Focal Length Quarry

[wallis23]

Hi. I am new to this site and this is my first post...

I have a simple question... I have just brought my 2nd scope. It's a 6" "short tube" reflector, and i've just done some (basic) research on telescopes.

Basically, i've read that the focal length of the 'scope is related to the distance from the primary to the secondary mirror. Does this mean that my "short tube" won't have as much magnification as a standard length 'scope? (If they are both 6", say...)

Also, it says my 'scope's focal length is "750mm or 1400mm (when synthesized by the correction lens.") - What does that mean? Is it saying it's 750mm standard, but 1400mm if i use a 2x Barlow, or a 2x Erecting Eyepiece? I didn't get an erecting eyepiece with my 'scope but maybe they supply others with this and use the same handbook?? (I did get a 2x Barlow...)

Another quick question - Will my "short tube" 6" 'scope have the same magnification as my old 4.5" standard length 'scope?

Any help will be greatly appreciated. Thanks.

[Andrew1972]

With regards the last bit, the magnification you get all depends on the focal length of the scope. The one with the longer focal length will give you higher magnification when using the same eyepiece. The aperture (4.5" or 6") doesn't increase or decrease the magnification - just the level of detail you can expect to see. If you use a 25mm eyepiece in your 750mm focal length refractor you will get 30x magnification (750/25 = 30), if your long tube refractor had a 1000mm focal length you would get 40x magnification.

With regards your first question, i would think if you've got a short tube refractor your focal length will be 750mm.

Welcome to SGL by the way!

Edited to say I've just seen that you're actually talking about Reflectors! Whoops!

[steppenwolf]

Also, it says my 'scope's focal length is "750mm or 1400mm (when synthesized by the correction lens.") - What does that mean?

I suspect that there is a 'corrector lens' built into the telescope focus tube. The primary mirror is probably spherical (as opposed to parabolical) and the 'corrector lens' produces an even plane of focus but in so doing also nearly doubles the EFFECTIVE focal length from 750mm to 1400mm.

What make is the telescope and is it described anywhere as 'catadioptric'?

[wallis23]

The aperture (4.5" or 6") doesn't increase or decrease the magnification - just the level of detail you can expect to see.

Ah, i see. I always thought a bigger mirror meant higher magnification for some reason. Thanks for clearing that up.

I suspect that there is a 'corrector lens' built into the telescope focus tube. The primary mirror is probably spherical (as opposed to parabolical) and the 'corrector lens' produces an even plane of focus but in so doing also nearly doubles the EFFECTIVE focal length from 750mm to 1400mm.

What make is the telescope and is it described anywhere as 'catadioptric'?

Ok. I think i understand that...

I don't know who the scope is made by. lol. It doesn't say on the box, in the book, and doesn't say on the Ebay ad either. To be honest i know it's not a good make because it only cost £90 new. i just got this one for the time being while i save up for a decent 8" 'scope.

It say's the mirror is concave, hard coated. The Resolving Power is 0.93", magnification is 30x, 115x, & 56x, 215X. The mount is a ET-8 Equatorial, but that probabaly doesn't help much, lol.

I can't see anything about 'catadioptric'. What does that mean?

Thanks for both of your replies, they were very helpful!

[steppenwolf]

As it is unbranded and bought from Ebay, I suspect even more that this is a catadioptric telescope (i.e. it is a telescope made up of both mirrors and lenses). The description of the 'concave' mirror is a bit of a give away too. Almost certainly this is a 'native' 750mm focal length and the corrector lens is giving you an EFFECTIVE focal length of 1400mm.

[wallis23]

The description of the 'concave' mirror is a bit of a give away too. Almost certainly this is a 'native' 750mm focal length and the corrector lens is giving you an EFFECTIVE focal length of 1400mm.

Does that mean if i get another scope, still a 6", with an actual focal length of 1400mm it would give me better magnification and allow for better detail?

Also, my old 4.5" scope had a focal length of about 1000mm or 1200mm, can't remember which, had a standard length tube, and was made by Tasco. Do you think that had roughly the same magnification and detail as my new 6"?

One more quick question (sorry). Are standard length scopes better than "short tube" scopes? (If they have the same aperture.) And do standard length scopes have a bigger focal length than "short tube" scopes?

(Sorry i'm a total newbie when it comes to this...) Your replies are extremly appreciated.

[blinky]

The description of the 'concave' mirror is a bit of a give away too. Almost certainly this is a 'native' 750mm focal length and the corrector lens is giving you an EFFECTIVE focal length of 1400mm.

Does that mean if i get another scope, still a 6", with an actual focal length of 1400mm it would give me better magnification and allow for better detail?

Also, my old 4.5" scope had a focal length of about 1000mm or 1200mm, can't remember which, had a standard length tube, and was made by Tasco. Do you think that had roughly the same magnification and detail as my new 6"?

One more quick question (sorry). Are standard length scopes better than "short tube" scopes? (If they have the same aperture.) And do standard length scopes have a bigger focal length than "short tube" scopes?

(Sorry i'm a total newbie when it comes to this...) Your replies are extremly appreciated.

A 6"'scope with a focal length of 1400 would be 1.4m long (Assuming it's just a 'normal' newtonian reflector).

The magnification of a scope is the focal length of the telescope divided by the EP Length. So for a 1000mm lenght reflector with a 10mm Eyepiece you would get X100 magnification.

[wallis23]

A 6"'scope with a focal length of 1400 would be 1.4m long (Assuming it's just a 'normal' newtonian reflector).

The magnification of a scope is the focal length of the telescope divided by the EP Length. So for a 1000mm lenght reflector with a 10mm Eyepiece you would get X100 magnification.

Thanks for the reply. I'm getting there. Slowly but surely. LOL.

[steppenwolf]

Does that mean if i get another scope, still a 6", with an actual focal length of 1400mm it would give me better magnification and allow for better detail?

Nope, the magnification would be the same (this is directly related to focal length, real or 'effective') BUT the image quality would almost certainly be better with a 'native' 1400mm.

Also, my old 4.5" scope had a focal length of about 1000mm or 1200mm, can't remember which, had a standard length tube, and was made by Tasco. Do you think that had roughly the same magnification and detail as my new 6"?

Really hard to say but catadioptrics like this don't normally give the best images!

One more quick question (sorry). Are standard length scopes better than "short tube" scopes? (If they have the same aperture.) And do standard length scopes have a bigger focal length than "short tube" scopes?

As mentioned above, it is better to use a 'native' focal length than an 'amplified' one using what is known as a 'relay' lens of 'corrector' lens like the one you have. The effective focal length of a 'native' 1400mm 'scope will be the same as the 'effective' focal length of a magnified 'short tube' reflector but note the word 'effective'!

[wallis23]

Many thanks for your reply. It has really cleared things up!!

So they're basically saying "it's focal length is 'effectivley' 1400mm, but don't expect much detail." LOL.