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Is a masked SCT like an APO for viewing Mars?

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#1 Guest_**DONOTDELETE**_*

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Posted 04 October 2005 - 08:00 PM

If I make a circular mask for my Schmidt Cassegrain scope that's off-center so that there is no obstruction in the circular area, is this going to make my scope perform like an APO? Specifically, I have a C-11 SCT, and I just measured the distance from the central obstruction to the edge of the aperture, and it's about 90 mm. So if I use a mask with a 90 mm hole in it, will this act like a 90 mm refractor? Even like an APO? For that matter, I could make a roughly elliptical hole with a minor axis of 90 mm and a major axis of maybe 120 mm, just to get more aperture. I realize that the f number will go from 10 to around 30, so that will be very different from a typical apo and prevent me from getting a wide FOV. But for viewing Mars, which one will give a clearer view? Yes, I'll try it, but any predictions? Also, if I use a non-circular hole to get a larger aperture in one direction, what shape is best? I could, for example, just use a mask with a straight edge on one side, and line up the edge with the edge of the central obstruction of the scope, producing something roughly like a semi-circle, if you get my drift... In fact, I could even slide the mask over so that a small amount of the obstruction is "exposed" to get more aperture. Surely someone has thought all of this through before. Also, the same considerations could be made for Newtonians and other scopes with a central obstruction, right?
Dale

#2 JerryWise

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Posted 04 October 2005 - 10:46 PM


Well Dale, my money is on this being uncharted territory. Sure will be interesting to get your report. There is a lot more that goes into an APO other than no central obstruction. The mirrors play a part as well as lens design, spacing, etc. My thoughts are the only thing gained would be a decrease in light gathering ability. Covering the central obstruction doesn't remove the central obstruction from the SCT design, just mask it which it is already doing for itself. But hey, maybe you are on to something. Post it up if it's a breakthrough.

#3 wilash

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Posted 04 October 2005 - 11:03 PM

The shape of the mask will affect the shape of the Airy disk. Whether the grain from less light and less resolution make up for hiding the central obstruction, you will need to test that yourself. I have read about many Newt owners that say the views are always better without a mask.

I am looking forward to your results.

#4 snorkler

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Posted 05 October 2005 - 12:50 AM

I recall reading about some experiments like you propose. Yes, they got APO-like contrast and resolution. OTOH, everything I've read says you'll see more detail with the full aperture of your C11.

Anybody who thinks he/she can beat a C11 with a 4, 5, or 6" APO is deluding himself/herself. Simply put, no 6" scope is going to resolve M13 to the core, while any decent 10 or 11" reflector or CAT will. Why would anyone expect an APO to do better on Mars, if it can't do better on DSOs?

I saw Comet Tempel 1 on Deep Impact night through 17.5, 12.5, and 10" reflectors, then through a 6" AP refractor. It was no contest. The 6" AP gave much worse views than the 10 and 12.5" scopes, and the 17.5 beat them all hands down.

#5 southmike

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Posted 05 October 2005 - 09:22 AM

IMHO i would think you are cutting down the light gathering ability, which is the advantage of a bigger ap. also the mirror is ground to reflect the entire surface off the secondary....that said it is not that hard to test ,with a simple mask..just be careful when you apply it , not to scratch the front plate.

#6 StephenW

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Posted 05 October 2005 - 12:08 PM

>Why would anyone expect an APO to do better on Mars, if it can't do better on DSOs?

DSOs require aperture, planets require resolving ability which is determined by aperture and by the quality of the optics. If you stop down your average SCT to the same size as your APO, chances are the APO will win based purely on the quality of it's optics.

I can confirm though that high quality large optics give very nice views of the planets :-)

Steve

#7 Art Jacobs

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Posted 05 October 2005 - 02:43 PM

Dale,

The reason that people have put off-axis apertures on telescopes has primarily to do with the effect of the atmosphere on resolution. Think of atmospheric turbulence as being "globs" that float through the line of sight of your telescope. When the seeing is bad the globs are small, and when the seeing is good the globs are large. In the case where the globs are larger than the aperture of your telescope the aperture of the telescope will set the limit to the resolution you can get. On the other hand, when the globs are smaller than the aperture of the telescope it is the globs, rather than the telescope aperture, which will limit the resolution. In this case, light which gets into the telescope which passes outside the glob will only serve to degrade the quality of the image.

So, when the seeing is good you should use as much aperture as you have available. When the seeing is bad you might benefit from using a subaperture mask. I am guessing that the people who have reported that the view is always better with the full aperture were either observing with modest apertures or under reasonably good seeing conditions.

As far as the shape of the mask is concerned, if you don't use a circular mask the resolution will not be symmetric. For example, suppose your mask is a long skinny rectangle, and you are looking at Jupiter, which has cloud belts oriented horizontally in your field of view. If you orient the mask so that the long axis is perpendicular to the cloud belts the cloud belts will be resolved. However, if you orient the mask so that the long axis is parallel to the cloud belts the belts will be unresolved. This phenomenon is related to the assymmetry in the shape of the Airy pattern which somebody alluded to in one of the other posts.

#8 BluewaterObserva

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Posted 05 October 2005 - 03:12 PM

I have tested this on a 16" Newt F/4.5 and it did allow me to split some tighter doubles cleaner one night. Atmospheric turbulance is your large aperture enemy on high power planets and doubles stars.

But most large aperture if for going deeper. But on the best nights when the atmosphere cooperates, full aperture should still be best.

#9 Don W

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Posted 05 October 2005 - 03:27 PM

The best views I had of Mars in 2003 were through my Nexstar 11GPS. I see no reason to reduce the aperture of an already good scope. Just make sure it's well collimated and cooled down. Don't give up on it.

#10 Psa19one

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Posted 05 October 2005 - 08:20 PM

I saw this idea explained in a book called "Astronomy Hacks" by Robert and Barbara Thompson. Honestly, this book is one of the most useful, fact-filled books I've encountered on amateur astronomy (and I'm a book nut!). It's brand new and EXCELLENT. In it, they talk about making your SCT or Newt "behave" like an APO -- and they say this trick works (though, of course, they also say that NOTHING is exactly like an APO but an APO!). For planetary and lunar viewing, this method essentially eradicates the central obstruction, thus promoting more pinpoint stars and sharper details (though, at the expense of light gathering abiliies). It's a trade-off, yes. But, I've only read about it...I've not yet tried it. So, I would be very interested in what you discover! I might just try it on my 12" LX200 sometime soon.

#11 jason_milani

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Posted 07 October 2005 - 07:52 AM

Aperture, Aperture, Aperture!! Did i get the point across? :)

#12 revans

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Posted 07 October 2005 - 04:17 PM

Dale,

For many years my main scope was a 16 inch F5 Newtonian that looked like a hot water heater. It was phenomenal for visual study of deep sky objects. I made a variety of aperture masks for it to study the planets. I think I made every size and shape you could imagine. I found that they were only useful on nights of particularly bad seeing. On a night of good seeing it was almost always better to use the scope without any mask. On nights of bad seeing the mask steadied the image and improved contrast.

Rick Evans

#13 Guest_**DONOTDELETE**_*

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Posted 07 October 2005 - 04:33 PM

Masking a SCT or any other obstructed telescope is IN THEORY like converting the scope into an "APO". The idea behind it is that you are converting the entrance pupil of the scope to a circle and hence the diffraction pattern of the star or extended object will conform to the "textbook" diffraction pattern corresponding to an unonstructed aperture - as opposed to one with ~35% CO.

Of course a reflector, an SCT,and to a slightly lesser degree, according to an optical designer I once spoke with, a Mak, are APOCHROMATIC to an equal or GREATER extent than APO refractors. They have either no refractive elements (except the eyepiece, etc) or "zero-power" correctors which introduce only very slight achromatism.

Also the F# goes up by the ratio of the full aperature to the masked aperature so an SCT goes to about F/30 or F/33! This is unlike any APO which are quite fast - however that in itself does not degrade quality of image formation.

What commercial APO's have on their side, beside no central obstruction is impeccable wavefront correction - superior to commercial SCT's and most Mak's. This is due mainly to attention to detail in the fabrication and figuring processes. This is what drives up the price (besides the cost of exotic optical glasses, etc.).This in addition to the unobstructed aperature, as well as excellent baffling gives them an edge in seeing low contrast detail.

If you mask your SCT you'll trade image brightness AND angular resolution for the smaller aperature. You probably won't come out ahead - especially if the seeing is good.

If you experiment with it make the mask so you can rotate it around the perimeter of the corrector and you can atttempt ot find the best corrected optical section that will actively form the image.

#14 BluewaterObserva

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Posted 07 October 2005 - 04:45 PM

I have an 8" mask for my 30", but have yet to try it out. I wanted to on Mars at least once this month.

#15 Guest_**DONOTDELETE**_*

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Posted 09 October 2005 - 08:29 PM

Guys, I appreciate all of the interesting comments. I have not had a chance to try this out, either because of other things going on or cloudy weather. If anyone does try this, please let us know what you see!
Dale

#16 Rich N

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Posted 11 October 2005 - 01:49 AM

I recall reading about some experiments like you propose. Yes, they got APO-like contrast and resolution. OTOH, everything I've read says you'll see more detail with the full aperture of your C11.

Anybody who thinks he/she can beat a C11 with a 4, 5, or 6" APO is deluding himself/herself. Simply put, no 6" scope is going to resolve M13 to the core, while any decent 10 or 11" reflector or CAT will. Why would anyone expect an APO to do better on Mars, if it can't do better on DSOs?

I saw Comet Tempel 1 on Deep Impact night through 17.5, 12.5, and 10" reflectors, then through a 6" AP refractor. It was no contest. The 6" AP gave much worse views than the 10 and 12.5" scopes, and the 17.5 beat them all hands down.


A 6" well made APO refractor will resolve lots of stars in M13. The problem is the brightness of the image.

Mars is very bright this month. The difference in the image between my AP 155mm f/7 EDFS and an 11" or 12" SCT is a fuzzy image vs a sharp image. Last week I had my 155EDFS aimed at Mars and a fellow with either an 11" or 12" SCT had his scope aimed at Mars. We were using about the same magnification. The image in the SCT was not as well defined. You could see that there was lots of detail in the SCTs image waiting to pop out. With a CCD and a sharpening program you can bring out much that detail. But, through the eyepiece the 6.1" APO the detail was noticably better resolved. It was a sharper, higher contrast image with more detail.

Where in the SF Bay Area are you? Do you get out to the TAC star parties? ( www.observers.org )

I'll be happy to meet you at one of the TAC star parties with my refractor.

I've seen a few C8s show nearly as much planetary detail as my 6" APO, but I've never seen a larger SCT show more planetary detail through the eyepiece. It may happen but it doesn't seem to happen very often. Not as often as my APO through its eyepiece.

On a really good night I've seen a little more detail through an AP 10" MC than my AP 180EDT (180mm f/9) APO but the seeing has to be very good and the MC very close to outside temps (several hours of being outside). That AP 10" f/14.6 MC had been lovingly hand figured. Well, all of Roland's telescopes are lovingly hand figured in the final polishing.

Rich

#17 Nocturnal

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Posted 11 October 2005 - 01:08 PM

I looked through the AP at a carbon star (Rich probably remembers which one, I don't know) and the image was superb. Much better than what I was able to do with 'my' C8. Now I'll admit that my C8 has a mirror that's a bit 'rusty' but it appears to be colimated properly. I could barely believe how sharp the image of that star was. No fringing or anything. Just an orange little point.

I'm no expert, yet, but from a quick observation I would prefer a big APO (say 6") for bright objects over a larger SCT. The other night mars was so bright in my C8 I needed to filter it down. Yet seeing (and scope?) were limiting the amount of detail I could see. Once you go to faint DSOs aperture requirements take over. But mars? The *Word deleted by the CN gnaughties gnomes* thing is so bright that what you need is superior optics so you can magnify as far as the seeing allows.

If I could afford it I'd buy an APO myself but I'll probably get an SCT to get started as I want to see DSOs. More " per $. If I was a planetary man I'd buy an APO I could afford.

Rich, I'll come see you again at the next star party and take a closer look at your rig and what visuals it allows.

Sander

#18 snorkler

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Posted 11 October 2005 - 01:58 PM

Where in the SF Bay Area are you? Do you get out to the TAC star parties? ( www.observers.org )

I'll be happy to meet you at one of the TAC star parties with my refractor.


That would be really interesting, a showdown between the big guns. I live in the north bay, and usually go to TAC-SAC observing sites. I went to Shingletown, but not to Calstar this year, so my affinity is north again.

What would we pair up? My mediocre 8" SCT and 10" SN against your 6", or my 18" against your 10"? What parameters? If we each chose 3 targets, you know mine would be dim and fuzzy. Let's start a shoot-out thread, and set up the details there.

#19 Rich N

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Posted 11 October 2005 - 02:13 PM

Hi Sander,

Great! I hope we have a night of good seeing.

Rich

#20 Rich N

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Posted 11 October 2005 - 02:33 PM

Where in the SF Bay Area are you? Do you get out to the TAC star parties? ( www.observers.org )

I'll be happy to meet you at one of the TAC star parties with my refractor.


That would be really interesting, a showdown between the big guns. I live in the north bay, and usually go to TAC-SAC observing sites. I went to Shingletown, but not to Calstar this year, so my affinity is north again.

What would we pair up? My mediocre 8" SCT and 10" SN against your 6", or my 18" against your 10"? What parameters? If we each chose 3 targets, you know mine would be dim and fuzzy. Let's start a shoot-out thread, and set up the details there.


You said, "Why would anyone expect an APO to do better on Mars, if it can't do better on DSOs?". My reply was trying to answer that question.

Aperture wins most of the time. I don't expect a 6" telescope to give better views of deep sky objects than a 10" telescope. However, when it comes to showing low contrast planetary detail it isn't unusual for a well made 6" APO to give better views of Mars, Jupiter and Saturn than say a 10" SCT.

Since you questioned the ability of a 6" APO to give better planetary images, I was suggesting comparing my 6" APO when looking at Mars.

Rich

#21 sixela

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Posted 11 October 2005 - 03:16 PM

However, when it comes to showing low contrast planetary detail it isn't unusual for a well made 6" APO to give better views of Mars, Jupiter and Saturn than say a 10" SCT.

Even the physics support it - for those objects, the contrast transfer function range of interest for rather large but low contrast (i.e. subtle) features is one where a 33% (linear) central obstruction scope won't be better than that of a refractor with only two thirds the aperture - even if the SCT is perfectly cooled and perfectly collimated.

Coupled with thermal gremlins that degrade the view in a much worse way in the SCT and its more demanding collimation, I'd be surprised if your statement weren't true.

I'd wager a bet, though, that SCT can still split doubles slightly better and resolve smaller high contrast features (if it's properly cooled down and collimated).


And I wouldn't want to pitch that 6" APO against a properly cooled down and well collimated 10" Newtonian with a 17% central obstruction...

#22 Rich N

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Posted 11 October 2005 - 10:38 PM

A 10" Newt with a 17% central obstruction must
have a pretty long focal length or give up some
of that 10 inchers light gathering.

I've yet to see a 10" Newt with a 17% central obstruction
at a star party.

I often take my 6" refractor to star parties. I've seen some larger Newts show more planetary detail, but it doesn't happen very often. I have no problem comparing images (double stars or anything else) with other telescopes. If another scope is giving a better image I will enjoy looking at that better image. I have a pretty good idea of the limitations of a 6" refractor.

Rich

#23 jrcrilly

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Posted 11 October 2005 - 10:54 PM

I've yet to see a 10" Newt with a 17% central obstruction
at a star party.


They aren't extremely common, but both my 10" F/7.5 (15% obstruction) and my 12.5" F/8 (17% obstruction) truss dobs made it to several star parties while I owned them. The 12.5" is still in town but I haven't seen it for a while.

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#24 snorkler

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Posted 11 October 2005 - 11:33 PM

You said, "Why would anyone expect an APO to do better on Mars, if it can't do better on DSOs?". My reply was trying to answer that question.

Aperture wins most of the time. I don't expect a 6" telescope to give better views of deep sky objects than a 10" telescope. However, when it comes to showing low contrast planetary detail it isn't unusual for a well made 6" APO to give better views of Mars, Jupiter and Saturn than say a 10" SCT.

Since you questioned the ability of a 6" APO to give better planetary images, I was suggesting comparing my 6" APO when looking at Mars.

Rich


I misunderstood you (do you know how badly I wanted to say "misunderestimated"?). I thought your M13 mention was meant to expand the 6" refractor's benefits away from just the bright planets and bring it to a par with the bigger Newts and SCTs.

Having re-read the thread, I have to agree with Sixela's and Sander's and your assessment that on the brightest planets, the contrast advantages of the refractor will probably help it overcome some of the aperture advantage of the bigger SCTs and Newts.

Should we ever meet at a star party or observing night and do our comparison, I'd be looking for subtle details in low contrast dark areas of the target planet where I'd expect larger aperture to have an advantage. While the APO may give more pleasing contrast in the bright areas, it's debatable whether that constitutes seeing more detail - rather, it gives more pleasing views of the same detail (i.e. the shape of Syrtis Major). Where your APO is likely to have an advantage is when a boundary layer on the mirror of the larger scope hurts its seeing.

I'm probably trying to be too academic and not doing a very good job of explaining. Let's hope we meet and enjoy a good night of observing together through both our scopes, each of which has its advantages.

#25 Rich N

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Posted 12 October 2005 - 04:29 AM

I find it is the low contrast detail that is usually easier to see and better defined in my refractor.

I hope you have a chance to have a look through my refractor sometime.

All the best,
Rich


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