Care to do a experiment

[fizzy123]

To my understanding, SCT have a obstruction of a total of 13% of total surface area, while APO are clear aperature. Thus when at planet works, the contrast is veri important thus APO will win but observing DSO u just need a big aperature as we do not go for veri contrasting details, as most DSO will appear as blurry in eyepiece. 8) :roll:

[IceAngel]

if i compare my 93mm arhcomat and the 80mm APO (the aperture looks not much difference to me),

1: will the Field of View of the APO better than the 93mm on DSO?? (if i wanna take some DSO picture)

2: then for planets viewing, will the 93mm archomat be better for camera image effects?? or the APO will have lesser false colour than 93mm... as i can see my two sets of moon photos, can still see some blue outline in the photo..

before I can answer this, what is the focal length of your 93 mm scope?

the focal length is 1000mm.. is a long tube archomat..

[weixing]

Hi,
I actually had read quite a few article on the effect of obstruction on the web. They use the Modulation Transfer Function (MTF) to study the effect of the central obstruction on telescope performance and found out that the telescope with 30% of central obstruction is delivering better contrast at higher spatial frequencies(high contrast object such as double stars, the rilles on the Moon, and etc) compared to the unobstructed telescope (see the graph)!

Anyway, their general conclusion are as below:

1) the general contrast of the images is lowered, the instrument (diameter D, obstruction d) has approximately the same efficiency as an unobstructed instrument of diameter Deff = D - d

2) the resolution power is not modified on high contrast structures:
Moon, double stars, Cassini division, shadow of a ring or a satellite, edge of a planet,...

3) the resolution power may be lowered on low contrast objects: surfaces of Mars, Jupiter and Saturn. The effective resolution depends on the contrast of the object and the technique used, it is at least the resolution of an unobstructed telescope of diameter Deff

Anyway, I think the effect of the central obstruction on a decent SCT/MCT/Newtonian telescope may be too small for most of us to detect.

Have a nice day.

PS: The below are some of the web pages that I read on this subject. It is very interesting... and don't worry, it is quite short and they explain it quite well(with some interesting graph), so should be easy to understand.

http://www.licha.de/AstroWeb/articles_f ... ?iHowTo=20
http://perso.club-internet.fr/legault/obstruction.html

[VinSnr]

if i compare my 93mm arhcomat and the 80mm APO (the aperture looks not much difference to me),

1: will the Field of View of the APO better than the 93mm on DSO?? (if i wanna take some DSO picture)

2: then for planets viewing, will the 93mm archomat be better for camera image effects?? or the APO will have lesser false colour than 93mm... as i can see my two sets of moon photos, can still see some blue outline in the photo..

1) You are talking about f/11 and a f/7.5 scope. Obviously the f/7.5 scope gives a wider field of view and better suited for astrophotography because it is faster.

2) I don't understand what you mean by camera image effects? However, the APO will of course has lesser colour......in fact no colour at all. That's what an APO for!

[weixing]

Hi,
I suddenly realised we have come back to the old topics on the subject: smaller aperture APO vs larger aperture SCT and obstructed telescope vs unobstructed telescope... I think this are one of the top 10 questions that most people interested in telescope would like to ask and the question will be ask again after sometime..

May be the "old bird" such as VinSnr could write an article on this subjects and post it in the FAQ section, so that next time those who interested can read up the article... or next time when I was asked the same question, I will have a "standard answer" to refer to... ha ha...

Have a nice day.

[kayheem]

Hi,
Recently, someone ask me this question again... "Does a smaller apeture APO can out-perform a bigger aperture SCT?? Because he read some review saying that a 80mm APO perform better than a 8" SCT."

Below is my understanding why people will have such a view:
the 80mm APO will give a more pleasing look than a 8" SCT, because the 8" SCT will grather more light and cause the atmostphere turbulence or the imperfect sky to be more obvious than in the 80mm APO

At the same magnification, they should be equally affected when it comes to poor seeing and the difference is only due to the optical design. There was an article in S&T some time ago that exposed this myth. I can't remember the exact issue.

Kay Heem

[weixing]

Hi,
I also see that article on the web (http://skyandtelescope.com/howto/scopes ... _505_3.asp) and I do agree with them that the light pollution will affect all scope regardless of aperture. But what I mean is whether will the larger aperture make the atmosphere turbulence more visible or not?

Remember seeing the "under water" effect when viewing the moon and planets when the weather is not so good. My point is whether a smaller aperture scope will make that "under water" effect less obvious or not? Since a smaller aperture scope gather less light and had lower resolution than a larger aperture scope, so will this make the view of the moon and planets more pleasing?

Have a nice day.

[MooEy]

interesting experiment. when we use an aperture mask on a larger equipment, we are decreasing it's aperture. but the focal length remains the same. what happens is that the focal ratio will increase by quite alot. dof has the effect of hiding flaws in optics.

if we compare 2 scope side by side that has the same quality of optics, at the same aperture, the one with the longer focal length will have a better view due to the dof.

so now if we take a 76mm f/8 apo and compare it with a c8 stop down to 76mm. we would have a scope that is f/8 compared to a f/26.7 equipment. how fair would the test be?

even in the event we compare a 76mm f/8 apo to a 76mm f/26.7 sct, the apo may still have advantage, due to having lesser surface, and being more well corrected in the first place.

but a test would be more convincing, ignore what i have said. pull out ur scopes tell me the results.

~MooEy~

[weixing]

Hi,

so now if we take a 76mm f/8 apo and compare it with a c8 stop down to 76mm. we would have a scope that is f/8 compared to a f/26.7 equipment. how fair would the test be?

Do you mean that telescope with larger(slower) f-ratio will perform better than a smaller(faster) f-ratio? Idealy, this is not correct, all telescope with any focal length should perform equally.... Then you may ask why longer focal length telescope generally perform better than it shorter length version?

Start of off-topics This is due to the fact that a longer focal length telescope is easier to manufacture than a shorter focal length telescope.... basically the longer focal length of lens/mirror are easier and cheaper to manufacture than a shorter focal length lens/mirror. As a result, the longer focal length telescope will be better and cheaper than a shorter length telescope of the same aperture from the same manufacturer.End of off-topics

OK... since we just add a mask to the C8, it won't change the optical perform of the new "C3" even the focal ratio is larger.... it still the same C8 physically.

Anyway, I think the new "C3" will still have disadvantage compare to a 76mm refractor, because lens and mirror combination will have lesser transmission of light than a lens only telescope.

Have a nice day.

PS: Those who interested in long vs short f-ratio in telescope may wish to read the following article:
http://www.brayebrookobservatory.org/Br ... atios.html

[ChaosKnight]

i did a brief analysis of the problem. My take on the issue is this: bad seeing DOES affect a big aperture more.

Weixing, can i suggest something regarding your experiment? When you mask your scope, can you make the hole at the center, directly in front of the secondary mirror? If you were to place your hole off-axis, i think the degration in image due to atmospheric turbulence is relatively slight.

A better alternative is to use a refractor, with the hole directly in the center. Diffraction due to the secondary will be eliminated.