80mm Refractors

[ollypenrice]

Aperture AND focal focal ratio are both important. Big aperture means more photons. Try out a 12" F4 scope and then compare it to an 80mm F4 scope.......

A 12 inch F4 has a focal length of 1,200 mm. An 80mm F4 has a focal length of 320mm. They take completely different photographs so what are we going to compare? One's a widefield, the other's an in-between sort of FL in amateur imaging terms, neither long nor short. You wouldn't use them on the same target, just as you wouldn't use a fish-eye to photograph a bird in a treetop.

Big aperture means more photons, yes, but if the focal length goes up then those photons from object 'x' are spread over a larger area on the chip. The big scope simply gets more of its light from a smaller part of the sky, if you like. F4 is F4. The only other player is the sampling rate in arcseconds per pixel and that's governed by pixel size and focal length.

You could usefully compare a 200mm F2 Hyperstar and an 80mm F5 apo because both have the same focal length. There is no doubt whatever that the Hyperstar wil be vastly faster but will it, in reality, out resolve the apo if the apo is a really good one? Not from the images I've seen posted, or not usually. In my experience optical resolution does not translate as decisively as one would expect into final image resolution. Focal length does do so, though.

Olly

http://ollypenrice.s...39556&k=FGgG233

[M L W]

Well, I think there we have 3 very good explanations of why needing the bigest scope possible for astrophotography is a misconception. It is for the reasons above that an 80ED scope is the most popular imaging scope going (along with being light, cheap and good quality).

Of course once you have some exposure time under your belt with an 80mm you'll want to complement with something else, which is why I bought a 6" RC which when reduced gives me a 900mm f/6 scope just right for picking out tiny planetary nebula and galaxies which are just too small in the fov in my 80mm refractor.

This explenation is as clear as day and very much appreciated and also ensures me that wanting 2 scopes in time for 2 different reason is not a bad idea. I will stick with the 80ED pro as I'm sure this will be a good place to start with imaging. I cant deny that I have been considering: Skywatcher Quattro f4 Imaging Newtonian also but understand that collamitation can be a put off with this scope. I have also looked at the Skywatcher Explorer 190MN DS-PRO and then looked at it again and again.....I'm really liking the sound of this scope and is a real contender for my second. I did consider it for my first buy but maybe I should not run before I can crawl!

[M L W]

A 12 inch F4 has a focal length of 1,200 mm. An 80mm F4 has a focal length of 320mm. They take completely different photographs so what are we going to compare? One's a widefield, the other's an in-between sort of FL in amateur imaging terms, neither long nor short. You wouldn't use them on the same target, just as you wouldn't use a fish-eye to photograph a bird in a treetop.

Big aperture means more photons, yes, but if the focal length goes up then those photons from object 'x' are spread over a larger area on the chip. The big scope simply gets more of its light from a smaller part of the sky, if you like. F4 is F4. The only other player is the sampling rate in arcseconds per pixel and that's governed by pixel size and focal length.

You could usefully compare a 200mm F2 Hyperstar and an 80mm F5 apo because both have the same focal length. There is no doubt whatever that the Hyperstar wil be vastly faster but will it, in reality, out resolve the apo if the apo is a really good one? Not from the images I've seen posted, or not usually. In my experience optical resolution does not translate as decisively as one would expect into final image resolution. Focal length does do so, though.

Olly

http://ollypenrice.s...39556&k=FGgG233

Thanks for this two Olly, I am slowly ready through these posts and the technical detailing is slowly but surely beggining to make sense. This information will help me when looking at telescopes in future which I have my eye on.

[ollypenrice]

The Quattro and the MN190 can sing, that's for sure, but they need to be fettled and sorted. The 190 is a particularly hefty scope, the more so because it has heavy glass at both extremities, creating a significant moment for the mount to control. A well sorted 190 can take on the best of the premium apos of comparable focal length and a very well sorted one can beat them.

Olly

[DaveS]

I don't want do derail this, but what level of mount would a 190 MN need? I know the skywatcher ilustration shows it on a NEQ6 but I'm thinking that it would warrent a league or two higher, especially with that huge moment arm.

[M L W]

The Quattro and the MN190 can sing, that's for sure, but they need to be fettled and sorted. The 190 is a particularly hefty scope, the more so because it has heavy glass at both extremities, creating a significant moment for the mount to control. A well sorted 190 can take on the best of the premium apos of comparable focal length and a very well sorted one can beat them.

Olly

I do like you analogy "they can sing" lol.... sweet. This was my other argument / thought. To me, the hobby is also about learning to get the most out of what you have and becoming better at using it, knowing what makes it "SING" even if this takes a long time... I suppose the longer the better as I do get board if I have a new toy and within a short time both I and the toy has reached the pinnicale of abilities. Having saying that, I shall humbely learnt to crawl and learn how to make what I have sing to its best of abilities... Still, I am liking the look of the 190 more so but also the quattro.

[laser_jock99]

Your 12 inch scope will produce the same brightness of an extended object as a 3 inch scope at the same f/ratio.

Example - I have an 80mm f/4 scope - focal length 320mm. Hypothetically a pixel on my camera receives 25 photons per second when imaging. Along comes someone else with a 160mm f/4 scope - focal length 640mm. The larger scope captures 4 times as many photons - 100 per second BUT due to the doubled focal length the 100 photons are now spread out over 4 pixels instead of just 1, so each pixel still receives 25 photons per second resulting in the same brightness and therefore the same exposure time is required.

If you fix the focal length to maintain the same field of view and double the aperture then you will collect 4 times as many photons and decrease your exposure time by a factor of 4, but to achieve this you have halved the f/ratio to f/2.

The exception to this are point sources such as stars too small to be spread out over multiple pixels when the focal length is increased, they will indeed by x4 brighter and quicker to capture.

Cleary focal length has to considered.

[Daddystu]

The ads for the equinox ed80 state that it is fast and flat ie needing no coma corrector. This would make it cheaper on the whole than the evostar ed80 which requires the expensive flattener. Am I reading that right? Also the equinox runs at f6.2 out of the box.

[Chris]

Some great advice and explanations above, no one scope will excel at everything but I do think the ED80 is a great starting point, its short focal length (around 480mm with 0.8 reducer) will track for longer than say the 800-1000mm focal length Quattro without smearing photons across your sensor. This will provide a nice gentle learning curve for AP. Also they are plug and play with no collimation, if you are going to start with a Newt instead of an ED80 I would recommend an f/5 like a 150pds (much easier to collimate, light, and the focal length is 750mm which is short for a newt, plus f/5 is fast trust me)

Going back to the refractor, I personally find it easier to identify, control, and resolve dew issues, it can be a real pain when your Newtonians secondary mirror dews up. Taking flat exposures are also more straightforward on a budget with a small aperture refractor, you can simply stretch a white t-shirt over the aperture and shine a torch uniformly over it, this is much more tricky at larger apertures.

I've just gone back to using an ED80 from a 200p, the main reason for me personally was more to do with my obsy pier being too high for a scope with a focuser at the top, having said this I'm not at all sad to be going back to the ED80, looking forward to it infact:)

[M L W]

Some great advice and explanations above, no one scope will excel at everything but I do think the ED80 is a great starting point

I've just gone back to using an ED80 from a 200p, the main reason for me personally was more to do with my obsy pier being too high for a scope with a focuser at the top, having said this I'm not at all sad to be going back to the ED80, looking forward to it infact:)

I agree with this that there's some great advice and explanations here but further to that, it provides newbies like me, confidence that were going in the righjt direction with the right equipment. I feel assured that the 80ED is a great scope and looking forward to presenting you guys n girls with feedback and images of my first attempt. "This may be some time yet"

Again, thanks all

Maz

[Chris]

Good luck, and enjoy:)

[nephilim]

The ads for the equinox ed80 state that it is fast and flat ie needing no coma corrector. This would make it cheaper on the whole than the evostar ed80 which requires the expensive flattener. Am I reading that right? Also the equinox runs at f6.2 out of the box.

As far as i'm aware (and i may be wrong) refractors dont suffer from coma, reflectors do. A field flattener is only really needed for AP using a frac.

[Daddystu]

As far as i'm aware (and i may be wrong) refractors dont suffer from coma, reflectors do. A feild flattener is only really needed for AP using a frac.

I know my mistake. I was looking at a bunch of refractors and reflectors at the time and got my terms mixed up. Couldn't find the edit post button.

Equinox 80ED and flattener on their way now though.

[nephilim]

Easily done mate, I can confuse myself with all this stuff no problem

[George]

I don't want do derail this, but what level of mount would a 190 MN need? I know the skywatcher ilustration shows it on a NEQ6 but I'm thinking that it would warrent a league or two higher, especially with that huge moment arm.

A MN190 does just fine on a EQ6, if its windy you wouldn't want to image regardless if you were using a bigger mount as the scope would act like a flag regardless.