Fast scope for observation?

[Altitude]

I am planning down the road to upgrade my telescope. So, I am wondering if I will benefit a lot from getting f3.9 fast 8" or 10" telescope which mainly will be used for observation. I read a lot about the fact that fast scopes are usually used for Astro photography. Is it really that "bad" for observation and why? I know I would need to use the coma corrector. If this is only a problem, what else is not suitable there for skywatching, or is the "speed" of my new telescope not that important for sky observation?

By the way, I have right now 6" f5 Reflector Newtonian.

[Stu]

The speed of the scope is far less important for observing than for imaging, but it does have some effects which are noteworthy.

Physically it keeps your tube length down. Focal length on a 10" would be 1200 at f4.8, 975mm at f3.9.

Faster newts have larger secondaries because of the wider light cone. This can rob views of contrast when observing. This is easily processed out for imaging but not ideal for visual.

Lastly you would get a wider field of view with the shorter focal length scope, something which may interest you.

A question..... What is making you consider the faster scope?

[Piero]

I am not an expert in telescopes, but I would say that a telescope so fast will need a very demanding collimation, excellent eyepieces, and excellent coma corrector. Due to the large secondary you should expect a significant decrease in contrast, but also a reduction in the real aperture.

Some of the mentioned problems are not important in ap, e.g. loss of contrast, but I would say that for observation, you are better off with a >F5 telescope. In all fronts this will also be cheaper.

Not sure on the following, but I think that even building a primary mirror for a f3.9 is really difficult. If so, either you go for a very good maker, or you might expect problems on this aspect too.

Surely other members can give you more detailed answers.

Best wishes,

Piero

[pete_l]

You will find that longer focal lengths (and therefore higher magnification) will make the sky background, behind your target, darker. This can actually help a visual observer to see fainter objects as the contrast increases.

Shorter F/Ls mean larger FoVs but also the image of your object appears smaller in the eyepiece - so less detail (this is true of imaging as well as visual use).

As said, the larger amount of sky and the sky glow that is therefore presented at your eye / CCD can be processed out for the imager - but for the visual observer it reduces your ability to see faint stuff.

I'm not sure if coma correction is much of an issue for visual observers. If you want to see an undistorted part of an object, you can simply move your OTA over to it.

[Stu]

You will find that longer focal lengths (and therefore higher magnification) will make the sky background, behind your target, darker. This can actually help a visual observer to see fainter objects as the contrast increases.

Shorter F/Ls mean larger FoVs but also the image of your object appears smaller in the eyepiece - so less detail (this is true of imaging as well as visual use).

As said, the larger amount of sky and the sky glow that is therefore presented at your eye / CCD can be processed out for the imager - but for the visual observer it reduces your ability to see faint stuff.

I'm not sure if coma correction is much of an issue for visual observers. If you want to see an undistorted part of an object, you can simply move your OTA over to it.

I get what you are saying Pete, but to add/clarify that scopes of different focal lengths but the same aperture will have the same exit pupil size if used at the same magnification eg a 20mm eyepiece in a hypothetical 1000mm f/l 200mm aperture scope will give the same exit pupil, and thus same inherent background brightness as a 15mm eyepiece in a 750mm f/l 200mm scope.

There would be a contrast difference caused by the larger secondary giving more light scatter, plus the coma would be worse.

Coma is definitely an issue in fast scopes for observing, particularly with very wide afov eyepieces such as the Ethos. A paracorr really helps to give pin point stars right across the field.

[michael.h.f.wilkinson]

Fast scopes also put more severe constraints on the eyepieces. Astigmatism in outer part of the field of view is a serious issue with cheap EPs in fast scope.

[estwing]

Yes your ep collection gets expensive!

[michael.h.f.wilkinson]

Yes your ep collection gets expensive!

Not that having slower scopes has ever held me back in that respect

[Moonshane]

I have 12" f4 and 16" f4 dobs. I have never had an issue with this mystical 'difficult collimation' - it's the same process for f4 newts as for f11 newts. OK you have to be more accurate to get best results with a fast newt but surely you always want to get the critical primary collimation in the optimal position for all newts?

I find that although I use a paracorr it is not critical at f4 even if you don't buy premium eyepieces. As above you can use cheaper plossls and obtain perfectly agreeable views with £50 or less eyepieces.

I find that with the exception of double stars (an issue with most sub f8 newts to some extent) a fast large newt is an ideal combination of tube size and aperture and I'd not be likely to go back to f5 having used these f4 scopes for some time. seated observing is definitely my preference.

for newts of 12" or more, I use an aperture mask and this creates an effectively slower, smaller aperture scope producing lovely tight stars and greater contrast / darker sky. e.g. my 12" f4 masks down to a 110mm f11 'dobfractor' with no secondary obstruction.

here's the one I use on my 16" f4 in case you have not seen one. http://stargazerslounge.com/topic/143780-aperture-mask-for-16-f4-dobsonian/?p=1445352

[Altitude]

The speed of the scope is far less important for observing than for imaging, but it does have some effects which are noteworthy.

Physically it keeps your tube length down. Focal length on a 10" would be 1200 at f4.8, 975mm at f3.9.

Faster newts have larger secondaries because of the wider light cone. This can rob views of contrast when observing. This is easily processed out for imaging but not ideal for visual.

Lastly you would get a wider field of view with the shorter focal length scope, something which may interest you.

A question..... What is making you consider the faster scope?

Fast telescopes always promoted by retailers and highly regarded by astronomical community. My knowledge in this field is minimal and I believed that if I want to upgrade the scope It needs to be much better than what I have now. Since the "speed" so highly valued, going from f5 to f3.9 would be a logical step.

My personal assumption was that faster scope would give me "more objects per square inch" in the view, and brighter DSO's. I have to mention as well, that I observe through a Binoviewer and not sure which way it will effect the whole "delicacy" of such a fast scope.

Another factor that slightly pushing me toward faster scope is potential AP in the distant future (when I will have more time...).

[Stu]

Fast telescopes always promoted by retailers and highly regarded by astronomical community. My knowledge in this field is minimal and I believed that if I want to upgrade the scope It needs to be much better than what I have now. Since the "speed" so highly valued, going from f5 to f3.9 would be a logical step.

My personal assumption was that faster scope would give me "more objects per square inch" in the view, and brighter DSO's. I have to mention as well, that I observe through a Binoviewer and not sure which way it will effect the whole "delicacy" of such a fast scope.

Another factor that slightly pushing me toward faster scope is potential AP in the distant future (when I will have more time...).

I think you are being misled by marketing in that case.

Let's leave AP out of the equation because a 10" f4 scope would not necessarily be the best choice for starting in AP.

A faster scope (and let's be clear, this means one with a shorter focal length for the same aperture), will not give you brighter objects when used at the same magnification. There is no difference between the image in an f6 scope vs an f4 scope when used at the same exit pupil/magnification.

A shorter focal length will give you a wider field of view when using the same eyepiece vs a longer f/l, and overall has the capability of showing more sky. The downside is that you will need shorter focal length eyepieces to show you higher powers, and her you can run into eye relief/comfort problems unless using more expensive eyepieces. You will also see more coma in wider field eyepieces.

At larger apertures you will naturally have quite a long focal length even at f4, so getting to high powers is less of a problem, but at 200mm f3.9 for instance you will need a 3mm eyepiece at x250. Quite possible but you need to ensure you have reasonable eye relief.

Don't get me wrong, I have nothing against fast scopes, I am just clarifying what I think is a misunderstanding you have when they are used for visual observing.

Stu

[John]

I'm not as skilled or experienced newtonian observer as Shane is but I did find accurate collimation and coma issues when I had F/4.8 and F/4.7 10" newtonians. Astigmatism was not an issue because I'd invested in Tele Vue eyepieces sometime before.

My fastest scope is now a 12" F/5.3 which seems to be easy to keep accurately collimated and shows no coma with 82 degree eyepieces and only a tiny amount with 100 degree ones.

As aperture increases beyond around 12" I realise that faster focal ratios keep the eyepiece accessible and the scope a more manageable size.

Personally I'd not chase a fast aperture for visual observing as a goal in itself though, regardless of what retailers tried to tell me

[Lowjiber]

Respectfully, you need a fast scope for visual work like a hog needs a songbook.  I'm not talking about "kinda fast" (f/47 to f/5).  I'm talking about f/3.x scopes.

Beginners often associate the word "fast" with good stuff like racing cars.  It means nothing but trouble for a visual scope, and there is absolutely nothing gained by using one.

Clear, Dark Skies

[estwing]

Given the choice between 28" F3.3 or a 28" F6 where's your money going?

[John]

Given the choice between 28" F3.3 or a 28" F6 where's your money going?

To be fair, the OP was referring to an 8" or 10" where the issues are substantially different.

[Stu]

To be fair, the OP was referring to an 8" or 10" where the issues are substantially different.

Yep, agreed. There are totally valid reasons to have as fast a scope as possible once the aperture gets large enough, but not at 8 or 10" for visual

[estwing]

Sorry I hadn't noticed what size the OP was talking about it was the post above mine that made me jump up and defend fast scopes! Obviously as aperture increases you may want a faster scope...

[Moonshane]

or a bigger ladder