a puzzling question with dobs / newtonians today

[garethmob]

iv been thinking about this for a while, ever since iv gotten more active with my coulter

from what i understand, back in the very early days of dobsonians and the like there was a lot of fast scopes (like the coulters F4) perfect for galaxy hunting and i understand that obviously eyepeices have to be decent, and collimation spot on

but how is it that the most popular makers (celestron, skywatcher, orion usa, meade) dont sell these fast scopes, their around f5 - f6+

is it because of cheapness? or is real galaxy hunting with a light bucket only for the big dobs these days?

[Capricorn]

Just done a search and it seems that the Coulter mirrors were reasonable but no more.

Coulter Reflectors

I would guess that although the chinese can turn out an f/4 mirror that customer expectations have also risen so "reasonable" is not good enough.

To produce a good f/4 10" parabolic mirror will probably be prohibitive, at least to the amateur market and an f/5 or f/6 is a lot easier. To get the edges correct takes more effort and so more expense, and when f/4 the edges need to be very accurate.

It will be more necessary to make the mirror around f/4 once the diameter gets to about 16", 410mm, or above. At least for a manual dobsonian.

[garethmob]

ahh so its the cost of reliably making a mirror repeatable

[swamp thing]

I have an old 1980's "Astronomy" magazine somewhere with the odyssey advert showing the old blue range topped by the 29" monster.

What an absolute beast that was.

They said the square end of the tube was 'big enough to sleep in'. It probably was.

Alas I can't find any pictures on the web to post of one.

Regards Steve

[Dunkster]

Orion Optics VX10 is a bit over f4.5 but VX12 is f3.9 unless my fingers are too chunky for my calculator

Not cheap like SW though.

[swamp thing]

Found one.

Check out the crew of nine plus length of 4x2 needed and open backed truck for quick sep up.

Those were the days, when Dobs made a man of ya.......or killed ya.

Regards Steve

[garethmob]

Wow that is a monster:P. how would you fit that in a corsa? Lol

I see Orion uk have a few but their stupid expensive. Is it really hard to make your own mirror at say f3?

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[JamesM]

Hey Swamp, where's the fuse to light that canon! Love to know how far away the safety net is? Would certainly help you make friends as a star party - wouldn't need a tent as you could just sleep in it when your observing session is over.

James

[garethmob]

So all of this comes to money? And mass production. I couldn't beleive the email I had off a specialised dob company that it's not profitable to make a 8"

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[ollypenrice]

Fast optics are harder, therefore more expensive, to make. We have a 20 inch F4.1 Dob with Beacon Hill mirror and I'm actually surprized by how good it is. While it isn't diffraction limited it is quite clean to the edge using good TV eyepieces.

There are now people making ever faster light bucket sized Dobs because the trouble with going bigger and bigger at F4 is that the FL goes up as well and you end up still looking at faint fuzzies, just smaller and more distant ones. What many people would like (myself included) is more aperture without more focal length, but that's difficult.

Olly

[garethmob]

ahh i see what you mean olly, like a F3 dob at say 1200mms is the focal length dictated in the mirror its self then?

[Moonshane]

now this is what I call a dob! 30" f3.

http://equatorialplatforms.com/Full%20scope%20at%20GSSP2sm.jpg

[Moonshane]

ahh i see what you mean olly, like a F3 dob at say 1200mms is the focal length dictated in the mirror its self then?

the focal length is determined by the curvature of the mirror - more curve = shorter focal length and more aperture for the same focal ratio = more focal length. inescapable unfortunately. for me my 16" f4 is a 'goldilocks' scope.

[garethmob]

ahh i see that explains how the coulter is actually shorter than the 200p

so the more curve the harder it gets to make it and the cost goes up

[Moonshane]

my understanding is that as the light cone is wider with a fast scope, the light is spread out more from the central focus point and you have to work harder on surface accuracy to reduce the effects of this. this takes up more time and hence cost.

physically, there's also more glass to remove via the various grinding processes as the 'bowl' in the centre is wider and deeper with a fast scope.

[swamp thing]

Fast optics are harder, therefore more expensive, to make. We have a 20 inch F4.1 Dob with Beacon Hill mirror and I'm actually surprized by how good it is. While it isn't diffraction limited it is quite clean to the edge using good TV eyepieces.

There are now people making ever faster light bucket sized Dobs because the trouble with going bigger and bigger at F4 is that the FL goes up as well and you end up still looking at faint fuzzies, just smaller and more distant ones. What many people would like (myself included) is more aperture without more focal length, but that's difficult.

Olly

Olly. Correct me if I'm wrong but if you go faster, all that happens is you are forced to use shorter focal length eyepieces. As I believe the exit pupil is the deciding factor in these matters. Your exit pupil being tied in with your aperture and mag. So in effect a fast scope with a 20mm eyepiece or a slower scope with a 30mm eyepiece. It seems to me the view will be the same if they are both using the maximum exit pupil they both will be at the same mag

Not sure I've explained this very well.

What we really need is bigger eyes.

Confused

Steve

[garethmob]

i think i understand what you mean steve i tried yesterday a test on the astral 500 (65mm)with my 30mm vixen an the field of view actually looked like a 25mm view in the megrez 72mm

so the faster you go the higher mag eyepeices / more expensive eyepeices you need

[swamp thing]

Yeah that's the way it looks to me too. The physical limitations of the human eye is the biggest factor. We could go 30" f/2 or 30" f5, the maximum exit pupil is still dictated by the human eye, therefore if the exit pupil maximum is the same on both scopes then it follows that the maximum FOV and minimum mag on both scopes will be the same. I think!!!

It's just how we get there (eyepieces) that will differ.

Obviously the f/2 will be lovely for that feet on the ground observing rather than 'whoa! This ladder is tall' effect.

Regards Steve

[garethmob]

ahhh wooo i understand somthing

so it looks for the near future that the faseter dobs will still be only in the arms of home made or really expensive scopes

does any one know what the cheapest fast dob is?

[John]

...does any one know what the cheapest fast dob is?

Cheap and fast together could be a recipe for a poor scope surely. Fast mirrors, as has been said, are difficult to make to any quality. There are lots of 200mm F/4 newtonians on e.bay which are cheap. I would not want to own one though. All credit due to Synta that the Skywatcher 250PX at F/4.7 is mass produced to a good and consistent standard

[garethmob]

lol to me john cheap is around 4 - 500 so say a 8" synta (200p is around 300? ) a F4 then at around 4 - 500? i know what yo mean tho with the cheap, flower pot scopes ones to avoide

obvious they might not sell many, but it would be smaller, and see more but the price i mentioend starts to go into th 10" as a 10" would be 100 more so into the 12" and then the conflicting aperture is king style, what would you rather

a 10" F6 or a 8"F4? which woul dsee more or go deeper?

[John]

..... what would you rather

a 10" F6 or a 8"F4? which woul dsee more or go deeper?

The 10" F/6 any day. It would go deeper (aperture), be easier to collimate (larger sweet spot), have a smaller by % secondary (more contrast, less diffraction) and do planets and the moon very well too

I wish Synta or GSO made a 10" F/6 - The Orion Optics one costs over £800

[garethmob]

ahhh i see

i guess the same answer would be between a F6 8" and a F4 8"

i didnt releise the higher the F the smaller the secondary

[Capricorn]

I wish Synta or GSO made a 10" F/6 - The Orion Optics one costs over £800

If you wanted to make your own Galvoptics do a 10" mirror at f/5.8 (£315)and f/6.4 (£309).

Suspect that the cost overall could be much the same as the Orion.

[swamp thing]

If you wanted to make your own Galvoptics do a 10" mirror at f/5.8 (£315)and f/6.4 (£309).

Suspect that the cost overall could be much the same as the Orion.

Not sure they're still offering mirrors Cap. A few months back I emailed them about a mirror and was told this was a service they no longer supply.

Maybe worth a shout they may have changed their minds.