Telescope myths

[Stu]

I found this article when trawling around the interweb for something and found it interesting so I thought I would share it.

A couple of these things such as effects of seeing and LP on different aperture sizes are regularly discussed on the forum. I do agree basically with what Gary is saying, although there are plenty of times when I have seen small scopes give better planetary images than larger ones. As he says though, ultimately this will also be related to cooling or collimation as well as seeing.

http://www.skyandtelescope.com/astronomy-resources/four-infamous-telescope-myths/

[chiltonstar]

An interesting discussion; some of his conclusions do seem to fly in the face of many observers' observations over the years, particularly wrt seeing. It would have been more convincing if his conclusions were based on objective evidence (images or actual data) rather than subjective (visual onservations, his) perhaps?

Chris

[Tzitzis]

It was a nice and fun to read article Stu. Thanks for sharing!

[MarsG76]

Very interesting.. I'm glad about the bigger aperture needing optimal seeing for planetary theory being false... I'd be disappointed if, when one day, I get a 14" SCT it would show less detail than my 8".

[Stu]

Very interesting.. I'm glad about the bigger aperture needing optimal seeing for planetary theory being false... I'd be disappointed if, when one day, I get a 14" SCT it would show less detail than my 8".

I think the reality is, as you go to larger apertures you do see more detail but in poorer seeing conditions you have to wait longer for the moments of clarity, and they are more fleeting than with a smaller scope. Ultimately you do normally see more with the larger aperture but that isn't always my experience!

Don't underestimate the cooling challenges of a big SCT either!

[laudropb]

Very interesting and thought provoking article. Thanks for posting.

[Paul73]

Good article Stu.

I wonder what other myths need debunking?

New Scope = Cloud

New Moon = Cloud

Or,

OIII is only usefull in a small scope.

Little APO's are only good for imaging.

Big Dobs = Spiral arms everywhere.

A UHC is a substitute for dark sky.

There is no point observing under moonlight.

Any more?

Paul

[Moonshane]

I read this a long while ago and it's pretty much a reflection of what I experience. My aperture mask for my 16" dob certainly improves the aesthetic quality of the views (probably in part due to slowing it down to some extent) of planets and doubles maybe in the same way as a refractor does. However I don't get the jaw dropping experience that I get with full aperture even though they are sometimes only occasional glimpses.

I agree completely on the other points.

[timwetherell]

I do think the aperture thing is a real phenomenon but I don't think it's anything to do with atmosphere. I've often heard that the explanation is that turbulence cells in the atmosphere are about 8" across so sub 8" apertures see through just one cell. However from a physics perspective that's nonsense because the atmosphere is more than 8" thick and it moves! The test he performed compared same scope, same conditions, different aperture, so I think quite effectively showed that atmosphere isn't really the cause. It may be more to do with cooling especially as poor seeing often occurs in conditions where warm and cool air pockets are swirling about. 

8" does seem to be about the point where increasing aperture yields diminishing returns in planetary observation but I think that's just because atmospheric seeing is rarely sub arc second at most amateur locations. One day not too long from now, adaptive optics will become available for amateur scopes and then we may all have to put our hand in our pocket for a bigger scope

[MarsG76]

I think the reality is, as you go to larger apertures you do see more detail but in poorer seeing conditions you have to wait longer for the moments of clarity, and they are more fleeting than with a smaller scope. Ultimately you do normally see more with the larger aperture but that isn't always my experience!

Don't underestimate the cooling challenges of a big SCT either!

Another thing to wonder is that if the aperture brought out the worst seeing etc etc etc, then why build telescopes the size of the Kecks or even the newly approved biggest telescope ever... I think called the Giant Magellan... Of course a bigger aperture will have better views and more resolution than a smaller one.

[Stu]

Good article Stu.

I wonder what other myths need debunking?

New Scope = Cloud

New Moon = Cloud

Or,

OIII is only usefull in a small scope.

Little APO's are only good for imaging.

Big Dobs = Spiral arms everywhere.

A UHC is a substitute for dark sky.

There is no point observing under moonlight.

Any more?

Paul

Nice ones Paul, I think you've got most of them covered!!

[Stu]

Another thing to wonder is that if the aperture brought out the worst seeing etc etc etc, then why build telescopes the size of the Kecks or even the newly approved biggest telescope ever... I think called the Giant Magellan... Of course a bigger aperture will have better views and more resolution than a smaller one.

Well to a degree they cheat the seeing by sitting them above the worst of the atmosphere, plus they also use active EDIT adaptive? optics on some of them to compensate for distortions in real time.

Us amateurs have to rough it at the bottom of the hill!

[John]

It's an interesiing article that I've not read for a while so thanks for the link Stu and the chance to re-read it

Over the years that I've been observing and having owned and used 25+ scopes I'd say that most of my personal experiences coincide with Gary's but I do differ from him a little in my personal experiences of aperture and mediocre / poor viewing conditions. What I've found regularly is that while my ED120 refractor very rarely beats my 12" on planetary details and sharpness, under average or less than average conditions, or where an object is not placed well in the sky (eg: Saturn this opposition) the performance "gap" between the 4.7" refractor and my excellent 12" dobsonian is much reduced - much closer than the pretty substantial aperture difference would suggest it might be.

Under good or excellent seeing conditions the 12" pulls away though and is basically "just cruising" at magnifications where the ED120 is running out of puff.

[baggywrinkle]

Excellent article...and some good comments above.

[ollypenrice]

Good article Stu.

I wonder what other myths need debunking?

New Scope = Cloud

New Moon = Cloud

Or,

OIII is only usefull in a small scope.

Little APO's are only good for imaging.

Big Dobs = Spiral arms everywhere.

A UHC is a substitute for dark sky.

There is no point observing under moonlight.

Any more?

Paul

Firstly thanks to the OP for the link. Interesting.

Secondly, I've read only the opposite claim about OIII filters, ie that they only work in big scopes. I've never agreed with either since I get considerable benefit on some targets (Veil and Rosette come to mind) from a basic Baader OIII in both a 70mm Pronto and 20 inch Newt.

Olly

[Paul73]

Secondly, I've read only the opposite claim about OIII filters, ie that they only work in big scopes. I've never agreed with either since I get considerable benefit on some targets (Veil and Rosette come to mind) from a basic Baader OIII in both a 70mm Pronto and 20 inch Newt.

Thanks Olly

You are, of course, correct. I was using a OIII quite happily with my little 80mm just the other day.

The other myth is that I am scrupulous in my proof reading before hitting the "Post" bottom.

How about the myth that you can't see the Veil using a UHC?

Paul

[Stu]

Here's one which will cause some debate...Can you see the Veil or NAN with the naked eye by holding an OIII up to your eye? I've heard it's possible...

[John]

....How about the myth that you can't see the Veil using a UHC?

Paul

Definitely a myth because the first time I saw the Veil was with an ED100 refractor and a Baader UHC-S filter, which is about the "mildest" UHC type filter you can get. Granted an O-III shows quite a bit more contrast, detail and breadth to the nebulosity but you can see it with a UHC. I've even managed to see the Eastern Veil segment with 15x70 binoculars and no filters on a really dark night last summer.

Likewise the Horsehead Nebula can, I understand, be seen with filters other than a H-Beta. UHC type filters such as the Orion Ultrablock  have done the trick for some. Thus far though even the Lumicon H-Beta has not quite done it for me but thats as much about my skies as anything 

Another "myth" that could use some examination I think is that of useful highest magnifications. On some targets under decent conditions I've been surprised how good using very high magnfications can be. On Neptune, for example, I found that 350x - 450x not only contined to show a crisply defined planetary disk but also enabled the moon Triton to pop into view wheras at a "more sane" 260x it was simply not visible.

[swamp thing]

Here's one which will cause some debate...Can you see the Veil or NAN with the naked eye by holding an OIII up to your eye? I've heard it's possible...

Not seen the veil like this but the NA is certainly possible, I have done it many times.

Remember surface brightness does not change with aperture so if an object is big enough (NA Neb) you can see it. Ive heard the rosette is also possible to see in this way. Not seen it myself yet though.......next time its up I will try from a nice dark site.

[ronin]

The smaller aperture and quality of images I can see being somewhat correct, although the object would need to be "bright".

In a refractor the bigger the aperture then in general the greater the spherical aberration so the worse the image quality, I presume there is similar from a reflector - the edges need to be more accurate as the aperture increases. After a while the cost just gets too much.

Concerning filters, OIII and UHC, that I guess come down to people and expectation. An OIII filter removes a lot of light, so in a small aperture the resultant image is reduced in brightness/intensity. I have the idea that we put in a filter that removes 90% of the light and yet expect an image as bright as pre-filter. In effect to only option to increasing the OIII throughput is to increase the aperture. Also the human eye is sensitive to contrast quite highly, not just brightness. So a nebula, naked eye and a filter is very likely to operate with this aspect quite highly.

[swamp thing]

Big Dobs = Spiral arms everywhere.

Seeing spiral structure is dependent on sky darkness and contrast. Without it, its a non starter in ANY scope.

It's gotta be daaaaaarrrrrrrkkkkkkkk man  

[michael.h.f.wilkinson]

Interesting piece, which corresponds very much to what I have observed. My C8 has taken any significantly smaller scope I have looked through to the cleaners on planets (and that includes a TEC140).

[John]

.. An OIII filter removes a lot of light, so in a small aperture the resultant image is reduced in brightness/intensity. I have the idea that we put in a filter that removes 90% of the light and yet expect an image as bright as pre-filter. In effect to only option to increasing the OIII throughput is to increase the aperture. Also the human eye is sensitive to contrast quite highly, not just brightness. So a nebula, naked eye and a filter is very likely to operate with this aspect quite highly.

The thing is though that a decent O-III filter allows through 95% or more of the light that is being emitted by the object we want to see making it stand out against the backgound.

[ollypenrice]

Thanks Olly

You are, of course, correct. I was using a OIII quite happily with my little 80mm just the other day.

The other myth is that I am scrupulous in my proof reading before hitting the "Post" bottom.

How about the myth that you can't see the Veil using a UHC?

Paul

I've seen it clear as day in a UHC. It was the first time I saw it. It was down at COAA about 15 years ago in their 12 inch Dob and I couldn't believe how easy it was. I had one before getting the big Dob going but it suffered a coating failure and I didn't feel like seeing that happen twice so I went for OIII instead, second time around. 

Olly

[cotterless45]

Unsure about the reduction or use of smaller aperture. My own experience suggests some increase in contrast using a slight aperture mask or comparing a 150 achro with a 102 and a 90. I think the 102 certainly is king with planets so far as fracs go.

The myth I'd like in room 101 is that spending gazillions will not produce results. The very best start I had an which gave me the best views was the eponymous 200p Dob, hurrah !

You cannot beat the contrast of dark skies, hopefully our burghers will be switching off the streetlights ,

Nick.