Mars with 3 different scopes - comparison

StarDust:
Best views of Mars found here!
http://mars.nasa.gov/mer/home/

Davoud:
Perhaps. But this forum is about astronomy and those photos have more
to do with robotics than with astronomy. Try comp.robotics.misc.

Newer toy telescopes hook up with the Iphone [sic], by wifi, BT, than [sic] the phone
pointed [sic] at the sky object and the telescope points there.
How retarded it is?

You mean the mount, not the telescope. My Astro-Physics mount, which
carries two research-grade refractors, a research-grade CCD camera and
a guide telescope with a combined weight of about 41 kg, is
controllable from my iOS devices. Some "toy!" A precision,
research-grade telescope mount controlled by a device that fits in a
shirt pocket. How "retarded" is that? Report back after you have done
it yourself. Until then you know nothing.

--
I agree with almost everything that you have said and almost everything that
you will say in your entire life.

usenet *at* davidillig dawt cawm

On Monday, July 11, 2016 at 11:57:59 PM UTC-5, Sketcher wrote:
Nice images! - and good for showing the advantages of aperture and the disadvantages of (large) central obstructions.

I seem to recall a 'rule-of-thumb' that says an obstructed aperture performs (on planets) like an unobstructed aperture that's equal to the obstructed aperture minus the obstruction diameter. Your images seem to provide evidence that "small" obstructions are less damaging and "large" obstructions more damaging than the 'rule-of-thumb' would imply - though in fairness other variables such as seeing and cool-down may have been factors as well.

Thanks for the great images! My last decent look at Mars was made with Excalibur shortly after watching "The Martian" ;-)

Sketcher,
To sketch is to see.


On Monday, July 11, 2016 at 4:09:27 PM UTC-6, Razzmatazz wrote:
I did some imaging of Mars last Saturday in fair seeing just after sunset with my 175 refractor, 250 Mak and 305 Mak. I started with my 175 refractor which seemed to give the best visual image, probably because the larger instruments had not settled due to the day's heat build-up. It has been quite hot during the day, around 90F inside the observatory, which then drops rapidly to about 70 when the roof is opened at sunset. The seeing was so-so, maybe 6 out of 10 with Mars changing shape rapidly but not tearing sideways.

After shooting Mars with the refractor, I went to the 305 Mak-Cass, but there were some tube currents. I opened the back of the scope to expose the quartz mirror and blew some air across the surface from an ordinary room fan placed some 4 ft away. This immediately stabilized the image and eliminated the plumes off the top of Mars. This actually worked better than an internal fan and added no vibration to the scope.

After some 15 minutes of imaging I switched the camera over to the 10" F14.6 Mak-Cass (back also open to get rid of the heat), and was surprised to see a very contrasty image of Mars with much surface detail. I was able to get some 8 video frame sets of about 1000 frames each before the seeing deteriorated (as it usually does after sunset).

The contrast of the 10" Mak-Cass is due to the very small central obstruction (23%) which makes it perform almost refractor-like. Both the 175 and the 10" were visibly more contrasty than the 12" Mak because this instrument was designed for wide-field imaging and has a 38% central obstruction. Theoretically it is much more affected by poor seeing, and you can see it in the comparison images.

The small central obstruction not only improves contrast, but also reduces the effects of seeing quite dramatically. This is quite easily seen when watching the videos taken with each of the 3 scopes.

Just for comparison I added a simulation of Mars from calsky.com, and sure enough the 10" Mak image clearly shows 3 of the volcanos as well as Olympus Mons to the right of middle. The albedos are different in the simulation (almost inverted), but you can clearly see the giant mountain and the trapezoidal area around it. Lots of other features are shown, with Mars being a puny 15 arc seconds in diameter and very low in the sky. I would love to see what this scope could do when Mars is at opposition, twice as large and high in the sky in excellent seeing.

http://www.astromart.com/common/imag...6.jpg&caption=

Camera is a cheap and cheerful QHY-5-II color camera

Poor seeing favors small apertures on double stars and planets. However, at some point when the seeing gets close to perfect, a larger aperture even with a large obstruction will show more detail.

On Tuesday, July 12, 2016 at 10:07:24 AM UTC-4, Razzmatazz wrote:

Poor seeing favors small apertures on double stars and planets.

I have never found that to be the case.

On Tuesday, July 12, 2016 at 10:54:00 AM UTC-7, wrote:
On Tuesday, July 12, 2016 at 10:07:24 AM UTC-4, Razzmatazz wrote:

Poor seeing favors small apertures on double stars and planets.

I have never found that to be the case.

It's very true. Once, I had my 8" SCT set-up looking at Jupiter and it's moons, good views only lasted for an hour or so, because the seeing got so bad, Jupiterturned to an orange blob. Switched to my 4" refractor and I still had good viewing of the planets through the night.
My first refractor was an old, scratched up Edscorp 3"F15 OTA, bought it on the flea market for $20, LOL! which showed wonderful views of the planets, even on days when my 8" SCT was useless. Use to ask my wife, do you want to see the planets tonight?
She's respond was - Only if you set up that old telescope! She didn't like my 8" SCT!
The old 3" refractor made me a refractor nut!

On Tuesday, July 12, 2016 at 7:10:20 PM UTC-4, StarDust wrote:
On Tuesday, July 12, 2016 at 10:54:00 AM UTC-7, wrote:
On Tuesday, July 12, 2016 at 10:07:24 AM UTC-4, Razzmatazz wrote:

Poor seeing favors small apertures on double stars and planets.

I have never found that to be the case.

It's very true. Once, I had my 8" SCT set-up looking at Jupiter and it's moons,
good views only lasted for an hour or so, because the seeing got so bad,
Jupiterturned to an orange blob.
Switched to my 4" refractor and I still had
good viewing of the planets through the night.

I have never seen anything like that happen. When comparing different sized scopes of (roughly) comparable quality side-by-side, the smaller scope might sometimes show a "prettier" planetary image, but most certainly not a more detailed one, AEBE.

On Tuesday, 12 July 2016 00:09:27 UTC+2, Razzmatazz wrote:
I did some imaging of Mars last Saturday in fair seeing just after sunset with my 175 refractor, 250 Mak and 305 Mak.

http://www.astromart.com/common/imag...6.jpg&caption=

Camera is a cheap and cheerful QHY-5-II color camera

Stunning images! Well done!

You can truly call them "All your own work!" :-)

You've come a very long way since the S&T article on "the refractor which bored holes in the sky." My home written ray trace software showed the prescription was wrong. You had changed the glass from the first lot of [unobtainium] KZNFS1 to KZNFS4 [from very fuzzy memory}. When I contacted Schott to buy some 6" blanks [to try to copy your prescription] they wanted a fortune!