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In the world of Dobs, does size matter?


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I understand the concept of dobson mirrors, and the bigger the mirror, then the more light can be gathered. I think this concept is true.

However if I had two dobsons, one 8 inch and one 5 inch side by side and only used one eyepiece to swap between them would the view I see from both telescopes be exactly the same size to my eye?

Or would the view from the 8 inch telescope be proportionally bigger?

Cheers

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Based on magnification being the same i.e. focal lengths of the two said tubes being the same, the apparent size would be equal.

However the difference comes from the light gathering ability, the larger of the scopes will gather more light and therefore give a greater resolution which means that at the eyepiece you will get a better clarity of the chosen object and more detail.

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The view from the 8 inch would be brighter, but have a slightly smaller field of view and would be slightly bigger (assuming both F/5), the 5 inch would be less bright with less magnification but would have a wider field of view (again, assuming both F/5).

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I see from your Tag that you have the same telescope as me. May I ask how you made your custom shroud for it?

It's nothing too fancy, all I got was a long roll of Neoprene, cut it to approximately the length of the flex tube (so about half the overall tube length), and then stuck it on the top with lots of selotape, then cut a circle out above the secondary mirror in the Neoprene so you can see :).

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The view from the 8 inch would be brighter, but have a slightly smaller field of view and would be slightly bigger (assuming both F/5), the 5 inch would be less bright with less magnification but would have a wider field of view (again, assuming both F/5).

Sorry mate, but none of that is correct, apart from the bit about being brighter. The answer above yours, is correct.

Russell

Edited by russ.will
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Sorry mate, but none of that is correct, apart from the bit about being brighter. The answer above yours, is correct.

Russell

I was assuming the same focal ratio not the same focal length.

A 5.1 inch telescope (Heritage 130P) with a 26mm 52 AFOV eyepiece gives 25x and 2.08 degrees TFOV

A 8 inch telescope (assuming 1000mm focal length) with a 26mm AFOV eyepiece gives 38x and 1.35 degrees TFOV

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Aperture is king! Just think of a light bucket collecting all that lovely faint light for the Mk1 eyeball.

When I was a kid a 6" mirror was out of reach for most, now we're spoilt with excellent fair priced aperture .Just ensure that you are able to comfortably move your gear !

Nick.

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It seems to me that both explanations have elements of truth. If 2 scopes with different appartures have the same focal length then obviously they have different focal ratios. So although the image that you see will be the same size its fov will be different. The 8" scope will have a wider field of view and be brighter. The increased resolution comes in when you take the magnification past the point that the 5" will go.( I read on somebody's tagline "if you can't explain it to a 3 year old you don't understand it") I think I need help. But in answer does size matter we are all agreed that yes it does.

Edited by rowan46
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But in answer does size matter we are all agreed that yes it does.

I'm getting confused. I understand that a bigger mirror collects more light and as such with a good lens will enhance the viewing pleasure of what is presented before me.

However my question is that if I use the exact same lens for two dobsons, for example my new Heritage 130P and either a 150 or 200 with the image presented before me be of the same size?

As I understand it with the 150 or 200 the image will be brighter because there is more surface area to capture the light but will the actual image be bigger in size when it hits my eye?

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Remember the magnification is the telescope focal length divided by the eyepiece focal length.

Now for Dobs the focal lengths tend to be about the same across the mid range of apertures for ergonomic reasons. Skywatcher's 150, 200, and 250 mm models all have the same 1200mm focal length for example, so would give the same size view with the same eyepieces. The Heritage 130, on the other hand, has a much shorter 650mm focal length so will give lower magnification with the same EP.

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Been away and had a think and couldn't put my finger on what was wrong with my answer. It's that using the same eyepiece bit. Of necessity you are changing magnification. Magnification = focal length of scope divided by focal length of eyepiece. the simple answer is yes you will get a different view How much different depends on the different focal length or the different focal ratio of the 2 scopes. 8" will give a wider brighter image, for the same focal length in other words what Foundaplanet said.

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No sorry misunderstood the question the eyepiece will give a different magnification dependant upon which focal length tube it is in.

example

200p dob 1200mm divided by 25mm = 48x mag

130p 650mm divided by 25mm = 26x mag

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As already said, the magnification will be the focal length of the primary mirror divided by the focal length of the eyepiece, regardless of the diameter of the mirror.

So a 200mm diameter mirror of focal length 1000mm (i.e. Focal ratio F5) will give a magnification of 100 times with an eyepiece of 10mm focal length.

The same applies with refractors, not just reflectors.

Dave

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technically I believe that a larger scope does not make anything brighter as there is nothing can make it brighter than the naked eye; this is why looking through a scope of any size, no matter how big, at the moon cannot hurt your eyes. the brightness of most things in space is 'fixed' (other than supernovae outbursts etc). our sun is the only example of something that CAN hurt your eyes.

as I understand it using a larger aperture will make the object appear bigger than a smaller scope when using the same magnification as you have more resolution and therefore see more detail in it. the net effect is the same as making objects brighter but it's not quite the same thing. it's a bit like an image taken at 1600asa (small scope) against an image taken at 64asa (big scope) covering the same area. There will be more inherent detail in the 64asa even though the images may seem similar at a small print size (low magnification in the scope). zoom in and Increase the image size (equal to more magnification in the scope) and the higher resolution will pay off more and more.

having said all that, the effect at the eyepiece is that DSOs do look brighter (and more detailed) with more and more aperture at the same magnification.

in your scenario using the same eyepiece, I'd expect you to end up with two different magnifications assuming the 8" has a longer focal length. however, using the above logic, you'd almost certainly see the DSO with more detail and more extent than in the 5" scope even though the magnifications are different. it's not really a 'fair' comparison as I would always recommend similar magnifications where possible.

In general terms more aperture is better for visual observing than less but it's not as simple as that as focal ratio comes into it (e.g. a slower focal ratio, say f8 vs f5, will have a smaller secondary and improved contrast over a faster focal ratio. also you get to a point when portability is an issue or where you cannot fit the objects you prefer into the field available. Exit pupil can be tricky too and all of these factors make it essential to meet with people with the scopes you are considering before you buy if this is at all possible.

waffled on a bit there (for a change) but hope you follow it. I am no optics expert and will gladly be corrected if the above is incorrect - we are all learning!

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I understand the concept of dobson mirrors, and the bigger the mirror, then the more light can be gathered. I think this concept is true.

However if I had two dobsons, one 8 inch and one 5 inch side by side and only used one eyepiece to swap between them would the view I see from both telescopes be exactly the same size to my eye?

Or would the view from the 8 inch telescope be proportionally bigger?

Cheers

If you had a 5" dob and an 8" dob that each had a focal length of 1000mm and was using a 20mm eyepiece in each of them your magnification would be 50x. Here, it has no meaning to how big or how small your objective lens is, 50x is always 50x.

The focal ratio of each though is different the 5" would have a focal ratio of f8 and the 8" would have a focal ratio of f5 but your field of view would be the same. So what I said in my first post.

If you had a 130p and a 200p side by side then this would change because they both have a different focal length. The 130p is 650mm and the 200p is 1200mm so your magnification in this situation using a 20mm eyepiece would alter to 32.5x in the 130p and 60x in the 200p therefore the object in view would alter in size as would your field of view..

Hope this with the others has helped..

:)

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