what 8 inch dob should i buy?

[astronobob (Bob)]

Agree with going the 10" Ryan, as mentioned the larger aperture will be more rewarding for sure

Originally Posted by Camelopardalis

Potato, tomahto

But not quite...the important distinction is that it is the focal length of the telescope that determines the maximum field of view, not the focal ratio. Whether it is fast or slow is irrelevant. Both an 8" Dob with 1200mm focal length (f/6) and a 100mm refractor with 1200mm focal length (f/12) will be capable of showing the same maximum FOV, yet one is faster and the other is slow

Also, the eyepiece IS important as that is what brings the image to focus for our eyes and the focal length of the eyepiece and telescope together determine the magnification (magnification = FL telescope / FL eyepiece). For all intents and purposes, an 8" Dob is just as capable at showing a decent planetary image as a similarly sized Mak, they just get there in different ways.

Originally Posted by Terry B

Sorry Bob but it isn't "common Knowledge". The "speed" of a scope makes almost no difference to the brightness of what you see "visually". Photography is different but this is mostly moot with a dob that is designed as a visual scope. Aperture changes the amount of light you collect not the focal length of the scope that determines the f ratio.
Your description of seeing galaxies and nebulas is the same except for the very largest objects and the only one that I can think of that will fill the field of my f8 200mm scope is the andromeda galaxy. You just use a longer focal length eyepiece.

Ok Ok I understand what you guys are saying, with EP's, FOV's ect.
Tho lets look at this situation ?
Hyperthectically, there are two scopes. 10" F10 & 10" F5

Between these two scopes you have 3 Eps, 5mm 10mm 20mm

10" F5 with each Ep gives magnifacations of 250, 125 & 62.5 respectfully

10" F10 ______________________________ 500, 250 & 125

What gives Go out and buy another Ep : say either a 2.5mm ( Hyperthetically) or 40 mm so the other scope can catch up but depend on which extremity - the other scope will out perform, yep, More magnification on a Planet or wider FOV for vast areas of Neb , ,
Why else do Planetary Imagers/observers get better results using Smit Cas F10's ect and Galaxy hunters/imagers prefer faster scopes. Look at Mike Sedonio images collected in just one night compared to Mega Data
Whether imaging or visual observing, Tomatoes, potatoes carrots and garlic - may as well stick some big wheel on my mini minor and go 4x4 mountain climbin' on the other hand, grab the semi trailor and go briz city shoe shoppin
thats the way I see it

[barx1963 (Malcolm)]

I think what Terry and Dunk are trying to put across is that there is a strong myth that has developed in some circles that a "fast scope" is better in terms of making a brighter image when used visually. So that will put the idea in some peoples head that you can substitute a small aperture fast scope for a large aperture slow scope. Or in other words an f5 is always "better" than a f10.
From a visual observing point of view that is simply not the case. A 10" scope is still a 10" scope and will put exactly the same number of photons on the focal plane regardless of it's focal length.
That is why I think it is very important when talking to beginners that we explain carefully what is meant by the term "fast" and "slow".

Malcolm

[Terry B]

Originally Posted by astronobob

Agree with going the 10" Ryan, as mentioned the larger aperture will be more rewarding for sure






Ok Ok I understand what you guys are saying, with EP's, FOV's ect.
Tho lets look at this situation ?
Hyperthectically, there are two scopes. 10" F10 & 10" F5

Between these two scopes you have 3 Eps, 5mm 10mm 20mm

10" F5 with each Ep gives magnifacations of 250, 125 & 62.5 respectfully

10" F10 ______________________________ 500, 250 & 125

What gives Go out and buy another Ep : say either a 2.5mm ( Hyperthetically) or 40 mm so the other scope can catch up but depend on which extremity - the other scope will out perform, yep, More magnification on a Planet or wider FOV for vast areas of Neb , ,
Why else do Planetary Imagers/observers get better results using Smit Cas F10's ect and Galaxy hunters/imagers prefer faster scopes. Look at Mike Sedonio images collected in just one night compared to Mega Data
Whether imaging or visual observing, Tomatoes, potatoes carrots and garlic - may as well stick some big wheel on my mini minor and go 4x4 mountain climbin' on the other hand, grab the semi trailor and go briz city shoe shoppin
thats the way I see it

Bob
You said the very important word above. "Imagers"
Visually it is makes little difference. Have you ever looked through a 2.5mm eyepiece?
They are not easy to use and are very expensive.
Galaxy hunters always use big scopes. The speed is just a convenience thing. An f/8 18" scope is might tall.
Cheers

[astronobob (Bob)]

I understand & have agreed with the EP situation, what is being said is indeed true, the resulting FOV & the brightnes of sky viewed with a certain aperture, whether be and F5 or F10 providing the appropriate EP is giving the same FOV, then the brightness will be equivelent, correct

What I dont understand is why are yous exercizing against me and Not seeing my point ? Be a little more BroadMinded fellas, I wasnt born yesty, in fact I was grinding & pollishing my own Mirrors, building Tubes, 2ndary spider cells, main mirror cells and home made focusers and putting all these together etc 15 odd yrs ago !
This thread is Ryans and Ryan asking for advice, true ?
If I was in need of advice, I would have also asked for it in another thread !
I have agreed to what your points are, so lets just leave it at that & I also agree with cheerful whistling