Focal length

[keithatrochdale]

Ok

My scope says it is 750mm and 1400mm:

1) What does that mean?

2) How can you choose which you use?

3) What are the advantages of either?

(more stupid questions to follow...........)

[Beardy Bob]

I'm guessing it means that the lens is 75mm diameter (aperture) (lots of people still use imperial in astro, so that's about 3 inches), and the focal length (distance between the lens and where you look through it) is 1400mm (55 inches). That seems very long, is that right?

If so, then you will probably get best results looking at nice bright objects like planets (jupiter is about at the moment) and the moon.

[Beardy Bob]

Oh no, sorry, I misread that. You say it's "750mm". Ah, well, I don't know then. Can you double check those numbers?

[keithatrochdale]

Hi

It is a 150mm (6") mirror, with a quote focal length of 750 and 1400mm.

I find it confusing that it can have 2 focal lengths.

[Beardy Bob]

Aha, I think I get it.

Yep, so your focal length (distance between the mirror at the bottom of the tube and essentially the top of the tube) is 750mm. 1400mm is exactly double 750mm so I'm guessing that you were supplied with a 2x 'barlow' lens. As you're probably aware the barlow lens goes into the eyepiece holder (and then the eyepiece slots into the barlow), which doubles the focal length (to 1400mm). It's a very technical way of saying that the barlow lens doubles the magnification. Your actual focal length is 750mm.

[arad85]

Maths is out there Bob... 750x2=1500

Which 'scope is it?

[keithatrochdale]

Which 'scope is it?

Hi, it is a Seben Big Boss 1400 -150 + motor drive

I am a bit confused, is it 750mm focus or 1500mm focus? If the second figure is with the Barlow lens then you could have a Barlow lens of 2x, 1.5x or 3x and quote a whole host of focal lengths.

[arad85]

Hi,

That 'scope has a focal length of 1400mm. Are you sure the 750 doesn't come from misreading the 1 in 150 as a 7

[Beardy Bob]

Ah, now I remember why I have a calculator built into my mouse mat!

[keithatrochdale]

Hi,

That 'scope has a focal length of 1400mm. Are you sure the 750 doesn't come from misreading the 1 in 150 as a 7

No, the instructions that come with it are very poor ... but it clearly says F=750mm & F=1400mm.

So I take it that it's f is 9.3?

Where would the 150 come from?

[keithatrochdale]

Hi,

That 'scope has a focal length of 1400mm. Are you sure the 750 doesn't come from misreading the 1 in 150 as a 7

No, the instructions that come with it are very poor ... but it clearly says F=750mm & F=1400mm.

So I take it that it's f is 9.3?

Where would the 150 come from?

[arad85]

150mm = 6" and is the diameter of the mirror. F ratio is 1400/150 which is 9.3 yes...

No idea where the 750 comes from - unless as said above, the tube is 750mm long....

[John]

I hope they are not implying that 750 is the max magnfication you could use

Tasco used to do this with their 60mm refractors - "650 x 60mm" the ads used to read

In reality of course it's much, much lower than that ....

John

[Doc]

What eyepieces and barlow did it come with.

If it came with a 4mm and a x2 barlow then that gets you pretty near 750.

[DAVPPK]

I wouldn't buy scopes off ebay though.....

[keithatrochdale]

What eyepieces and barlow did it come with.

If it came with a 4mm and a x2 barlow then that gets you pretty near 750.

6.5mm and 25mm, I think giving 215x and 56x and a 2x Barlow

[dark knight]

Sounds like a bit of dark matter got into the instructions to me... It is a Catadioptric Newtonian telescope. So maybe the tube length is 750mm and the effective focal length is 1400mm, meaning the mirror maybe set up the tube a bit, and the front negative doublet (transfer lens) and secondary slightly down the tube. Just a theory...