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Scooot

Big Dobs & Planets

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I've read on here a few times that people don't use or get big Dobs to look at planets, and hence don't need a top of the range mirror. If I had a bigger Dob, say a 16", why wouldn't I want to use it to look at planets when they're about, in the same way that I'd currently use my 10"? I acknowledge tracking is a great help, and an EQ platform helps in this regard, any other reason?

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You can but it won't be as good as a mak. Because there so close and bright I find it washes detail out.

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You can but it won't be as good as a mak. Because there so close and bright I find it washes detail out.

Would a filter help with that?

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The general concern is that larger aperture has finer optical resolution. Finer optical resolution is more affected by 'seeing' conditions so you don't bet any benefit over smaller aperture that doesn't have that finer resolution to begin with. In practise, you may find that a large scope, with an aperture mask gives you the best of both worlds. Light grasp for faint objects and with the aperture mask, you can tune the scope to match the seeing conditions so you get diffraction free views with good contrast and the best clarity the atmosphere will allow. I have had better planetary views from a 10" Dob than a 180 Mak!

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I originally ordered my Sumerian as an all rounder and hoped it would be the only scope I needed for planets and DSO's but the 16" mirror really does suffer if the seeing conditions aren't good.

The best views of the planets I have ever got have come from my 16", nothing has ever come close to the views of Jupiter at Mars that I got last year but those times only happened when I was out of town at dark sites and the seeing conditions were excellent.

I see aperture masks / apodising masks as a last resort though, my 10" will give better views than my 16" masked.

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I see aperture masks / apodising masks as a last resort though, my 10" will give better views than my 16" masked.

Might that be because your 10" has a better mirror Mike?

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The general concern is that larger aperture has finer optical resolution. Finer optical resolution is more affected by 'seeing' conditions so you don't get any benefit over smaller aperture that doesn't have that finer resolution to begin with.

Presumably it's no worse though? I follow that you'd get better contrast with the aperture mask and no central obstruction.

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big mirror comes with its own problems too.One of them is temperatures and climate.If the mirror cant cool down to its best or the weather around is not at its best,that big mirror will never show you its best side.I have noticed that myself on my 14".There where nights where i was struggling to see anything in good quality,from another hand,when the seeing allows,you will be rewarded with great images.

planetary wise,you dont really need massive mirrors as planets are bright objects and as previous posters mentioned,that big mirror will wash out the views due to too much light gathered.i agree with Mike that if you do want to use your big dob on planets,mask it down.Apperture mask can be made home with very little effort.

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In defense of Dobsonians, I've not had many problems with the Skywatcher 300p on Jupiter.

When the mirror has cooled and conditions allow (which is not that often :D), I can see quite a lot of detail in the barges, whorls, spotting and cloud bands. One time I recall seeing quite a few shades of colour; white, greys, creams and browns. You'll be surprised at how versatile these dobs are. Perhaps I have had a happy accident with finding good quality plossls as a winning combination. The only regret I have is that I didn't sketch what I saw so I can prove it.

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I've seen some fine views of the planets with 12" dobs and I bought my current one very much with the moon and planets in mind. It's slightly slower focal ratio (F/5.3), proportionately smaller secondary obstruction, curved secondary supports and relatively high quality primary mirror all contribute to various degrees in delivering high quality planetary and lunar images. My ED120 refractor comes quite close when the seeing conditions are poor or mediocre but the 12" dob draws away when things are good and can show detail that is simply beyond the reach of my smaller aperture refractors.

I've owned SCT's to 8", mak-cassegrains and mak-newtonians too but a well cooled and collimated Newtonian with decent optics is very hard to beat :smiley:  

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Might that be because your 10" has a better mirror Mike?

Well my 16" is 1/6th pv and my 10" is 1/10th but to be honest you will only see the benefits of a well figured mirror on the planets in good to excellent seeing conditions.

My 10" just isn't effected by seeing conditions compared to my 16", masking the 16" down to 4.75" gave lovely high contrast sharp views but lacked the resolution to pull out fine details.

I guess a lot depends on just how interested you are in the planets and how much detail and patience you have in observing them. For a quick fix and great views an aperture mask on a 16" will work very well but I don't think that will better the views you can get through your current 10".

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If I increase the magnification too much with my 10" in poor seeing the image deteriorates. If I was viewing a planet at the same magnification with both scopes, and the same quality mirror in poor seeing, would the view through the smaller aperture be better? If so would I get an even better image with a 5"?

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masking the 16" down to 4.75" gave lovely high contrast sharp views but lacked the resolution to pull out fine details.

Mike, Have you tried to mask down to 6"+? I'm under impression that 16" with 88mm 2:a can be masked down to somewhere around 6.2".

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Mike, Have you tried to mask down to 6"+? I'm under impression that 16" with 88mm 2:a can be masked down to somewhere around 6.2".

No I haven't, the mask I made was pretty rubbish to be honest, when it blew off the dob and landed in a puddle I gave up on it. 

For me at least it all comes down to resolution and image scale and that was the lesson I learnt when I saw the planets though my 16" unmasked and the reason I bought a 10" as a dedicated planetary scope. I prefer to have a mushy view the size of a 5p and wait for moments of better seeing than a sharp view the size of a pea. :)

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For what it's worth :-

Many decades ago when I was considering purchasing a mirror to make my first Newt I had to think carefully cos the mirror and flat were going to cost upwards of 2months wages ( yes, a very long time ago :) )

So I did a lot of reading and it (resolution) seemed to be all down to atmospheric "cell size"  and in the UK we couldnt often expect more than 4" cell width of 'clear' air to look through, sometimes perhaps 6" ( a bit like the gaps between the ripples on the bottom of a swiming pool) Any more and you then look through 2 or more cells _more of the time_ which is even worse and confuses the image.

So I chose a 6" mirror set by David Hinds.

So, that would suggest a mask to stop down to what is good for the night.

If your mask can be rotated you could then choose between 4 positions between the spider vanes and select the best bit of your big mirror ?

Light grasp on fuzzies is summat else ;)

Thinks : if you have 4 holes and a rotating shutter you could blink your big mirror and select the best quadrant ? ??? Ok I'll get my coat , ,  :)

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Point it at the planet and have a look, it's a scope.

Scopes are for observing objects in the sky so point it at one and observe.

I recall looking at Saturn through a 36" reflector 4 years ago - magnificent.

Your 16" won't come close to that.

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in answer to your question, there's absolutely no reason not to look at planets with big dobs. in poor seeing I mask to 170mm. either option gives great views and I can still remove the mask to soak up the photons for faint stuff.

that said, at home I most often use my 12" f4 unmasked and views are fab. double stars improve with a mask for sure.

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Point it at the planet and have a look, it's a scope.

Scopes are for observing objects in the sky so point it at one and observe.

Lol, I will when I get one. - :)

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You can always stop a large scope "down", never a small scope "up".  :smiley:

I can "stop up" my small scope - with the lens cap :D

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