telescope length ~ focal length?

[kheong sann chan]

Hi,

I've been reading up on the focal length for reflector telescopes. I recently purchased a Visionking 1400150 from China. The numbers mean focal length=1400mm and aperture=150mm with F=9.33. However the telescope itself is less than a meter long. It is not the Schmidt-Cassegrain type.

From my understanding so far, the telescope length should be approximately equal to its focal length if the light is not folded inside the scope. So is it possible that my scope is 1400mm given it is quite a bit shorter than 1400mm? How do I confirm that the focal length is 1400mm?

I haven't actually measured the length of the telescope yet, but just eyeballing it, it looks less than 1m. I'll do that when I'm home tonight.

Thanks.

[kheong sann chan]

Firstly, I want to correct my previous post that the telescope model number is 1501400 rather than 1400150.

Secondly, I've found the issue after going to the visionking website. The telescope focal length is actually 750mm, not 1400mm. The seller's website advertised it as 1400mm but the manufacturer website states its 750mm...

750mm is much closer to the telescope's physical length, so I believe that one.

Anyway, just a warning to anybody else who is purchasing this scope online, it isn't 1400mm.

However despite the mis-statement on the focal length I'd still recommend this scope for it's price.

But my previous questions still stands: is there a quick easy test I can do to check the focal length of the scope?

[steppenwolf]

Your telescope may have an 'effective' focal length of 1400mm as opposed to an 'actual' focal length of 1400mm. This is accomplished by the use of a Barlow type lens (often referred to as a Bird-Jones lens) built into the focuser.

[pixelsaurus]

These scopes seem to be sold under various guises. I have a blue one sold in NZ under the Eden Optics brand and my wife bought me a green one in the US which was sold as a Baytronix. Some of the American reviews are quite scathing of them however I have always felt the they do offer some value for money. When I bought my Eden Optic's, I did so in the knowledge that I couldn't build the OTA for the price, let alone the mount as well.

It is 1400mm effective focal length. The primary mirror focal length is usually around 700mm but as it is not parabolized, to improve image quality, a 2x "barlow" is screwed into the base of the eyepiece holder. A spherical mirror working at f/9.3 requires little, if any, parobolization. The big drawback of this optical arrangement is that collimation is difficult as you can't tilt or move the "barlow". However, it should be enough telescope to whet your appetite astronomically.

[kheong sann chan]

Thanks for the info Steppenwolf, pixelsaurus. And you are both correct, there is a built-in lens after the after the secondary mirror that should be converting the 700mm into 1400mm. This is my first scope and I've had to do a lot of reading over the past few weeks to get caught up with the physics, optics, terminology and usage of the scope. 

One of the things I read is that I should not go above 50x per inch of aperture in magnification, above which the quality degrades. I got a 5mm eyepiece at 1400mm it should give 280x which is less than 6inches * 50x per inch. But the performance of this eyepiece is still a little disappointing. Is this due to the build in Barlow you think? Or might it be because the mirror is not parabolized? Just to confirm, pixelsaurus, what you meant, the mirror in this scope is spherical, not parabolic is that right?

Since this is my first scope, I don't have a point of reference. Is the image quality from this scope poorer than other similar aperture scopes? I do find the picture a little muted, or low-contrast, but like you say, I find it to be great value per dollar when I'm not stressing it to its limit. The 25mm EP that comes with scope produces a better picture than the 5mm I bought later. The 10mm EP (that also comes with scope) seems a little flaky.

I was thinking of getting the Celestron Luminos 7mm since it looks good and I hear good things about it. Would you recommend it for my scope?

[kheong sann chan]

I guess I should say what I'm wanting the EP for. I would like to see as much planetary detail as I can with this scope.

[John]

If this scope is anything like the Baytronix or the Seben Big Boss (both of which are 150mm / 1400mm and both use a bird-jones corrector) I don't think you will be able to use anything like 280x as the optical quality of the scope will not be able to support that.

A practical maxmimum power of 150x will give crisper views - for that you would use something like a 9mm eyepiece. 

[pixelsaurus]

Just to confirm, pixelsaurus, what you meant, the mirror in this scope is spherical, not parabolic is that right?

Correct.

I have 2 of these , or similar, scopes and I wouldn't want to spend too much on eyepieces etc. My personal opinion is these scopes, as they stand, are just not up to it. A long term plan for me is to strip the coating off one of the primary mirrors (the one that seems to have suffered some environmental damage) and parobolize it myself. I can then ditch the corrector, upgrade the focussing unit, and install a bigger secondary mirror (the current one is quite small) and effectively turn the scope into a 150mm f/4.6 Rich Field Telescope (RFT). Such instruments were quite common prior to the 1980's.