Jump to content


Photo

Central Obstruction question I have always had!

  • Please log in to reply
20 replies to this topic

#1 cpsTN

cpsTN

    Mercury-Atlas

  • ****-
  • Posts: 2502
  • Joined: 26 Apr 2007
  • Loc: Rutherford Co, TN

Posted 09 February 2013 - 07:02 AM

I have been looking up now for more that I quarter century. ..but here is something that I have found to be inconsistant among Planetary observers over the years:

If you talk to Newt owners, most of them will say they want the smallest CO they can get, preferably no more than 20%. BUT, if you look at how many Mak and SC users there are who are planetary observers, whose scopes commonly have obstructions of 30-35%, they aren't complaining about large COs. I have used scopes that have even larger COs and the views were fine. Am I missing a key point to this or is it all in our heads?

#2 Eddgie

Eddgie

    Voyager 1

  • *****
  • Posts: 12960
  • Joined: 01 Feb 2006

Posted 09 February 2013 - 01:36 PM

A central obstruction lowers contrast. The effect of a CO is to take light out of the Airy Disk of a star and throw it into the rings around the star.

When pointed at a planet, every point on the surface of the planet behaves like a star. Light that would be focused into this point in a telescope with no obstruction would be removed from that point and spreand around it.

The result is that a bright feature will spread some of its light to the surrounding area makeing it dimmer (by comparison) and larger, and slighly harder to see.

A dark featrue will have light from a nearby lighter feature scattered into it, making is smaller and brighter, and once again, slightly harder to see.

This is caused by diffraction and there is no escaping it.

But notice that I said it make is "Slightly" harder to see. In most cases, all of the details visisble in a perfect 6" scope will still be visible in a scope with even a fairly large obstructiion (33%). But they won't appear as bright (or dark) and they won't stand out with quite the ease as in the unobstructed scope of the same aperture. There will be some details that start out with so little contrast against their backgrounds that they will not be visible in the unobstructed scope, but the fact is that most details will still be visible.

But they won't be as "Sharp."

"Sharpness" is just a simple way to describe "Contrast" and an unobstructed scope will have slighly better contrast than a scope with a small obstruction, but if you keep the obstructino to less that 20%, the difference is so small that no one can see it. This is why Newt owners that like to do planetary observing will keep the seoondary obstruction to below that size.

Once the obstructions gets to 33%, the diffraction is serious enough that just about any good observer can see the difference between a scope of a given aperture with a big obstruction and a scope of the same aperture with no obstruction.

My EdgeHD 8" does not give quite the same contast as my 6" APO, but I can still easily see most of the detail in the C8 as I can in the 6" APO. But dark features stand out better in the 6" APO. Again, I see almost all of the same features in the 8" scope, but the darker features do stand out more in the 6" APO.

My C14 though has a 30% obstruction, but it still has far more clear aperture (the diameter of the aperture minus the diameter of the obstruction) than my 6" APO, and because it has so much more clear aperure, the contrast is better.

So the obstruction only penelaizes it to perform more or less to the same contrast level as a bit smaller aperture. Most of the detail will still be visible, but it will not be quite as easy to see.

#3 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10430
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 09 February 2013 - 04:41 PM

Hi Charles,

They aren't complaining - kinda - some do. At any rate the overwhelming success of the designs that incorporate these large COs is so positive it outweighs the negatives to the point they sell quite well. I've relaxed my intolerance of big COs since ive been enjoying my C6 as its a great performer and makes Quite an argument in defense. It doesn't have the clarity and definition of a 6" f/8 reflector but the design has so many pluses elsewhere I can live with the *deficienies* .

Pete

#4 Ed D

Ed D

    Gemini

  • *****
  • Posts: 3077
  • Joined: 30 Apr 2010
  • Loc: Sunny South Florida

Posted 09 February 2013 - 08:16 PM

Charles, glad you asked that question. I have often wondered the same thing.

Ed D

#5 Rick Woods

Rick Woods

    Voyager 1

  • *****
  • Posts: 14808
  • Joined: 27 Jan 2005
  • Loc: Inner Solar System

Posted 10 February 2013 - 03:10 AM

I think that the bottom line is, SCT owners have SCTs; they have a larger CO; and those are the facts of life. We make the observations we can, and life goes on. Yes, a 12" Newt owner with a 20% CO will get better low-contrast detail than a 12" SCT owner with a 35% CO; but the SCT owner may have a better eye. Or not.
We all use what we have; and nothing else matters. There's no key point to miss. Different scope designs are, well, different. If someone has a Newt that shows a better planetary image than my SCT, fantastic! - I'd love to look!

Get what suits your situation the best, and use what you have.

#6 Jure Atanackov

Jure Atanackov

    Vostok 1

  • -----
  • Posts: 118
  • Joined: 04 May 2010

Posted 10 February 2013 - 09:53 AM

This is a very complex topic. The final overall image quality of a system with a large CO will depend a lot on its optical quality - a system with high-quality optics will still produce a good image, but a system with mediocre optics will produce a poor image. It also depends on seeing. If you live and observe in an area with very good seeing, the system will still perform quite well. Under poor seeing the large CO will amplify its effects.

My personal preferences are high-quality small-CO Newtonians. I mostly use a 10’’ f/6.3 Newtonian with very good optics (0.993 Strehl primary) and 18% CO. On planets its better than any other telescope I’ve ever used. At my regular observing location (Krim, 1100 m asl, ~1.0’’ median seeing) it regularly beats an otherwise fine 12’’ f/5 SkyWatcher Newtonian (with ~25% CO). It produces far better views of planets than a good C11 I used to use (it star tested at 1/5-1/6 wave).

Buf if my observing location was better, with regular sub-arcsec seeing and small temperature variations during the night, things might be different. I used a Meade 14’’ f/10 SCT for a couple of years. This was at another site with significantly poorer seeing. The telescope never produced good planetary images. Collimation was always as close to perfect as I could get (using a star at very high magnifications), the optics were cooled, but still the planets looked soft and washed out. Until I observed with it from Lastovo island in the southern Adriatic. It’s a small isolated island with a central hill about 400 m high. This location regularly produces excellent seeing, probably sub-arcsec most of the time. The 14’’ would go up to 500X on Jupiter with lots of fine detail. The Gallilean moons were clearly resolved disks, although no surface features were seen. So from this experience I suppose that under really good seeing even a telescope with a large CO produces good planetary images.

For me, high-quality Newtonians work best. If I lived somewhere with regular excellent seeing a large, but compact telescope (=larger CO), would probably be quite appealing.
CS!Jure

#7 Ed D

Ed D

    Gemini

  • *****
  • Posts: 3077
  • Joined: 30 Apr 2010
  • Loc: Sunny South Florida

Posted 10 February 2013 - 10:06 PM

Get what suits your situation the best, and use what you have.


My sentiments exactly, and this applies to scopes, binoculars, eyepieces, etc.

Ed D

#8 Carl12

Carl12

    Explorer 1

  • -----
  • Posts: 76
  • Joined: 26 Sep 2014

Posted 18 February 2013 - 06:56 AM

I'm another one left confused by the discussions about contrast and central obstruction.

My question: does lower magnification overcome contrast reduction in an SCT? In other words, setting up a 6" apo next to an 8" SCT, would the view be as detailed and contrasty in both at the same relatively low magnification, say 200x?

In my opinion this should be the case, but it's something I never tested properly. If my thinking on this is correct, then it must follow that for visual observations, an 8" SCT is a better choice than a 6" apo. The tube is shorter and can be more readily used on a fork, auto driven alt-az. Plus you have the added bonus of (slightly) greater light gathering. The only drawback being tube currents and seeing.

#9 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10430
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 18 February 2013 - 09:17 PM

No to your first question. A sct doesn't need lower magnification to get better contrast. There are different kinds of contrast too so in some cases low magnification is preferable other times as in double stars for example at the diffraction limit, exceedingly high magnification can be beneficial. Sct don't have tube current issues any more than a refractors tube. What they do have is a slower thermal equilibrium waiting time since the mirror housed inside is somewhat insulated from the outside air and do the tube must cool first, then the air inside the telescope then the mirror inside the air inside the telescope. A fan helps.

Pete

#10 t.r.

t.r.

    Skylab

  • *****
  • Posts: 4485
  • Joined: 14 Feb 2008
  • Loc: 1123,6536,5321

Posted 19 February 2013 - 11:24 AM

What everyone is dancing around is this...a typical diffraction-limited SCT with double the aperture WILL (less than double MAY) show a better image than a high quality apo of half its aperture if the seeing allows the SCT to perform to its potential and it is properly cooled and collimated. The seeing, cooling and collimation is the kicker. But, apo aficionados do over emphasize this to drive home their point. So, an 8" SCT made to 1/4 wave should beat a 4" apo made to 1/10 wave IF, IF, IF the conditions are right for it. An 8" SCT will about tie a 5" apo.

IME living in the NE with fast moving weather fronts and a Jet stream that pivots over head, only on less than average seeing nights, will the smaller scope prevail, due to two factors: It is looking through a smaller column of air (thus less turbulence) and the higher quality allows it to keep its diffraction limit ability in these average seeing conditions, where the SCT's will not. Simply stopping down the SCT to a smaller aperture helps some, but can't compete apples to apples with the refractor in this mode. End result, 50% of the time the SCT gets the bang for the buck award! 50% of the time the apo is worth its weight in gold! :p I will always own both to compliment each other. ;)

I prefer the view of the planets with my C11XLT to my AP 130 GT when the conditions allow it. At other times, and admittedly more often the seeing prefers the 130. In addition, I prefer the view in my C6XLT to my previous (all sold because the C6 kicked them out!) 90mm apo refractors most of the time. The reason for this is that the 6" is looking through less atmosphere than the 11" and can hit its performance more of the time. Its quality (1/8 wave) is a tad better than the C11 (1/5 wave).

#11 stanislas-jean

stanislas-jean

    Vanguard

  • -----
  • Posts: 2049
  • Joined: 22 Oct 2008

Posted 19 February 2013 - 02:24 PM

May I suggest to have a look on the below graphs for the assessment of perfect apertures in different seeing levels expressed in terms of FWHM.
For having the actual figure of personal scopes with a PTV level and a known central obstruction it is necessary to combine the corresponding FTM curves.
Surely not exact for some cases of observation but this helps to assess a scope in given conditions and for making comparisons between scopes on the paper.
Contrasts under seeing are lowered and globally quantified and generally the resolution ability is kept but with degraded contrasts more or less.
In average sites, a 6" is well adapted, may be more if some "holes" in the seeing appear oftenly.
An observation report for pertinence shall give data about the seeing well quoted.
References here are coming from "telescopeoptics.net" and the De Suiter book the "star testing astronomical telescopes".
Hope this is an help.
Stanislas-Jean

Attached Files



#12 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10430
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 19 February 2013 - 07:22 PM

Geepers - I wasn't dancing around it - I figured it was a given.

The NE seriously has crummy seeing in the colder months, some good moments in spring and fall and some phenomenal seeing on those godawful hot muggy heat wave summer nights. I haven't done it but seeing how even my 8" can have this diffraction pattern at 500x just lay there with barely a quiver in tired summer heat I'd bet those same nights a C14 would hit its full Dawes limit res . It's a hard way to go other times of the year and virtually impossible in winter here Id wager, but some of those summer nights - I'd bet it'd happen. Maybe even for a 16"?

When things line up - and it's not crazy rare just frustrating in winter for me anyway - I'd bet a C14 would beat any apo commercially made to 7" aperture. A real testimony to sct resolving power is had daily in the solar system imaging forum. No 7" apo CCD can even come near an f4 10" reflector. It's unfair to compare CCD to eye but the simple facts are that the larger sct's trounce the apo images because the apps aren't flat out able to reach the resolution . I've NEVER seen a 7" apo come near a C11 image and frankly Ud bet the same would hold true of a C8 image. Visually the 7" apo would beat it, but I'd bet you big there isn't a CCD image in that forum going however far back that has a 7" apo Jupiter image that's better than the best c8 image. It'd be an interesting thing to uncover, but I've seen some scary good c8 imaging and only so so with the mentioned sizes apo.
But that's the CCD realm anyway. Real as rocks in its own way though.

Pete

#13 Carl12

Carl12

    Explorer 1

  • -----
  • Posts: 76
  • Joined: 26 Sep 2014

Posted 20 February 2013 - 07:10 AM

I wish I could understand the graphs posted by stanislas, they look interesting but I don't know what the axes represent.

Thinking of a light and portable system my choice is between a 4" apo and a 6" SCT, and I think I'd go with the latter. The small SCT won't suffer from the usual SCT problems so much, and its performance should be comparable to the 4" apo.

That said, it would still be useful to hear from those who have personal experience of looking through SCTs and apos side by side, say at a star party.

I continue to have a nagging suspicion that although 6" SCT views of the planets my be as good as in a 4" apo, the stars will not be as tight, even at the same magnification. Is there a way of mathematically determining the size of airy disk and the visible diffraction rings around it, so we could confidently answer this question: "If looking at a star through a 4" apo at 200x magnification, the size of the visible airy disk and rings is XYZ. What aperture of an SCT would we need to have at the same magnification to attain a star image of the same size or smaller?"

#14 leviathan

leviathan

    Ranger 4

  • -----
  • Posts: 326
  • Joined: 29 Nov 2011
  • Loc: Azerbaijan

Posted 20 February 2013 - 07:49 AM

I would choose 4" APO. It will come to termal equilibrium sooner than SCT + it has less light loss (usually 88% main mirror + 88% secondary + diagonal for SCT). C6 has 37% CO - too much for me.

#15 t.r.

t.r.

    Skylab

  • *****
  • Posts: 4485
  • Joined: 14 Feb 2008
  • Loc: 1123,6536,5321

Posted 20 February 2013 - 08:00 AM

If the choice is 4" apo or 6" sct, I would as well. If it were 4" apo or 8" sct, it gets more complicated and I would go for the sct. When you double the aperture going with an sct, it is hard for any apo, no matter how well made, to keep up. Modulation Transfer Function (MTF) is reason for this. The apo may show a more aesthetically pleasing image (tight, sharp and unwavering)but the larger sct will show more detail when seeing allows. I received my first apo in 1993 and have been comparing various apo's (3" to 7") to scts (5" to 11") side by side ever since. My direct comparisons involve 90mm apos to 6" sct, 101mm apo to 5", 6" and 8" scts, 130 & 140mm apos to 6", 8" and 11" scts and finally two 7" apos to an 11" sct.

THe point about star images is a good one and there is fact to apos producing the better image due to the dispersion of energy into the diffraction rings...the bane of scts! That is why many folks have both an apo refractor for stellar viewing and an sct for brute force to bring in light and detail. I assume all of this applies to newtonians as well, however I have no experience with them and will not comment on comparisons there. Another one of my rules of thumb for what its worth(nothing really)...if you read through all the various reports of this apo vs. that mirrored scope of double the size, one can come away with a recurring finding...approximately 75% of what is in the larger mirror can be seen in the smaller apo. If you can live with this, you can get by with just one scope.

In a nutshell...if you have the means for both, get both because they compliment each other beautifully! If you can only have one, I recommend the largest mirrored scope you can get and handle comfortably. If you are a "refractor man" and you know if you are deep down...you know what you have to do and what you have to accept! ;) The best thing anyone can do is get to a star party and take a look and make comparisons for yourself...then you'll know what you can live with and without! :p

Pete, sorry if I came off rude...it was more of a sarcastic blurt...I get excited during these discussions! :lol: ;)

#16 nirvanix

nirvanix

    Surveyor 1

  • -----
  • Posts: 1755
  • Joined: 07 Jun 2007
  • Loc: Saskatoon, SK

Posted 20 February 2013 - 09:09 AM

I read a very comprehensive analysis of central obstruction effects several years ago, but I don't have the link anymore. It stated that CO below 25% would not be detectable by the visual observer.

#17 stanislas-jean

stanislas-jean

    Vanguard

  • -----
  • Posts: 2049
  • Joined: 22 Oct 2008

Posted 20 February 2013 - 09:59 AM

These are FTM curves for 3 different perfect apertures
represented in bolt color blue green orange.
Y axis is the representation of the the contrast levels
X axis the frequency for featuring until the resolution limit of the considered aperture with a transfer ratio of 0 at the ultimate limit (power resolution).
The FTM gives say for a feature size (expressed usually in cycle/mm or inch) the response through a scope lead by the light diffraction laws.
Seeing is considered as a filter expressed in terms of optical roughness that degrade the contrast transfer, as the optical acuracy does also.
The curves are defined for features having a 1:1 contrast level at the entrance in scope. For assessing a feature of 0.10 contrast level just modify the curve with the factor 10% for each frequency.
For a central obstruction scope ratio CO, the related FTM curves are sufficiently published widely, therefore combine the FTM curves and for PTV ratio (optical acuracy) combine also the FTM curves.
The given curves of the previous post combined with the curve for PTV and for the CO will be significantly affected negatively. 1" FWHM seeing equals about images 7/10 in Danjon scale for a 6" aperture, involve also a degradation of the final curve as shown on graphs.
Do the exercise, this is instructive. From a night to an other quote the image of a star close to a planet and perform correlations on a period with a 4" a 6" a 8" with the planet blurring.
You will see at final, big guns are not workable during long times in average sites, not new but quantified.
Indeed the no obstruction scope and the best PTV optics will go deeper with less degradation and blur and the question can be: if a 7" apo actually is well workable if the seeing is average or bad mostly with regards the investment? Somebody can answer as: stay with a 4-5" only for optimum results in your average site.
Stanislas-Jean

#18 stanislas-jean

stanislas-jean

    Vanguard

  • -----
  • Posts: 2049
  • Joined: 22 Oct 2008

Posted 20 February 2013 - 10:11 AM

Refer to
http://www.telescope-optics.net/
you will find many answers through.
From my opinion 25% CO is moderate but still sensitive (see the 1st diffraction ring intensity on a star at high power compared with a refractor same diameter).
I acquired recently a MN 127 with 16% only this is sensitive the difference between the 2 (16% and 25%, practically un- perceptible between 16% and 0).
Stanislas-Jean

#19 gfeulner

gfeulner

    Ranger 4

  • *****
  • Posts: 308
  • Joined: 23 Nov 2007
  • Loc: Bergen county, New Jersey

Posted 20 February 2013 - 12:06 PM

This is an excellent post. A very good question and a lot of very good answers. I'm lucky enough to have a newly acquired older C8 with fantastic optics, a C11 and an AP 6" f8 apo. The C11 will always show more planetary detail than the other 2 when seeing permits. My C8 gives an image similar to my 6" but with a little lower contrast. I use the 6" when I want to enjoy the absolute pristine and beautiful star images that this scope will produce. Gerry

#20 leviathan

leviathan

    Ranger 4

  • -----
  • Posts: 326
  • Joined: 29 Nov 2011
  • Loc: Azerbaijan

Posted 20 February 2013 - 12:59 PM

gfeulner, have you made side-by-side comparison of 6" APO and 8" SCT on planets ? It would be great to read about this.

#21 gfeulner

gfeulner

    Ranger 4

  • *****
  • Posts: 308
  • Joined: 23 Nov 2007
  • Loc: Bergen county, New Jersey

Posted 20 February 2013 - 01:11 PM

I just got the C8 last month and haven't gotten around to setting up both the C8 and 6" to make a comparison but I will do that. I've had the 6" since 1986 and know what it can do on the planets and it looks to me that the C8 has a little less contrast even though it has the starbright coating. I'm waiting until Saturn rises a little earlier to make a comparison. It's not as bright as Jupiter and will hold magnification better. Gerry






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics