Jump to content

sgl_imaging_challenge_2021_annual.thumb.jpg.3fc34f695a81b16210333189a3162ac7.jpg

SCT focal length - how does that work?


Recommended Posts

My 8se has a focal length of 2030mm, but basically....it doesn't look long enough -(even taking the folding of the light path into account). From the end of the corrector plate to the mirror is about 400mm. If the light enters, bounces back to the secondary and then bounces back again, that only makes about 1200mm. Where does the extra 800mm+ come from?

I don't get it. :)

Please treat me like an idiot and speak in words of one syllable(!)

I've looked at the diagram, but I still don't get it.

post-15346-133877421922_thumb.jpg

Link to comment
Share on other sites

Good point Lulu.

I've been puzzled about this SCT focal length vs tube length mystery for a while now and I'm looking forward to a logical and comprehensible explanation, especially now that I'm contemplating buying an SCT in the not too distant thingy.

glossary

SCT = Schmidt Cassegrain Tardis :)

Edited by Astronut
Link to comment
Share on other sites

The lenses have a focal multiplier - much like a convex lens. The 6,8,11 and 14 have a primapry mirror that has an f/2 focal length (or so I'm led to believe). It's this that gives the SCT a long focal ratio. HTH

Link to comment
Share on other sites

The key to this is not just the folded light path design - you must factor in the 'magnification' imparted by the Convex secondary mirror which increases the 'apparent' focal length.

OMG you're clever!

Now explain it to me in Idiot-Speak(!) (Pretty please):D:)

Link to comment
Share on other sites

The big primary mirror is 200mm diamter and brings the starlight to a focus 400 mm in front ( with me so far....) at about 350mm from the main mirror is sitting the secondary mirror ( about 75mm diameter) which is a mirror with a "hyperbolic" convex shape, so it does two things:

1. It reflects the beam back down the tube and through the baffle/ hole in the main mirror to the focus.

2. It also acts as a x5 magnifier so the 400mm focal length of the main mirror becomes 400 x 5 = 2000mm

( note also that the f ratio of the primary mirror is also multiplied by 5 so the final "system" become f10.)

This is very similar to the optics in a Cassigrainian telescope.

Hope this helps.

Ken

Link to comment
Share on other sites

Idiot-Speak - I'm your man but be warned, I also do other kinds of speak as well and males cows come to mind here.

The light from the distant object is 'corrected' by the corrector plate at the front of the lens but this is not a factor in the magnification. However, this corrected light then hits the primary mirror and is reflected back up the tube to the secondary mirror, This secondary mirror is convex - i.e. it is shaped such that it bends outwards. When the light hits the secondary mirror it is dispersed 'outwards' and down the optical tube again towards the primary mirror and out through the hole in the centre of the mirror. This 'dispersion' causes the magnification.

That has to be the poorest explanation on Earth but I hope it makes some sense. Calling an SCT guru to tell the tale properly .....

Link to comment
Share on other sites

Oooooooooooookaaaaaaaaaaay.

I think I'm getting there.

I may need to go and draw lines on the picture....

Thanks.

Edit: Ah! Ta Johninderby. I've physically turned my computer screen upside down to view your diagram and it makes much more sense. :) (Yup, it may be loopy to turn it upside down, but like Frank Sinatra, I did it my way. :D)

Edited by lulu57
Link to comment
Share on other sites

Astronut - thought you were a die hard reflector fan :) Cant criticise though as I have given in as well to and have a TAL 100 in the bag plus a completely mental scope that I bought on a whim cos it will look nice in the living room :D

Divorce papers probably going to be served as a result but hey ho ;)

Link to comment
Share on other sites

Astronut - thought you were a die hard reflector fan :D Cant criticise though as I have given in as well to and have a TAL 100 in the bag plus a completely mental scope that I bought on a whim cos it will look nice in the living room ;)

Divorce papers probably going to be served as a result but hey ho ;)

Astronut is obviously something of a dark horse. :hello2::)

I'd like to see a picture of this 'mental scope' - and the TAL, come to mention it! Intriguing...

Thanks for the focal length explanations, guys. I get it now. ;)

Link to comment
Share on other sites

Astronut - thought you were a die hard reflector fan :) Cant criticise though as I have given in as well to and have a TAL 100 in the bag plus a completely mental scope that I bought on a whim cos it will look nice in the living room :D

Divorce papers probably going to be served as a result but hey ho ;)

I am a reflector fan but I'm also a fan of SCTs, especially after a recent viewing of Mars with an 8se. Truth is, as much as I love the dob, when I first posted on SGL I'd had a hankering for a C9.25 click this then this The original plan was to get the C9.25 after a year or so but now 3 years have gone by. The price reductions are very attractive so I think I'll take advantage before it ends. ;) Maybe, just maybe I might be dipping my toes into astrophotography in the not too distant..... ;)

So whats this "completely mental 'scope" about to adorn your living room? Is it an antique brass refractor or something?

Edited by Astronut
Link to comment
Share on other sites

I guess the seconday mirror is a bit like a barlow lens on a refractor that makes the focal length longer than the light path.

The primary on an SCT is typically f/2, something has to get it up to f/10 which is done with the secondary, a bit like a barlow.

Edited by sgazer
Link to comment
Share on other sites

The big primary mirror is 200mm diamter and brings the starlight to a focus 400 mm in front ( with me so far....) at about 350mm from the main mirror is sitting the secondary mirror ( about 75mm diameter) which is a mirror with a "hyperbolic" convex shape, so it does two things:

1. It reflects the beam back down the tube and through the baffle/ hole in the main mirror to the focus.

2. It also acts as a x5 magnifier so the 400mm focal length of the main mirror becomes 400 x 5 = 2000mm

( note also that the f ratio of the primary mirror is also multiplied by 5 so the final "system" become f10.)

This is very similar to the optics in a Cassigrainian telescope.

Hope this helps.

Ken

Mainly correct, but just one niggle:

<schoolmaster-mode>

The original Cassegrain designs had a parabolic primary (like fast Newtonians) and hyperbolic secondary mirror, which yielded perfect on-axis images, but quite distressing coma and astigmatism off-axis. The SCT (Schmidt-Cassegrain Telescope) replaces both primary and secondary by spherical mirrors. These show much more uniform behaviour on- and off-axis, but require a Shmidt corrector plate to obtain good on-axis performance.

</schoolmaster-mode>

Link to comment
Share on other sites

Hmmmm

The newer versions have non-spherical secondaries....

To quote:

"For instance, typical ƒ/2/10 SCT with spherical primary, σ1=0.4 and k=0.25 would, in an aplanatic arrangement, require secondary conic K2=-0.77 for cancelled coma; its astigmatic P-V error would be W=-0.000155(αd)2. Change of the secondary magnification to m=4, corrector separation σ to 0.375 and relative marginal ray height on the secondary k to 0.3 (for identical back focus distance), would require zero-coma secondary conic K2=-1.176 and would have the P-V wavefront error of astigmatism W=-0.000374(αd)2 - greater by a factor of 2.4 (despite that, best image surface curvature would slightly improve, due to the lower Petzval curvature)."

Unquote.

See Schmidt-Cassegrain telescope (SCT)

for even more information.

Ken

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.