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Sweet spot f-ratio for visual observation with a newtonian design?


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So if we're looking at newtonian telescope of let's say 8-12" aperture, what's the sweet spot f-ratio if you don't care about imaging, but do want to observe more than just the Moon and Planets? We also want to be able to observe DSOs and would like a decent field of view, not a keyhole. 

Let's assume we're going to get Orion Optics to make the scope to order. So that we can assure mirror quality and select our f-ratio as we wish. 

I think we can get OO to set up it for both dob and GEM mounts so let's assume we can make that choice and the scope may be used for either. 

Let's also assume 'typical' UK viewing conditions. So contrast is definitely something we're interested in optimising for. This isn't going to be used up a mountain in a desert someplace, but from a suburban back garden or similar. 

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if you have a luna plantary bias a longer F ratio and smaller secondary is the way to go.

the only issue you get with the longer F is the lenght of the tube, if your looking at OO give them a call, they were more than willing to make me a F10 10 inch but it would of needed a bit of a russ section as they only make the tubes to a maximum lenght.

a F10 8 inch will be a great scope for luna observing.

Edited by Earl
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Would have said around f/7, say f/6.5 to f/7.5, even here I think f/7 - f/7.5 not so much the f/6.5.

Making the mirror profile accurately is not difficult, so final image quality should be good..

You have the option to drop a 5mm eyepiece in for getting to higher magnifications, or keep it lowish, 20mm to 25mm, for a wider view, eyepiece are easy (using present options a GSO/Vixen/BST/X-Cel will work pretty good).

With the good span of magnifications available you can view DSO's or planets - this aspect being aperture dependant.

The "problem" is on say a 10" or 12" f/7 the focuser+eyepiece can be too high.

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Personally I feel that the 10" F/6.3 Orion Optics with a 1/10th wave PV primary mirror, would be a really good choice for observing the moon and planets. It would be very good on deep sky objects too of course loosing just a little in potential field of view terms to the F/4.8 in that aperture. The 10" F/6.3 is feasible on something like an HEQ5 mount for visual observing wheras the 12" F/5.3, which I have on a dob mount, would push even an EQ6 to it's limits.

I realise that F/6.3 and F/4.8 are "stock" focal ratios for Orion Optics but I was also thinking of the coast and timescales for them to make a bespoke focal length. Sometimes even their "stock" focal ratios take quite a while to put together !

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Going from a long focal length C11 to a much faster newt of around f/5 may not be a favourable experience for you as coma can take some getting used to.

Taking this in to account I would have thought a middle of the rd f/7.5 might suit you as this would be easy on eyepieces, give you a better FOV over the C11 yet still perform on planets. But a long focal length newt has other factors that you have to take in to consideration. I think other than the obvious stray light issues with this scope http://www.ebay.co.uk/itm/222mm-8-75inch-F7-5-Newtonian-telescope-/191467303463?ssPageName=ADME:X:AAQ:GB:1123 it went for pennies because 1) it is simply too impractical a scope to use on an EQ mount 2) unless you have an EQP it's pointless as a dob as at f/7.5 my guess is it's better suited to planetary observing and tracking at high magnifications would no doubt soon become a chore & 3) not that it really matters but it was dam ugly :D

I guess my point is, if I were going to go in to the expense of having a bespoke scope from OO I would make sure it was a useable size or included in the budget an EQP.

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12" f4 is my goldilocks scope. It is awesome. Compact and light, great on all objects, wide field for aperture (1.6° at 50x with paracorr and 26mm nagler), very rigid and stable, cools quickly, seated observing in a garden chair, no balance issues.

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Hmm food for thought.

The 'custom scope from OO' thing was a bit of a thought experiment rather than a serious plan btw. Just wanted to avoid limiting the discussion to actually available f-ratios. 

What got me thinking was this article about super-fast monster dobs used with a Paracorr - http://www.televue.com/pdf/Astronomy_2011_Jan.pdf

... and various other stuff I've read extolling the virtues of long f-ratio newtonians ...

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My personal preference is for a slightly slower newtonian for a number of reasons, one perhaps being that I'm probably not as adept at collimation as Shane is ! :smiley:. Another is that I like ultra wide eyepieces but I don't want to have to use a coma corrector.

When they get much over 10" though there are mounting and eyepiece accessibility challenges with long newtonians so practicality kicks in. I don't want to start having to use a ladder to reach the eyepiece really :smiley:

In reality I've never had the funds to commission a scope entirely to my spec but if I did it would be a slow newtonian with curved secondary supports, the smallest secondary that would illuminate my eyepieces and a top quality primary and secondary.  But thats just what would "float my boat". Others will have different preferences of course :smiley:

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I must admit I'm not too experienced with newtonians of any description, so I'm a bit hazy on the trade offs involved with paracorrs and the like. 

My refractor is optically gorgeous, but lacks the aperture to deal with heavy light pollution and my C11, while much better at piercing the murk has a rather restricted field of view. 

So I'd sort of vaguely wondered about the practicalities of 10" dobs. Hence this thread. 

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  • 1 month later...

A couple more specific questions for people who have them if I may? 

Do you need a Paracorr or similar with the 10" f4.8 version?

Roughly what is the eyepiece height from the two 10" dob options, f4.8 and f6.3?

Can you feasibly buy a Losmandy dovetail and stick it on an Az-Eq6 with the hope of imaging? 

Would being in an observatory of some kind make the difference to the GEM imaging scenario?

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A couple more specific questions for people who have them if I may? 

Do you need a Paracorr or similar with the 10" f4.8 version?

Roughly what is the eyepiece height from the two 10" dob options, f4.8 and f6.3?

Can you feasibly buy a Losmandy dovetail and stick it on an Az-Eq6 with the hope of imaging? 

Would being in an observatory of some kind make the difference to the GEM imaging scenario?

Eyepiece height of my OO f4.8 10" is 122cm at zenith. I use a Paracorr but I'd say I only really notice a difference using my 26mm Nagler, the others are ok without it. The 26mm Nagler is not an eyepiece you'd use a lot with it.

I find this a great all round visual scope, the views of planets and the moon are tremendous as are many clusters. Galaxies aren't so good where I am but that's because of light polution.

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A couple more specific questions for people who have them if I may? 

Do you need a Paracorr or similar with the 10" f4.8 version?

Roughly what is the eyepiece height from the two 10" dob options, f4.8 and f6.3?

Can you feasibly buy a Losmandy dovetail and stick it on an Az-Eq6 with the hope of imaging? 

Would being in an observatory of some kind make the difference to the GEM imaging scenario?

The eyepiece height of my OO 10" f6.3 on just the dob mount, and pointing up at the zenith is 152 cm.

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A sweet spot? Yes - the one that means the scope is sufficiently long that you're not breaking your back and snapping your neck trying to view something at 30deg elevation, but maybe that's just me getting older and less flexible.....

Joking aside, it encompasses a lot different issues, especially if you don't want a scope for every occasion and/or like to hit a lot of different types of targets in a night.

For my money an 8" F7.5 would be a good blend of big enough to do to some fuzzies, without running up against UK seeing at higher mags on planets on all but the worst nights. At F7.5, it will pony up 1.5deg FOV and 43x from a 35mm EP with a 4.7mm exit pupil and still manage 250x with a 6mm EP at a manageable 0.8mm exit pupil. There will be no need for a coma corrector. Assuming the modus operandi is Dobsonian, it will be long and short enough for the majority of people and you will be able to lift it easily.

That's my Swiss Army Knife of a Newt, or a scope of the common man, if you prefer.

I saw that article before and note that it does ignore two minor facts. For one, whilst you may be able to stand at the EP of a 22" F3 Newt, the sticker price for entry into that club is high enough to make a Takahashi/APM fetishist blush. Two, as I alluded to above, it's a good job it's a low mag uber light bucket; based on my experience of  a 12" Newt where it only really sang on planets on about 3 nights a year, the 22" should be useful only once a blue moon, under typical UK skies, for that purpose. Mind you, if you have that sort of folding to spend on your hobby, you can afford the good frac you'll need to partner it!

Russell

Edited by russ.will
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Yep, would be a first class scope and accessories for anyone prepared to make a trip to Cornwall.

Sad to learn that Mike73 is stepping out of the hobby though and will certainly miss his sketching projects.

+1!

I hope Mike73 won't mind me posting it: but his sketch archive is really great. In the same vein that Turn Left is so relevant, his expansive collection of sketches give a sense of what can be seen in a scope. After I've seen something new, I often search out sketches from others for comparison. Thanks for sharing/building the website Mike73 + best of luck.

http://www.pbase.com/mike73/root

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It seems you can make a case for every F ratio! F5 lies somewhere near the fast end of the middle with plenty of widefield possibility and fair tolerance in terms of colimation, coma and choice of EP. At the apertures you're suggesting, length wouldn't be too much of an issue other than on an EQ for the longest options. I have a 20 inch F4.1 and a guest brings a 20 inch F5 from time to time. The difference in EP height borders on the terrifying!!

Olly

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