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Removing built-in Barlow from focuser??

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#1 maroubra_boy

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Posted 14 March 2013 - 06:21 PM

Hi all,

I'm considering getting a short tube reflector, but the one in question has one of these blasted barlow lenses built into the focuser. As it is the f/ratio is 7.7, giving an effective focal length of 1000mm. I'm suspecting a spherical mirror, but as this scope will be used for video, could focusing still be a problem at f/3.85 on the small area that is the GSTAR-EX chip (very similar to the Mallincam Jr)?

Alex.

#2 maroubra_boy

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Posted 14 March 2013 - 06:37 PM

I should note that I have a Celestron FirstScope, being f/3.9, very similar to the scope I'm looking at. It has a spherical mirror, and other than a imperfect final figure, it does give a very nice image at low power using half decent eyepieces, such as a 25mm plossl.

From this I know that a spherical mirror does work with the human eye (at low power), but how would this be with the chip of a video camera considering it is not as adaptive as a human eye?

#3 jgraham

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Posted 14 March 2013 - 09:22 PM

Unfortunately, that's not just a Barlow, it's also a corrector lens. The mirror in these short tube Bird-Jones Newtonians is spherical and won't work without the corrector lens in place.

#4 GlennLeDrew

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Posted 14 March 2013 - 10:04 PM

A spherical mirror at low power doesn't seem so bad only because of the small image scale. A video camera has a field of view about equivalent to a 6mm eyepiece... See how visually obvious the spherical aberration is with such an eyepiece installed! Your video camera, even though it hasn't near the eye's resolving power, will readily enough show such SA.

#5 maroubra_boy

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Posted 14 March 2013 - 10:13 PM

Thanks John.

After reading your reply I've been reading up on this Bird-Jones design. The old Tasco is such a scope with the corrector lens sitting between the primary & secondary. This scope shows when done correctly it works very well.

Thing is, this scope has this lens placed in the focuser tube, so it's not a true Bird-Jones design. How annoying!

When you say "it won't work", what do you mean? Focus it should, despite the spherical shape. The actual focal point would be well outside the travel (in or out) of the focuser?

I just want to really understand the situation of how & why :)

Alex.

#6 maroubra_boy

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Posted 14 March 2013 - 10:15 PM

Ahh, thanks Glenn.

I saw your reply only after I posted mine.

#7 ccs_hello

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Posted 15 March 2013 - 04:39 PM

Flex the OTA mirror to get a better result:
http://www.cloudynig...ts/convert2.pdf

More on Bird-Jones (properly corrected ones) vs fake (Barlwo only) cheapies: CN linkie.

#8 maroubra_boy

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Posted 18 March 2013 - 04:10 AM

Fixe the OTA to get a better result:
http://www.cloudynig...ts/convert2.pdf

More on Bird-Jones (properly corrected ones) vs fake (Barlwo only) cheapies: CN linkie.


What a fantastic link, the first one!!! Just the sort of thing that also appeals to me to undertake, :) Thanks so much for that CCS.

I reread one of my earlier posts, an I noticed that I had omitted the model of Tasco reflector I was talking about, the 8V - that is a true Bird-Jones reflector.

#9 Dragon Man

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Posted 18 March 2013 - 09:14 AM

G'day Alex,

I hope you get it sorted.
Sorry I haven't been able to help much via email, but there are a few people in here that will steer you right :waytogo:






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