 2.5-4
s&^%$d: less than 2.5

give or take half a mag. See http://cleardarksky.com/lp/LAXCAlp.html for impact.

OK ... I think need something between the puny 80mm achromat and the 11" SCT for times like now where it's too hot in the shed to keep the SCT out on wheels and too hot in the yard to assemble it.

I'm thinking medium-focal ratio 5 to 6 inch scope (absolute minimum: 4). Must be carriable in its entirety.

Should be decent for planets (capable of a good image at around 200x); I don't expect perfection but it should perform to what that size optics the particular design is reasonably capable of.

Also, I'd like it on a goto mount - enough light pollution in the yard to make star hopping not much fun and it is a grab-and-go, not grab-and-search scope. That means a dob won't really work out of the box. If a suggested OTA is light enough, maybe I get a 114GT at Costco for 160 bucks and use that mount.

I don't mind it "equilibrating" the scope for a few minutes.

Prefer 2" focuser.

Before you say APO - no way am I spending that kind of money on that small a scope. But, a good achromat would be an option for sure, and so would be any reflecting scope.

Suggestions? How much?

Thanks!

H

OK, which digital camera is recommended for multi-use

- normal pictures
- occasional eyepiece/prime focus astrophotos of planets etc.

fitting the following requirements:

- 2 Megapixel or better (don't need 6)
- under $500 or so
- straight forward operations and meaningful menus - often used stuff should be easily accessible (some cameras are a total mess, even with an engineering degree they are next to impossible to figure out)
- shutter time somewhat controllable for astro
- easily and firmly attachable to a telescope
- good picture quality
- Generic FlashCard or MMC, not some proprietary stick for
memory
- Rechargable battery with reasonable life span (preferred, not required)

Thanks!

Hinrich

I'm sure they meant 247. But hey, I ordered one. At 156 shipped, that isn't so bad!
We'll see what happens ...

Of course this is nearly a religious discussion. Here's what I think : if you compare apples to apples, i.e., similar money invested, an SCT will give you much more aperture in a package of about the same portability (especially at larger apertures), good performance on planets (higher resolution, but less contrast) and hands down much better performance on DSOs. They do complement each other nicely, but since you asking about choosing one over the other, I'd go SCT no question, except if you never want to look at anything but planets and double stars.

Why? If you want a refractor to perform comparable to a much larger aperture SCT on planets, you have to get a good one. A decent 4" refractor, like a Vixen 102, will already cost you as much as or more than a decent 8" SCT and the SCT will perform better - aperture does rule. A really good 4" APO refractor, like a TV or Tak 102, costs - without a mount - as much as fully loaded 10+ inch SCT with goto and the works, and still won't outperform it especially holding it free hand . With another grand into a mount, the stars may be floating on a "velvety" background and the refractor will have better contrast, but you won't see that much more through it as the SCT will have 2-3 times the resolution.

To get decent DSO performance, you'll need at least a 6" refractor and for it to still be refractor-great on planets, you're talking say 10 grand (like a TAK FS-152) which will now easily buy you a 16 inch SCT, or a huge dob. Neither one is very potable, the dob the most still since it comes apart. There are cheaper 6" refractors, but they can't compete with a larger SCT on anything.

The bad rep SCTs sometimes get for planetary viewing is IMHO because many are not collimated right. Doesn't take much being off collimation to underwhelm on planets. It's not hard to fix, though.

That's not to say a really good APO isn't a super scope, and yes, in terms of contrast, they can't be beat. It's just that you are paying a lot of money for a relatively subjective improvement in performance (i.e.,. the "velvety sky" high-contrast thing) in a small area of observing, while giving up a lot in other areas, including DSO and money. Every scope has its niche, what you're willing to pay for it is in the eye of the beholder.


That all said, you'll enjoy any good telescope. Someone else mentioned MAK-Cass's and Schmidt- or MAK- or just plain Newtonians: All still have central obstructions, though maybe a little smaller, so a refractor will still have better contrast. MAK-Casses are pretty pricey compared to SCTs as well, Newtonians may be cheaper but much larger at a given aperture.

There is no perfect scope. I think:
Best compromise: 10”+ SCT, especially if is has a 3 or 4" relatively fast refractor buddy (widefield, so doesn’t really have to be APO).
Planetary only = long focal lenght 3 or 4" APO
DSO only = huge dob
On a budget: Newtonian

First, ask yourself if you want a goto scope or not. There are very strong, almost religous, opionions on both sides of that discussion Not that I want to open this can of worms, but at least if you have noticeable light pollution I wouldn't go without a goto mount.

If you decide to get a goto scope, I'd look at the Celestron NexStar 80GT, a tad over $300 new or used on Astromart for much less. I think Tasco made a very similar one too, some people found these ridiculously cheap a while ago.

I bought one used and like it a lot as a grab-and-go scope, the mount is quite stable, pointing works well, and its optics are surisingly good for such a cheapo tube. Plus, the OTA rides piggyback on my NX11 so if you keep your C-8 (which I would do) then you can do that too

H

I guess the eyes count as "equipment"

I had it done (LASEK, actually) and am sure glad I did. It didn't hurt my night vision any, practically no side effects.

Just make sure your maximum iris diamter is smaller than the treatment area or you will have night vision problems.

H

Tim, what scopes do you have right now? Some may work extremely well with a binoviewer, IMHO much more user-friendly (and cheaper) than a "big" binoscope.

You're on the right track - with two eyes, a lot of the issues you have as a cyclops will be "calculated" out of the picture by the brain!