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Brad737

Not your typical 8" vs 10" Dob question...

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Hello,

I've owned my 90mm Skywatcher Virtuoso Mak for less than a week.  It was specifically purchased as a grab-n-go scope, and camping trips.  And as you may have seen in my other post, I LOVE IT!  I knew from the get go that I'd be looking to buy a nice Dobsonian scope for home use.  Like all newbies, I'm wrestling with the decision of an 8" Dob, or a 10".

I know now that everyone says "APERTURE RULES!!!"  And I know it's because the larger aperture gathers more light. But here's what I don't really understand yet. What exactly is the benefit of greater light gathering?  I've read that it will allow you to find dimmer DSOs.  But will the image through the eyepiece be bigger, using the same eyepieces?  Will the image be clearer?

As for my intended usage, right now we're giggling like school kids looking at the Moon, Staurn's rings, etc. I really don't intend on mapping out all of the DSOs out there. But I would love to see some things like nebulae, galaxies, etc.

I've tried googling images showing the comparison of 8" vs 10" images, but they almost always just show photos of the actual telescopes. i like the idea of storing a smaller scope, but if the views are much better with a 10", I'd be happy to pay a little extra.

Besides just saying "aperture rules", can you please tell my exactly why YOU would recommend a 10" Dob?

Thanks,

Brad

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It all depends on what you want to view for me its the wide expanse wide field view and aperture is a big hindrance if only you could have both low magnification and aperture at the same time.

Alan 

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Just to help you answer yourself two of your questions:

- The size of the image depends on the magnification and the field of view. Using the same eyepieces (same EP focal length and apparent FOV) the difference will hang on the difference of focal length of the telescopes. Try simulating eyepiece view in Stellarium to compare with the telescope choices you consider.

- Another very important aspect to consider is telescope size and ease of use. If you are not too worried about seeing fainter DSO, think that a 10" scope is necessary bigger, heavier and clumsier than an 8", but perhaps not by a too large margin though!

Good luck!

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Welcome to SGL and to the vast and fun world of astronomy! 

In my opinion the first thing you should consider before buying a scope (putting budget aside) is how dark skies you have in the place you live. Bigger apperure, fainter DSO's for sure BUT only if you live under reasonably dark skies. If your skies are light polluted there will be a limit in what you can see and HOW  you can see it (details,luminosity etc)

As for the view in the eyepiece (EP) you have to consider the focal length (the distance between the primary and secondary mirror f. e 1200 mm for a 8"dob). That diameter / the EP mm gives you the magnification of the EP. There is a limit though in how much mag you can actually use and be able to see clearly.

If you live in nice dark places then go for it. If you don't then think about it. Listen to the opinions here in SGL. 

How much light a mirror can gather is very important. Photons travel million miles to come to you mirror and the bigger it is the more photons it can gather and focus them to your secondary mirror that gives you your view. So a bigger apperture gives you brighter images because it gathers and focuses more photons of the same object :) 

Edited by Tzitzis

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Deep sky objects will look brighter, more extensive and have more contrast in them with a 10" aperture scope compared to a 3.5" scope. Also a range of objects that are simply not visible in the smaller scope will become visible.

Add to that, planets and the moon showing more detail and detail that is at the limit of a 3.5" scope becoming more routinely viewable and thats what additional aperture brings.

I've not mentioned the ability of the larger aperture scope to handle higher magnifications because I don't regard that as a high priority but if conditions are decent then 180x, which is at the limit for the 90mm mak, would not have a 10" scope really pushed at all.

I had similar reservations myself and for quite a few years a 4" scope was my maximum but when I got an 8" dobsonian I realised what aperture brings - and I was glad to have it !

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When hunting for deep sky objects, a larger aperture means a brighter image. It means that everything will appear clearer, as there's more light getting to your eye, and this results in an easier find. Increasing the aperture is like increasing your pupil size - a larger pupil (like when your eye adjusts to low light) means that it's much more sensitive. We want a larger aperture because it enables us to see the fainter things out there, however the difference between an 8" and 10" isn't going to be massive. I think you should get the 8" then save some money for other accessories - better finder, books, collimation eyepiece ect.

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A large scope cannot actually make an object brighter than you see it with the naked eye. I know that is counter intuitive, but it is true.

What a large scope allows you to do is maintain image brightness at a larger image scale ie same brightness but larger image. Your eye perceives contrast better with larger objects, so that 's why the object appears brighter in the larger scope. Technically it is related to the exit pupil achieved. This is calculated by the aperture divided by the magnification, or alternatively the focal length of the eyepiece divided by the focal ratio of the scope.

The Skywatcher 150P, 200P and 250PX all have the same focal length of 1200mm which means with the same eyepiece they will produce the same magnification.

A 25mm eyepiece gives x48 in each scope, which gives exits pupils of 3.1mm, 4.1mm and 5.2mm, meaning the image will be the same size in each, but will be increasingly bright as the aperture increases.

An alternate way of looking at this, would be to see the magnification you can achieve in each, giving the same exit pupil and brightness.

Let's pick a 3mm exit pupil. This can be achieved with magnifications of x50 in the 150P, x66 in the 200P and x83 in the 250PX. Note that these are hypothetical magnifications because they may not match up with available eyepieces, but the principle is correct. Basically the 250PX will deliver the same brightness at the higher power, allowing your eye to perceive finer detail and better contrast due to the larger image scale.

 

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Hello. The chaps on here have already gave good advice. The larger the aperture the more light that comes into the scope making  faint items brighter and therefore improved contrast. That's why they refer to large aperture scopes sometimes as light buckets, because the larger the aperture the more light that is able to get into the scope and image. One point I will bring up which may also help your decision if you are deciding between the 8" and 10" aperture is the what if factor. If you buy the 8" will you be thinking for a bit more money I could of got the 10"  which would of squeezed out that bit more performance  ( skys permitting ) . I had the same decision between a 10" or 12" dob. In the end a used  14" came up for sale and I have never regretted going for aperture ?.   

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29 minutes ago, Stu said:

 

..... the object appears brighter in the larger scope.....

 

Isn't that the salient point though ?

 

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1 hour ago, John said:

Isn't that the salient point though ?

 

Not necessarily John, the ability to maintain brightness at larger image scale is what allows you to perceive the detail better because your eye detects the contrast more easily on larger objects.

Some of the smaller galaxies become visible by the fact that the larger scope can maintain its brightness whilst increasing the size sufficiently to make it visible by allowing your eye to detect the contrast more easily.

The object will appear brighter at the same magnification in a larger scope, but that's missing the point I think. Having seen M82 at high power in a Meade 12" SCT recently, the image scale and detail was amazing.

Are you disagreeing with what I was saying in my post?

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1 minute ago, Stu said:

....Are you disagreeing with what I was saying in my post?

Probably not - I just didn't understand it ! :rolleyes2:

I'm sure Brad will though.

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45 minutes ago, John said:

Probably not - I just didn't understand it ! :rolleyes2:

I'm sure Brad will though.

Ok John, it's clearly not of benefit sharing this stuff so I've deleted my post to avoid confusing people.

EDIT I've unbidden my post because I believe it's a concept which is very important to understand to get the best out of your scope, or understand the reasons for upgrading. Just because things might be challenging to understand, shouldn't stop us trying.

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15 minutes ago, Stu said:

the ability to maintain brightness at larger image scale is what allows you to perceive the detail better because your eye detects the contrast more easily on larger objects.

 

This in a nutshell is exactly the benefits of aperture. :) Image scale.

 

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27 minutes ago, swamp thing said:

This in a nutshell is exactly the benefits of aperture. :) Image scale.

 

Cheers chap, I knew you'd get it ??

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56 minutes ago, Stu said:

Not necessarily John, the ability to maintain brightness at larger image scale is what allows you to perceive the detail better because your eye detects the contrast more easily on larger objects.

A anology to this is reading newspaper on a wall: Standing 5m away, you'll have difficulty to see any small letters clearly, but in 2m, you'll see much better. There's no brightness difference in these two distances, only the image scales. that's exactly the case of two scopes of twice aperture difference and the same exit pupil.

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2 minutes ago, YKSE said:

A anology to this is reading newspaper on a wall: Standing 5m away, you'll have difficulty to see any small letters clearly, but in 2m, you'll see much better. There's no brightness difference in these two distances, only the image scales. that's exactly the case of two scopes of twice aperture difference and the same exit pupil.

That's a great way of explaining it Yong. ??

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For me there are three factors to consider in your 8 v 10 inch Dob question.

1. Size: an 8 will be easier to store and move, although some 10" Dobs have a collapsible tube.

2. Focal ratio: larger Dobs tend to be "faster" so you may need to spend considerably more on good eyepieces. This is represented by Focal Ratio, the length divided by the aperture. For example the Skywatcher 200 is about an f6 while the Skywatcher 250 is about an f4.8.

3. Price: would it be better to invest the difference in better EPs etc.?

Yes, a 10" should give better views, but the difference won't be "gob-smacking". Personally I feel that an 8" Dob will give you many years of excellent sky exploring. Maybe eventually you will be drawn to a very specific niche of astronomy and want a specialised telescope, but until then I don't know that a 10" has vast superiority.

As said, eyepiece magnification depends on focal length, not on aperture. The Skywatcher 200 and 250 for example are both 1200mm and will give the same magnifications wit the same EP.

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@Stu - yes, a bigger aperture gives more mag for the same surface brightness.  Interesting point you make about this appearing brighter because of perception of contrast.  Can I ask - is that in turn related to the fact that SB over a larger area produces a greater overall/total/integrated brightness?          Doug.

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12 minutes ago, cloudsweeper said:

 

 

@Stu - yes, a bigger aperture gives more mag for the same surface brightness.  Interesting point you make about this appearing brighter because of perception of contrast.  Can I ask - is that in turn related to the fact that SB over a larger area produces a greater overall/total/integrated brightness?          Doug.

Hi Doug,

I risk toppling off the cliff of my own ignorance here ?..... @acey 's post may help

My basic understanding is that if you increase the size of the image, whilst maintaining the same contrast, our eye/brain finds it easier to see. This may be due to an increased number of  cells detecting photons in the retina but...... expert input needed.

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To get an impression of what you can see, search for "sketch" and then the telescope type.

You can also take a look at www.deepskylog.be, sort observations on aperture and look at the descriptions that people have given, and at the observations that have sketches.

I personally prioritized light weight and transportability when I chose my scopes.

If you camp at dark places, then dark skies rule even more than aperture. A smaller scope under a dark sky will see more galaxies than a bigger scope under a light polluted sky.

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Hi. Another point which may effect your decision when talking about 8" v10" scope debate. Is that are you talking about like for like scopes. As my understanding is that the mirror which is at the heart of the scope is also very important. This is where the quality of the mirror and pv rate comes into force in regards the light, contrast and therefore quality of image. If you compare a 8"  scope with a 1/12pv rate mirror(very high quality) with a 10" scope with say a 1/3pv(a lower quality mirror) rate mirror then the 8" scope could very well give you superior views/image quality due to the quality of the mirror ,and so aperture(light gathering ability) is important  to the quality of image but the quality of the scopes mirror also has a marked effect also. 

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I wrestled with the 8 or 10" dob question for some time, and in the end went with the 10" and haven't regretted it for a moment. Looking through my friend's 8" I believe the differences between the two are nevertheless subtle. You'll read in advertising that a 10" has 56% more light gathering than the 8", but that doesn't mean it's 56% brighter and better. What it means is that objects which are clear in the 8" will be a touch brighter and detailed in the 10, and some things which are invisible in the 8 can just about be picked up in the 10. It allows you to see that little bit deeper. For me that's an important point because searching for DSOs is what I do, and even under my moderately light polluted skies I've seen well over 100 galaxies so far. If I had the 8" I doubt I'd have seen anything like as much as many of them have just been very faint smudges. So it's just a question really of whether very faint smudges excite you!

You'd also see the benefit in star clusters with more fainter stars being evident. Looking at the moon and planets though I doubt you'll spot much if any difference in the view as they're quite bright enough as it is. If you only intend to do casual DSO observation, sticking with Messier objects and the brighter things in the sky, you'll be very happy with an 8". If you want to go hunting for the faint stuff, you'll benefit from every inch of extra aperature you can afford.

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2 hours ago, Stu said:

Hi Doug,

I risk toppling off the cliff of my own ignorance here ?..... @acey 's post may help

My basic understanding is that if you increase the size of the image, whilst maintaining the same contrast, our eye/brain finds it easier to see. This may be due to an increased number of  cells detecting photons in the retina but...... expert input needed.

Got it, @Stu - it's that business about magnification reducing background surface brightness, thereby increasing contrast.

I love all this physics!

Doug.

Edited by cloudsweeper

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1 hour ago, Linda said:

 

I personally prioritized light weight and transportability when I chose my scopes.

If you camp at dark places, then dark skies rule even more than aperture. A smaller scope under a dark sky will see more galaxies than a bigger scope under a light polluted sky.

An excellent policy Linda :) 

You are going to go far as a deep sky observer.
The only trouble is. You will find that the more dark sky trips you make the stronger the "I need a bigger scope" feelings become. :D 

Bigger cars, 4x4's, vans, trailers, trucks :eek: suddenly start looking like suitable family cars after all :D 

Have fun out there :) 

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1 hour ago, cloudsweeper said:

Got it, @Stu - it's that business about magnification reducing background surface brightness, thereby increasing contrast.

I love all this physics!

Doug.

It's a bit more complicated than that.

Higher magnification does reduce background sruface brightness, but it reduce extended DSO(galaxies, nebulas) too, and to exactly the same amount, there'll be no gain in contrast.

Point source DSO like open clusters and globular cluster, their brightness will not be reduced with higher magnification, therefore becomes brighter with higher mag, and more constrasty, that's why big scopes looks deeper, they see more smaller DSO(point sources or point-like planetary nebulas).

Following picture of M42 is copied from the excellent knowledges source:

http://starizona.com/acb/basics/equip_eyepieces_understanding.aspx

M42_in_different_mag.jpg

In first glance, we can easily see that:

1. The larger image looks brighter

2. There're far more faint stars visible in the larger image.

Does it mean magnification increase brightness or contrast?

Yes, definitely for the stars which are point source, and it enhances the perception that higher mag gives better contrast. But if we look carefully at M42's outer part, many faint outer parts got lost in higher mag, e.g. the whole left arm is almost lost, This is the real brightness change. What you gain with higer mag are the small features in the brighter center part, where the image scale is bigger enough for our eyes to recognize.

What I can draw from looking at these two pictures are: For hunting faint fuzzies (galaxies and none-stellar nebulas), dark sky is the only factor which deternines contrast, larger apertures are very good help to get larger image scale, therefore see more smaller DSO, some balance between magfication and exit pupil has to be made to see threshold DSO, different magnifications might be needed for larger faint fuzzies depending on what features you're looking at.

I'm sure there're many useful conclusion can be deducted from the pictures above.:smile:

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