1st Planetary scope: Refractor vs Dobsonian

[cantab]

Would I be right in thinking that a smaller Newt is likely to have a proportionately-larger obstruction, other factors being equal?

[jkwhinfrey]

I think that slow scopes are easier to focus, and that makes a bigger difference than resolution limits under UK skies most of the time. But you can get slow Newtonians as well as Refractors )

[great_bear]

Focal ratio is not relevant it's the size of the obstruction that matters.

I think this is oversimplifying things a bit.

All eyepieces perform better in just about every respect at longer focal ratios, and also it's easier to make more accurate lens surfaces - for both eyepieces and telescope objectives - at longer focal lengths.

[great_bear]

Not looking to do any imaging, hence tracking is not required

Tracking scopes aren't specifically for imaging - they're for planetary visual too. At 240x magnification, Jupiter doesn't stay in the view for very long!

OK - so on a dob you can use widefield eyepieces and track manually - however, widefield eyepieces that work well at dob-like focal ratios are very expensive. On a tracking scope, you can use a world-class, minimum-glass planetary eyepiece like a TMB supermono with ease - and it will cost you less than a quality widefield.

Edited November 1, 2011 by great_bear

[michael.h.f.wilkinson]

Tracking is very useful for visual work. I love the little RA motor in my EQ mount. It allows me to concentrate on observing faint DSOs.

[Capricorn]

Since a Dobsonian is a Newtonian asking 6" or 8" is somewhat redundant. Especially as the poll misses out SCT/Mak.

So we have a poll with 2 scopes of the same type, 6" Newtonian or 8" Newtonian, and does not including a type that many astronomers own and use.

[rightguard]

Since a Dobsonian is a Newtonian asking 6" or 8" is somewhat redundant. Especially as the poll misses out SCT/Mak.

So we have a poll with 2 scopes of the same type, 6" Newtonian or 8" Newtonian, and does not including a type that many astronomers own and use.

i disagree.

difference from 150 to 200 is actually quite alot.

secondly, arent sct's and maks generally more expensive?

[blusky]

Since a Dobsonian is a Newtonian asking 6" or 8" is somewhat redundant.

Dob is about a particular type of an altazimuth mount, so that makes them different for me.

Especially as the poll misses out SCT/Mak.

Just to be clear - am not interested in other types. Those wanting to learn more about various telescope design may read here: List of telescope types - Wikipedia, the free encyclopedia

[blusky]

Tracking is very useful for visual work. I love the little RA motor in my EQ mount. It allows me to concentrate on observing faint DSOs.

Thanks. Dob, (almost by definition) being not an equatorial, is not well-suited for imaging work on DSOs, esp. using extra-long exposures, as the objects distant from the centre of the frame do not remain exactly stationary, correct?

[Moonshane]

incidentally it looks just like that in my 16" f4 (masked to 170mm f11) at this very moment! not quite as big in my eyepiece though.

[vince 66]

If you put a 8" newt and a 6" newt side by side the difference is not overwhelming you have to look hard to see the difference.

[rightguard]

If you put a 8" newt and a 6" newt side by side the difference is not overwhelming you have to look hard to see the difference.

i found a big difference with saturn

[John]

i found a big difference with saturn

Did the 8" show you features of Saturn that you simply could not see with a 6" or was it that the 8" showed the features slightly better ?.

I can see that the 8" would make seeing one of the fainter moons like Enceledus easier but I'm interested what the differences were on features like the Cassini Division, the Crepe Ring, ring segment brightness variations and cloud band / features on Saturn itself.

Just trying to quantify the differences I guess

[rightguard]

Did the 8" show you features of Saturn that you simply could not see with a 6" or was it that the 8" showed the features slightly better ?.

I can see that the 8" would make seeing one of the fainter moons like Enceledus easier but I'm interested what the differences were on features like the Cassini Division, the Crepe Ring, ring segment brightness variations and cloud band / features on Saturn itself.

Just trying to quantify the differences I guess

well for a start i could acutally see the cassini division which i couldnt with the 150, but perhaps seeing had something to do with this, who knows.

secondly, the image was noticably larger and more detail was there.

However, i don't know too much about the planets or anything to do with space, i just enjoy the view.

as such, I have no idea what the other features on saturn you mentioned are!

[Moonshane]

I think the magnification (due to differing focal lengths?) could also be a factor with two different scopes.

[dweller25]

My Skywatcher 8" F/6 Dob beats my 5" F/8 Apo on the planets ......

[umadog]

I think this is oversimplifying things a bit.

All eyepieces perform better in just about every respect at longer focal ratios, and also it's easier to make more accurate lens surfaces - for both eyepieces and telescope objectives - at longer focal lengths.

Yes, I agree. You'll notice more astigmatism from poorly corrected eyepieces in a faster scope, more coma, etc. The benefit of longer focal ratios is that you get a nice well-corrected view to the field edges without needing a coma-corrector or expensive eyepieces. A slow scope can get away without a fine-focus knob on the focuser. These are the benefits of a slow focal ratio. They are real benefits and are most apparent when looking at star fields.

For planetary observing, however, the focal ratio is one of the least relevant things. Don't forget that at high power with a small object in the centre of the field there are no problems with eyepiece astigmatism or coma. However, what I was really getting at is that when the issue of "planetary scopes" comes up one often sees comments about long focal ratio scopes being better for planets because the central obstruction is smaller. This is such a over-simplification that it's usually wrong.

Edited November 2, 2011 by umadog

[rory]

i disagree.

difference from 150 to 200 is actually quite alot.

secondly, arent sct's and maks generally more expensive?

yes but the point is the poll is asking you to pick a 6 or 8 inch newtonian . well all would agree an 8 inch is better for gathering light.

so asking 6 inch newt or 8 inch dob in a (what can i see capacity) is redundant in that respect .

[michael.h.f.wilkinson]

Yes, I agree. You'll notice more astigmatism from poorly corrected eyepieces in a faster scope, more coma, etc. The benefit of longer focal ratios is that you get a nice well-corrected view to the field edges without needing a coma-corrector or expensive eyepieces. A slow scope can get away without a fine-focus knob on the focuser. These are the benefits of a slow focal ratio. They are real benefits and are most apparent when looking at star fields.

For planetary observing, however, the focal ratio is one of the least relevant things. Don't forget that at high power with a small object in the centre of the field there are no problems with eyepiece astigmatism or coma. However, what I was really getting at is that when the issue of "planetary scopes" comes up one often sees comments about long focal ratio scopes being better for planets because the central obstruction is smaller. This is such a over-simplification that it's usually wrong.

The effect of central obstruction is generally exaggerated, agreed. However, even on-axis, spherical aberration in the EPs depends heavily on the width of the light cone. It does not show up as a gradual degradation of the image quality from centre to edge (as with coma and particularly astigmatism, but in an overall lowering of contrast and sharpness. Dealing with a wide light cone means the EP runs into similar problems as fast camera objectives.

[michael.h.f.wilkinson]

yes but the point is the poll is asking you to pick a 6 or 8 inch newtonian . well all would agree an 8 inch is better for gathering light.

so asking 6 inch newt or 8 inch dob in a (what can i see capacity) is redundant in that respect .

I brought up SCTs only to highlight the (lack of) problems with large central obstruction.

[Moonshane]

Yes, I agree. You'll notice more astigmatism from poorly corrected eyepieces in a faster scope, more coma, etc. The benefit of longer focal ratios is that you get a nice well-corrected view to the field edges without needing a coma-corrector or expensive eyepieces. A slow scope can get away without a fine-focus knob on the focuser. These are the benefits of a slow focal ratio. They are real benefits and are most apparent when looking at star fields.

For planetary observing, however, the focal ratio is one of the least relevant things. Don't forget that at high power with a small object in the centre of the field there are no problems with eyepiece astigmatism or coma. However, what I was really getting at is that when the issue of "planetary scopes" comes up one often sees comments about long focal ratio scopes being better for planets because the central obstruction is smaller. This is such a over-simplification that it's usually wrong.

I am sure that you know much more than I do Umadog but I can only say what I see. This is almost certainly a result of seeing conditions as I live in the UK, rather than scope type, focal ratio or anything else. This backed up by my comment below re my 12" dob.

When using a large fast reflector (e.g. 12" f5 or 16" f4) I have found that the planetary views are much brighter and therefore the contrast is more difficult to 'control' to the point where you see more detail. With a slow small reflector (e.g. 6" f11) or where the aperture of a large fast reflector is masked to create a small slow reflector (e.g. 6.7" f11 unobstructed), the views are slightly dimmer but very much (more consistently) sharper, have more contrast and detail visible and focus is much easier to find.

As mentioned I believe these to be in the main due to seeing as on those rare occasions when seeing is excellent, the view through a large fast reflector (given the better aperture resolution) is incredible. Another good thing about a longer focal length / slower speed is of course that the image produced is larger for a given eyepiece and therefore requires to be 'magnified' less.

[michael.h.f.wilkinson]

I am sure that you know much more than I do Umadog but I can only say what I see. This is almost certainly a result of seeing conditions as I live in the UK, rather than scope type, focal ratio or anything else. This backed up by my comment below re my 12" dob.

When using a large fast reflector (e.g. 12" f5 or 16" f4) I have found that the planetary views are much brighter and therefore the contrast is more difficult to 'control' to the point where you see more detail. With a slow small reflector (e.g. 6" f11) or where the aperture of a large fast reflector is masked to create a small slow reflector (e.g. 6.7" f11 unobstructed), the views are slightly dimmer but very much (more consistently) sharper, have more contrast and detail visible and focus is much easier to find.

As mentioned I believe these to be in the main due to seeing as on those rare occasions when seeing is excellent, the view through a large fast reflector (given the better aperture resolution) is incredible. Another good thing about a longer focal length / slower speed is of course that the image produced is larger for a given eyepiece and therefore requires to be 'magnified' less.

Apart from the effects of spherical aberration in the EP as I suggested earlier, the effects you describe could have other causes. If only the amount of light is the problem, a moon filter would help more than stopping the scope down. This is because resolution should be higher, and the amount of light the same. Another factor is of course seeing. In smaller scopes, seeing predominantly shifts the entire image of a stellar object, whereas in larger scopes blur, but less motion is the result (because multiple seeing "cells" contribute to each star image along the line of sight). The human mind can deal more readily with motion than with blur. The tipping point is somewhere near the 6" mark. Finally, the figure of the outer part of mirrors is quite frequently less accurate than the centre. I know of planetary observers who stop down a 10" scope to about 9.5" just to get rid of the outer ring of the primary.

Edited November 2, 2011 by michael.h.f.wilkinson

[umadog]

We too live under the jet stream and have bad seeing. Very frustrating! I've looked through a variety of scopes around here and so far a well made 18" f/4 has given the best Jupiter views. I too found this rather surprising to be honest, but it's true. Also, it's not better by a little bit, it's better by a lot and it's better consistently. Even first-timers can see the difference. It's true the view is brighter, but I find this helps one to see colours, such as the purple streaks along Jupiter's equator.

I've also had some very disappointing views through larger scopes where everything looks mushy and unpleasant. So I know what you're talking about. Making a large scope well is very hard and some of these instruments may have defects. Also, having a fast scope correctly dialled in takes a little more effort. Cooling is very important with a large mirror. When these things aren't done right the instrument gives bad views and this is blamed on the seeing, the image brightness, or any number of other things.

It is true, of course, that to make a faster scope work well you may want a Paracorr and you do need nicer eyepieces. So that costs more. However, the quality of Chinese mirrors is pretty good now. I had some rather good views through a 12" f/5 Orion scope.

[umadog]

seeing predominantly shifts the entire image of a stellar object, whereas in larger scopes blur, but less motion is the result (because multiple seeing "cells" contribute to each star image along the line of sight).

Hmmm... I see plenty of motion shifts in Jupiter's moons through an 18". Even double images of the moons for fractions of a second. I have not experienced a situation where a smaller aperture gives better results. A well cooled and well collimated larger scope has always done better. I suspect this larger scope and seeing thing is a myth. S&T even ran a piece on it a while ago: Four Infamous Telescope Myths - Visual Observing - SkyandTelescope.com It's true that masking out a bad edge can help, though.

Larger and faster scopes have their problems, but these problems are collimation, cooling, and quality control during manufacturing. All of which are more critical when you push the envelope. Secondary obstruction size, and perhaps seeing and focal ratio, are often blamed for poor performance when in fact they aren't the root cause anything.

Edited November 2, 2011 by umadog

[michael.h.f.wilkinson]

Hmmm... I see plenty of motion shifts in Jupiter's moons through an 18". Even double images of the moons for fractions of a second. I have not experienced a situation where a smaller aperture gives better results. A well cooled and well collimated larger scope has always done better. I suspect this larger scope and seeing thing is a myth. S&T even ran a piece on it a while ago: Four Infamous Telescope Myths - Visual Observing - SkyandTelescope.com It's true that masking out a bad edge can help, though.

Larger and faster scopes have their problems, but these problems are collimation, cooling, and quality control during manufacturing. All of which are more critical when you push the envelope. Secondary obstruction size, and perhaps seeing and focal ratio, are often blamed for poor performance when in fact they aren't the root cause anything.

You still see motion, even in quite big scopes (much depends on the size of the seeing cells). The doubling effect is in fact a result of different parts of the wavefront being influenced by multiple seeing cells, and is more likely in intermediate size scopes (for professional astronomers, 18" is not that much;)). In the limit to very large scopes (think Hale) motion is neutralized by averaging out the motion over very many seeing cells. Blur is then the net result. Adaptive optics is the cure for that.

Many myths also stem from comparisons between say a TEC 140 and some cheap 1/4 PV 10 Newtonian (not necessarily well-collimated). In that case a TEC might win. However, a high quality, well-collimated 10" Newtonian should leave even a TEC 140 in the dust in planetary observations.

I agree with your last point: for me a bigger scope has always done better.