Question Telescope Focal Length

[ednyfed]

So I've got 2 Newtonian Refractors (Celestron AstroMaster 114 & 130). The 114 has a shorter physical tube with a Focal Length of 1000mm and the 130 has a significantly longer tube but a Focal Length of 650mm.

If Focal Length is measuring the physical distance light travels in the tube, I'm not understanding how the longer tube (with a longer distance between the mirrors) has a shorter focal length.

Any ideas?

 

[Pogo]

I looked up both scopes at Celestron's site. The 130 states that the tube length is 610 mm which makes sense for a 650 mm fl. The 114 page states the fl is 1000, but does not state the tube length, I find that kinda interesting they don't state that. The focal ratio seems to be correct in both cases assuming the fl is correct. I did see some youtube videos of both, from what I could see, they are both actual newtonians. I was thinking maybe the 114 was more like my SCT with a folded light path, but, doesn't seem so. Just realize that the the focal length is measured from the mirror to the focal point of the eyepiece. So, a 1000 mm mirror and a 25 mm eyepiece at focus, it should measure 1025 mm from center of the mirror, up to the center of the secondary, plus out the side to the glass of the eyepiece. The 114 sure doesn't appear to be a meter long like that. Incorrect advertising? Otherwise, not a bad looking scope. Enjoy. Not sure about the usefulness of the 9mm eyepiece they advertise, that might be stretching the capability of the scope a bit. I got a 60 mm f/20 refractor when I was about 9. It was advertised as a '600x' scope, came with 25 mm down to 4mm, but the 9, the 6, and the 4 were pretty much useless for such a small scope. Still have it and still use it, but, just for low mag.

 

[Catastrophe]

Can you buy a nice big telescope, with autofind and all that stuff, linked to a tv screen - so you can sit in comfort in your living room and explore the Universe? Worth a couple of hundred dollars, I think!

Cat

 

[Pogo]

Sure can. All the major manufactures make ‘GoTo’ scopes, once you use a two star alignment, it’s computer has a catalog of everything you may want to see. Then you can buy a camera to replace the eyepiece which can send images or videos to a computer, some by Bluetooth. Just be prepared to spend some $$. You can browse Meade, Celestron, Orion to start with, many good scope makers I’m not familiar with. Just stay away from the cheapies.

 

[Catastrophe]

Yeah, but I said a couple hundred dollars? What do you suggest?

 

[Pogo]

Hard to find computer scopes at bargain prices, but a quick search, I found a Celestron 114LCM on eBay $305, new condition. Celestron sells for $369. 114 mm is about 4.5 inches, 1 m focal length, it comes with 25 mm & 9 mm eyepieces, which gives you 40 and 111 magnification, not bad. Just point it at three known stars and tell it what they are, then it knows where everything is. Later you can get an inexpensive WiFi or Bluetooth cam to replace the eyepiece after alignment, then you can see stuff on your devices. If get this or one like it, you should learn collimating and stuff like that to get and maintain your scope. Like anything else, there’s always stuff to do to maintain your new toy. Hope you live under a pretty dark sky. My old 60 mm refractor, I found quite a few objects, but didn’t memorize everything, but when I got my goto scope it knows where stuff is that I could never find myself.

 

[Catastrophe]

Pogo, thank you very much for the detailed reply. I have to admit that my question was a little tongue in cheek as, according to that scenario, I would not have had the experience of actually seeing the Galilean moons wobbling around through a not very 2 and half inch refractor about 70 years ago. Nowadays I am prepared to imagine the cold (well, some of it) and otherwise experience the wonder of the Universe vicariously through some of the largest telescopes in the world. But, thanks for your effort which is appreciated.

Cat :

 

[Pogo]

Any time, here to help. Hopefully this thread spurs a couple of folks into the hobby. I enjoy the discussions on this forum.