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Dobsonian doubts...

Sonia

By Sonia
in The Astro Lounge

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MartinB

MartinB

Posted
Posted

Sonia, the atmosphere is the limiting factor for everyone. 300x magnification is about the limit. The ETS 90 theoretically shouldn't be worth pushing beyond x180. The dobsonian will provide a much brighter and more detailed image at 180 than the ETX. You can comfortably go up to x300 so you will get more magnification when you need it e.g. planets. Whats more on deep sky objects at any given magnification the light catching abilites of the dob will mean that you will catch much more of the detail when looking at distant nebulae and galaxies.

Remember, the Mona Lisa is much smaller than anything painted by Rolf Harris.

Get the Dob Sonia!!!!!!!!! :lol:

Martin

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Naz

Naz

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Go for it Sonia, the dob will give you lots and lots of light to play with, the detail on the deep sky objects will convince you. Also the planets will take on whole new feel. Your astronomy will become much more enjoyable.

honest :lol:

nabban

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MartinB

MartinB

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I am asking this out of genuine ignorance - why will a 100mm scope with a focal length of 2000 be able to produce better magnification that a 200mm scope with a focal length of 1000. Using a 6mm EP will the 100mm produce a better image than the 200mm using a 3mm EP or 6mm with a barlow?

I thought that as a rough rule you could maginify aperture in mm x 2. The field of view of the 100mm scope would need to be 1/4 or less than the FOV of the 200mm scope for it to produce a better image I would have thought.

I have a vague grasp of the physics but don't know how it works in real life :?

Martin

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Kaptain Klevtsov

Kaptain Klevtsov

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The main thing that a telescope does is collect light. The ETX 90 that you have funnels the available light from a 90mm diameter hole to your eye pupil. The result is as if your eye pupil is 90mm diameter, all things being equal, the Dob would give you 250mm worth of light. Compared to the 90mm diameter that you currently have that's 7.7 times as much light gathered.

Once you have got the light you do whatever you want with it. You can use more magnification without the image going too dim, so things look bigger, or you can see fainter, more distant things at the same magnification.

The size of things in the eyepiece depend on the focal lengths of the telescope and the eyepiece. Divide the focal length of the telescope by the eyepiece focal length and you get the magnification, easy sums.

A good rule of thumb is that the useable magnification of a telescope is about 2x per mm of aperture (This copied from the instruction manual that will come with my telescope when it comes so it has to be true)

The 90mm ETX therefore will do about 180X magnification, the Dob. will do 500X. How is that smaller? Its nearly 3 times as big!

Wait until those who really know chip in on this as well but it seems that you have been misled WRT the size issue. In this case size does matter.

PS Put a link in Equipment Discussion in case the Gurus are stuck in there.

Captain Chaos

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GazOC

GazOC

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PS Put a link in Equipment Discussion in case the Gurus are stuck in there.

Captain Chaos

I think the problem is that the "Gurus" :lol: can't give Sonia the answer she wants to hear...in the UK you are stuck at x300-x350 even on the best nights of seeing with the biggest scopes...theres no getting around this,

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Kaptain Klevtsov

Kaptain Klevtsov

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PS Put a link in Equipment Discussion in case the Gurus are stuck in there.

Captain Chaos

I think the problem is that the "Gurus" :lol: can't give Sonia the answer she wants to hear...in the UK you are stuck at x300-x350 even on the best nights of seeing with the biggest scopes...theres no getting around this,

I thought that i said that the ETX would do 180X Mag. as a theoretical limit? How is that answer spoiled by the fact that 300 or 350X is the Max. possible in the UK?

The Dob. will do UP TO the 300 or 350X mag. whereas the ETX will not.

Sonia's original point was that she had heard that the Dob. would give smaller images. That is not the case, unless I'm missing something here?

Captain Chaos

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GazOC

GazOC

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Posted

PS Put a link in Equipment Discussion in case the Gurus are stuck in there.

Captain Chaos

I think the problem is that the "Gurus" :lol: can't give Sonia the answer she wants to hear...in the UK you are stuck at x300-x350 even on the best nights of seeing with the biggest scopes...theres no getting around this,

I thought that i said that the ETX would do 180X Mag. as a theoretical limit? How is that answer spoiled by the fact that 300 or 350X is the Max. possible in the UK?

The Dob. will do UP TO the 300 or 350X mag. whereas the ETX will not.

Sonia's original point was that she had heard that the Dob. would give smaller images. That is not the case, unless I'm missing something here?

Captain Chaos

Capt, my comment was nothing to do with your answer (it's spot on) its a carry over from another thread (one of those Steve?), Sonia still seems unsure of what the Dob will be capable of and how aperture, focal lenght and magnification will affect what she sees through the eyepiece.

No offence meant... :(

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Naz

Naz

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Sonia, try this link and put in the details for your ETX and then the dob, it will give an idea on what to expect.

http://www.scopesim.com/telescope-calculator.html?lid=banner-start

everyone has different opinions / expectations for their equipment, otherwise there be only one scope spec made, only one type of car, etc.

nabban

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FLO

FLO

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PS Put a link in Equipment Discussion in case the Gurus are stuck in there.

Now I know you meant that tongue-in-cheek CC but, Astroman aside, there are no Gurus here; just people with different experiences offering opinions/help/advice and generally promoting amateur astronomy :lol:

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FLO

FLO

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OK, lets be opiniated :(

The unspoken truth is... There is no such thing as a general purpose telescope!

(From an earlier post):

For what its worth... I feel contentment lies in using a telescopes strengths and accepting its weaknesses. Maksutovs are fantastic, high-power scopes that deliver contrast, resolution and acutance in bundles - which is perfect for planetary, lunar and solar (with a solar filter!).

For the cost of a quality 2" diagonal, focal reducer and megawide super-duper eyepiece, you can buy a large/fast Newtonian.

IMHO, 1x Maksutov [or other compound telescope] + 1x Newtonian + 1x 'grab & go' refractor = astro heaven :lol:

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