Why such a big difference in focal length?

[Razsky]

Hi there,

Was hoping you guys can ans some qns for me.

I have been looking to buy a scope for quite some time now. After much considerations, etc ( budget, size, portability), i am now left with 2 options.

1st) Orion StarBlast 4.5 EQ Reflector

http://www.telescope.com/control/produc ... t_id=09798

2nd) Celestron Firstscope 114 EQ Short Newtonian Reflector

http://www.astro.com.sg/telescopes/cele ... 114eqs.php

I actually prefer the Orion starblast scope. However, i cant help but notice the huge difference in the focal length of these 2 scopes.

Can someone enlighten me on this?

Why the huge difference? I feel that a scope with 1000mm focal length will provide comfortable power/magnification with that aperture. An 450mm, i feel, handicaps a scope of that aperture in terms of the power/magnification it can provide. Given that i use a 9mm eyepiece on both scopes.

Am i right to think this way?

I stated 9mm as i have looked through a 9mm eyepiece before and i find it comfortable. I won't really go for a 4mm eyepiece. Don't you think it's too small for viewing pleasure?


Thanks in advance!

Razsky

[Area503]

There's a HUGE difference. First thing to note would be the f ratio, which the focal length divided by the diameter of the light collection medium, which in this case is the mirror. Orion star blast is a f4 scope, compared to the f9 Celestron short tube. So technically, the images from the orion star blast is about ~2 time brighter than celestron short tube. (Just so u know, it is NOT 9/4 = 2.25 )

One more thing to point out about the celestron short tube. Its 19" tube is about 482.6mm, which is shorter than the focus length of 1000mm. So if u recall sec school physics, there is something funny about the focus length. The reason why they can do that is because there is a barlow in front of ur eyepiece, which is NOT removable. Barlow makes images dimmer (then it alrdy is). So in terms of true focus length, they are both roughly the same.

I would suggest that rather than buying the Celestron short tube, u might be better off buying the Orion star blast, and then buy a barlow. At least with this configuration, u can remove the barlow anytime, and still get the 1000mm focus length u are after.

[Razsky]

Wow. Thanks alot!

I really didn't know that the celestron scope came with an attached barlow. Thought that it was a different kind of reflector.

Haha.

I really preferred the Orion scope in the 1st place. I guess looks does matters.

Do you guys have any more recommendations? Last time i checked, this setup costs less than $600. I think thats the cheapest scope i can ever find.

Thanks!

Razsky

[jermng]

Do you guys have any more recommendations? Last time i checked, this setup costs less than $600. I think thats the cheapest scope i can ever find.

Look in the Buy & Sell section. Weixing is selling his 8" Newtonian at a good price.

[Razsky]

Yeap.

Done that.

Just too big la. I dun think i can even make it to the obs site carrying all those stuff.

That said, i got another qns. I remember seeing someone putting up a WTS for his Celestron 114 GT.

If i am not wrong that scope is on an motor driven mount.

The thing is that with these motor mounts, am i able to use it if i dun power it up? Must i use the motor to turn the scope? Do i still have the option of adjusting the scope manually? Will it like damage the mount or something, if i adjust it?

Thanks alot!

Razsky

[weixing]

Hi,

Orion star blast is a f4 scope, compared to the f9 Celestron short tube. So technically, the images from the orion star blast is about ~2 time brighter than celestron short tube. (Just so u know, it is NOT 9/4 = 2.25

Err... what you mention only apply to photographic, but not visual. Brightness of the object seen through telescope depend on aperture of the scope and magnification used... the larger the aperture, the brighter the object will appear at same magnification or the lower the magnification, the brighter the object will appear if aperture is the same.

The Orion StarBlast is a pure (the length of the scope is similar to focal length of the scope) and very fast Newtonian while the Celestron Firstscope 114 EQ Short Newtonian is a hybrid Newtonian (the length of the scope is usually a lots shorter than the focal length of the scope due to a built in corrector lens). I and I think most of us here will not recommend the Celestron Firstscope 114 EQ Short Newtonian as this type of telescope usually are not that good.

By the way, Orion StarBlast is a very fast Newtonian, so the collimation will be very critical to get good image at higher power. Also, you'll need better eyepiece to get good low power, wide field view for fast Newtonian.

Anyway, review for Orion StarBlast are quite good, so I think it might be a better choice of the two.

Have a nice day.

[Meng Lee]

The Celestron 114 GT is the one with the built in "barlow" lens, please avoid it if you still want to enjoy astronomy.

Without power, the GT is unusable and manually pushing, will damage the motors.

[rlow]

It would be advisable for beginners not to go for a very fast scope (eg, f/4) for observing, as you will usually need premium eyepieces (read that as 'expensive' eyepieces) to get sharp-to-the-edge images, unless you are doing imaging, or you have deep pockets. For a scope with focal ratio of f/8 to f/10, cheap eyepieces will work well enough.

If you do want a fast scope after all, you can also consider a second-hand Vixen R135SS, which is available here for a good price.

[acc]

Razsky its great that you are doing your homework here before buying a scope The starblast is not a bad scope but as otehrs pointed out, its collimation is critical and you need better eyepieces to bring out its potential. Best bet is to be patient (i know that's very difficult...) and look out for fantastic deals on used scopes that pop up here once in a while. If you buy from places like astro scientific at the omni theatre, u will be paying much much more due to their high (and sometimes very high) markup.

cheers
cc

[Razsky]

Hi all. Thanks for your comments!

Ok. Now i am stuck again. I have been on a look out for quite a few months. I have considered the Orion Starblast for quite sumtime. In fact, i am actually waiting for astrobargains to get back to me and if everything goes well, i hope to be obbing by this weekend.

So here goes.

Premium=Expensive
How expensive?

I choose the Orion Starblast because i intend to do imaging sometime down the road.

Another thing about it is i find it cheap. Below 600 for the whole set, eq mount and so on.

In terms of ampertureVSportabilty. i think 4.5' (reflector) is a comfortable number.

One thing that i am hesitant, however, is collimating. I have not colimate a scope before. I did however, read a scope's instruction manual and its seems a little simple compared to what i have been told.

I have looked thru wei xing's R135 and twice for sam's C5.

I can't say much about R135 as the weather was bad that day. But as for sam's C5, i am happy with what i saw.

Another thing i learned is the importance of a good mount. The R135 was shaking just from the wind that day as it was on a camera tripod. The vixen porta (i think tats the name) mount that the C5 was on was so simple and easy to use. I believe the Starblast is on a decent mount.

I do not want to limit myself to just planets and bright star clusters. Galaxies, Nebulae are also on my "to observe" list. I dun mind if the image is not super duper clear. Just a clear enuff one for me to enjoy will do for now. Just like the Eskimo nebula, Jewel Box, Saturn and its moon, was seen thru sam's C5. Will the default eye pieces from Starblast be abl to do that?

OMG. I think i had typed too much.
Comments and opinions are greatly appreciated.

THANKS!

Razsky