All About Telescope Focal Lengths

[bornfree]

Hello all.

I am a newbie to astronomy and reading up on alot of how to get your first scope, what to get etc. here, i would like to summarise all i have digested about the focal length of the scope to help other newbies like me. i hope the experts here can fill in the blanks i missed out. While i was going through the info i can find online, i stumbled on the term fast scope and slow scope. this brought me to the need to understand focal length of the telescope. since for sidewalk astronomy, i suppose we tend to have scopes with aperture preferably less than 6 inches, unless you like to carry a huge cannon around or has the means of transportation.

Here is my summary:

Long focal Length
- Long Tube Length
means a longer length of the OTA, in MCT, SCT or Newt because mirrors are in there to let the light bounce across the tube, they are generally shorter than the refractors. those telescopes body length doesnt reflect their actual focal length
- Larger Focal Ratio.
Focal Ratio = Focal Length / Aperture. Please use the same unit of measure when dividing. 600mm / 3 inches doesnt give you a focal ratio of 200 have to convert 3inches to mm which is about 76mm then 600mm/76mm to give u focal ratio of about 7.9
- Slow Scope
i figured it was because light had to travel a longer distance down the longer tube length making it "slower" hence slow scope. I am wrong here. its is because for photography it requires longer exposure time therefore slower. hence, it is a slow scope.
- Smaller field of view
My logic tells me that it is like through a tunnel the further from exit , the smaller the view i can perceive. like a frog in a well? the deeper the well, the smaller the sky it can see. is this how to properly explain this? hmm..
- Better contrast
Due to the smaller view, more light is focused at a point, hence better details ( better for planet viewing, splitting double stars maybe?)

Short Focal Length
- Shorter Tube Length
Means its going to be lighter and increased in portability
- Smaller Focal Ratio
- Fast Scope
- Wider field of view
Better at looking at star clusters
- lesser contrast

These are have not taken into account of the EP used.

while surfing around i found this thread from another forum the clearest when explaining.
http://www.astronomyforum.net/astronomy ... scope.html
This youtube also helps.
http://www.youtube.com/watch?v=gpnB5w_gZsI

Some formulae commonly used in relation with Focal Length

Focal Ratio = Focal Length / Aperture
Magnification = Focal Length of OTA / Focal Length of EP

i hope this can help other newbies to understand more about focal length.

[Gary]

Hi bornfree. Welcome to the forum! Thanks for sharing the info. Feel free to join public stargazing sessions in Singapore to have a deeper hands-on understanding and appreciation of the concepts you have mentioned. Have fun exploring the the different pros and and cons of various types of telescopes.

[cloud_cover]

Hello! Welcome to the forums
Its really great that you're doing your research. here are my thoughts:

- Long Tube Length
means a longer length of the OTA, in MCT, SCT or Newt because mirrors are in there to let the light bounce across the tube, they are generally shorter than the refractors. those telescopes body length doesnt reflect their actual focal length

Actually the length of the light path must equal to the focal length. In designs like refractors and newtonians, the light path follows a fairly straight line (its bent 90deg in the Newt only so that your head isn't looking down the front of the telescope) so these designs tend to have a tube roughly as long as its focal length. Some newtonians are designed as variants with an integral barlow so their tube length is much shorter but most "normal" Newts have a length about the light path distance. That's why for larger newtonians, such as 14-16" Dobsonians and above, you might need a step ladder to reach the eyepiece!
MCTs and SCTs are designs where the light path is folded hence their much shorter tubes. On a technical note, most SCT mirrors are f/2 and the secondary is curved so it has a magnifying effect to achieve the f/10 focal ratio. That's why despite a C8 having a 2000mm focal length, its tube is about 600mm in length.

- Smaller field of view
My logic tells me that it is like through a tunnel the further from exit , the smaller the view i can perceive. like a frog in a well? the deeper the well, the smaller the sky it can see. is this how to properly explain this? hmm..

Yes, that's right. for a given eyepiece focal length, a longer focal length scope has a smaller field of view compared to a shorter focal length scope, regardless of aperture. The reason for slower scopes which have a longer focal length per unit aperture is historical and production related: refractors benefit from higher f ratios because the amount of chromatic aberration is dependent on the glass as well as the ratio of the focal ratio to aperture. Since modern ED glass has only been available for a few decades, most of the older instruments were at best achromats and the faster they were, the more false color rendering the contrast less. Due to their greater depth of field (the range of focus positions where the image remains in focus), slower scopes are also more forgiving of optical errors on their lenses or mirrors making them easier to produce. The lens/mirror figure on a slower scopes is also flatter making it easier to produce.
However, magnification is not a good reason to get a "long" scope. Even short scopes can handle magnification well if they are well made. They key limiting factor to magnification, other than atmospheric considerations, is aperture. The bigger the aperture, the better its resolving power hence the higher a magnification it can accept. You merely need shorter focal length eyepieces. As a rule, we use 50x/inch aperture as a telescope's maximum practical magnification. This is for decently good optics, of course, which most scopes today are.

- Better contrast
Due to the smaller view, more light is focused at a point, hence better details ( better for planet viewing, splitting double stars maybe?)

This would unfortunately be untrue. Contrast is a function of the optical figure of the telescope (as in how smooth and accurately the lenses or mirrors were made), lens coatings as well as (to a much less extent) the central obstruction. The reason why people regard long focal length instruments to have better contrast is because in slow NEWTONIANS (note also that its not the focal length per se, its the focal ratio), a smaller secondary mirror can be used since the light cone is narrower. This improves the contrast as compared to a similarly made newtonian with a faster focal ratio which would require a larger secondary mirror. Likewise in refractors since all things being equal (i.e. same kind of optical glass used for the lenses, same design and accuracy of lens figure) a slower refractor will have less false color (which disperses part of the incoming light) hence it is regarded as having better contrast. Practically though, since most modern apochromatic refractors have near perfect color correction, there's no need to makes these scopes very slow.

As an example of how it works practically, Gary and I had a small comparison between his 127mm Mak (an f/12 or f/15 scope) vs my 101mm APO refractor (an f/5.4 scope) on Jupiter. We both agree the contrast in the 101mm APO was better than the 127mm Mak, despite the Mak being a slower scope. (That's not to say the Mak made terrible views. They were actually pretty nice)

I think as a newbie looking to get his first scope, while these thoughts are certainly important and its great that you are doing research, perhaps a better way to approach it would be "For the amount you can spend, how big (aperture) and how bulky/heavy would you be willing to accept".

One extra note about faster scopes: because the light cone coming in is steeper in a faster scope, better quality eyepieces, especially if you're doing widefields are required for faster scopes. So while an F/10 or f/15 (typical SCT or MCT) should have no issues with the cheapest decent eyepieces, a fast f/5 or f/4 (typical newtonians) will require better eyepieces

[bornfree]

I have been to your TPY Library talk on July as well as a Bishan Sidewalk sesion and saluting to the Iridium Flare.

For now, i got the First Scope to try get a hang of things first. For now it is mainly moon viewing from my bedroom and any stars i can kind of locate from my window. the eye pieces are the generic H20mm H15mm H6mm and the H4 mm lenses. not the best out there but just nice for starters.

I am kind of eyeing on the Astromaster 114AZ at the moment.. but reading some mix reviews, still looking and testing, but ultimately just want to get more info. Looking forward to the next sidewalk. Hopefully, the next time round i can setup my scope and see what the rudimentary scope can do. if possible, play around with EP and see what the scope has to offer with the more knowledgeable around.

[Gary]

Hi Bornfree. Congrats on getting your very first telescope!

Very happy to know you are able to manage to your expectation of this telescope and explore its limits. I am also having fun trying to explore mine between the free telescope loans.

Here's a pic of the Moon shot through it with a hand-held phone last Friday at Bishan Park:



The magnification is about 47x which was achieved by a 32mm plossl eyepiece on a 5x tele-extender:

[bornfree]

Hello! Welcome to the forums
Its really great that you're doing your research. here are my thoughts:

- Long Tube Length
means a longer length of the OTA, in MCT, SCT or Newt because mirrors are in there to let the light bounce across the tube, they are generally shorter than the refractors. those telescopes body length doesnt reflect their actual focal length

Actually the length of the light path must equal to the focal length. In designs like refractors and newtonians, the light path follows a fairly straight line (its bent 90deg in the Newt only so that your head isn't looking down the front of the telescope) so these designs tend to have a tube roughly as long as its focal length. Some newtonians are designed as variants with an integral barlow so their tube length is much shorter but most "normal" Newts have a length about the light path distance. That's why for larger newtonians, such as 14-16" Dobsonians and above, you might need a step ladder to reach the eyepiece!
MCTs and SCTs are designs where the light path is folded hence their much shorter tubes. On a technical note, most SCT mirrors are f/2 and the secondary is curved so it has a magnifying effect to achieve the f/10 focal ratio. That's why despite a C8 having a 2000mm focal length, its tube is about 600mm in length.

- Smaller field of view
My logic tells me that it is like through a tunnel the further from exit , the smaller the view i can perceive. like a frog in a well? the deeper the well, the smaller the sky it can see. is this how to properly explain this? hmm..

Yes, that's right. for a given eyepiece focal length, a longer focal length scope has a smaller field of view compared to a shorter focal length scope, regardless of aperture. The reason for slower scopes which have a longer focal length per unit aperture is historical and production related: refractors benefit from higher f ratios because the amount of chromatic aberration is dependent on the glass as well as the ratio of the focal ratio to aperture. Since modern ED glass has only been available for a few decades, most of the older instruments were at best achromats and the faster they were, the more false color rendering the contrast less. Due to their greater depth of field (the range of focus positions where the image remains in focus), slower scopes are also more forgiving of optical errors on their lenses or mirrors making them easier to produce. The lens/mirror figure on a slower scopes is also flatter making it easier to produce.
However, magnification is not a good reason to get a "long" scope. Even short scopes can handle magnification well if they are well made. They key limiting factor to magnification, other than atmospheric considerations, is aperture. The bigger the aperture, the better its resolving power hence the higher a magnification it can accept. You merely need shorter focal length eyepieces. As a rule, we use 50x/inch aperture as a telescope's maximum practical magnification. This is for decently good optics, of course, which most scopes today are.

- Better contrast
Due to the smaller view, more light is focused at a point, hence better details ( better for planet viewing, splitting double stars maybe?)

This would unfortunately be untrue. Contrast is a function of the optical figure of the telescope (as in how smooth and accurately the lenses or mirrors were made), lens coatings as well as (to a much less extent) the central obstruction. The reason why people regard long focal length instruments to have better contrast is because in slow NEWTONIANS (note also that its not the focal length per se, its the focal ratio), a smaller secondary mirror can be used since the light cone is narrower. This improves the contrast as compared to a similarly made newtonian with a faster focal ratio which would require a larger secondary mirror. Likewise in refractors since all things being equal (i.e. same kind of optical glass used for the lenses, same design and accuracy of lens figure) a slower refractor will have less false color (which disperses part of the incoming light) hence it is regarded as having better contrast. Practically though, since most modern apochromatic refractors have near perfect color correction, there's no need to makes these scopes very slow.

As an example of how it works practically, Gary and I had a small comparison between his 127mm Mak (an f/12 or f/15 scope) vs my 101mm APO refractor (an f/5.4 scope) on Jupiter. We both agree the contrast in the 101mm APO was better than the 127mm Mak, despite the Mak being a slower scope. (That's not to say the Mak made terrible views. They were actually pretty nice)

I think as a newbie looking to get his first scope, while these thoughts are certainly important and its great that you are doing research, perhaps a better way to approach it would be "For the amount you can spend, how big (aperture) and how bulky/heavy would you be willing to accept".

One extra note about faster scopes: because the light cone coming in is steeper in a faster scope, better quality eyepieces, especially if you're doing widefields are required for faster scopes. So while an F/10 or f/15 (typical SCT or MCT) should have no issues with the cheapest decent eyepieces, a fast f/5 or f/4 (typical newtonians) will require better eyepieces

Hi cloud_cover
Thank you for correcting my mistake. For now, it cleared any doubts i have. n with a f4 newt still hanging on the original cheap Hyugen lenses, maybe time to get some better EP..

[Gary]

As an example of how it works practically, Gary and I had a small comparison between his 127mm Mak (an f/12 or f/15 scope) vs my 101mm APO refractor (an f/5.4 scope) on Jupiter. We both agree the contrast in the 101mm APO was better than the 127mm Mak, despite the Mak being a slower scope. (That's not to say the Mak made terrible views. They were actually pretty nice)

For the benefit of newcomers or value-conscious readers, here are the approximate retail pricing of this 2 telescopes:

(1) Televue NP101 OTA - USD $3995
http://www.optcorp.com/te-npf-4056-np-1 ... scope.html

(2) Sky-Watcher SkyMax 127 OTA - USD $383
http://www.firstlightoptics.com/maksuto ... 7-ota.html

So the question is, is the planetary contrast in the NP101 ten times (or more) better than the Mak127 at the same magnifications?