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Not reaching focal point?

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#1 Metalmanstan

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Posted 11 December 2012 - 09:51 PM

So I was looking at Jupiter tonight and I wanted to see closer. So I slapped my 2x barlow with a 10mm eyepiece swung it over and tried to focus down and I couldn't go down anymore, wouldn't even reach focal point. Went back up and it made it more distorted. I had a situation like this before but with a different telescope and I needed more height to reach the focal point. Now I don't have enough. The focus is fine without the barlow. Any suggestions? :grin:

#2 panhard

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Posted 11 December 2012 - 10:05 PM

Have you had this problem with the same eyepiece and the same scope before?

#3 Metalmanstan

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Posted 11 December 2012 - 10:27 PM

No sir. The other scope that had this sort or problem was an old Edmund scientific but that needed more length to focus. The orion xt12i that I'm using only does this with a Barlow on and only when I'm focusing inwards(at the end I'm still not focused) . Every other eyepiece without the Barlow works fine. I'm just confused I guess...

#4 panhard

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Posted 11 December 2012 - 10:54 PM

What where the sky conditions like at the time?

#5 JamesL

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Posted 11 December 2012 - 11:21 PM

What kind of barlow is it? Is it a shorty barlow?

#6 sopticals

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Posted 11 December 2012 - 11:26 PM

You may need to move your primary mirror forward by uniformly screwing in the collimation screws, to give that little bit more infocus needed.

Stephen.(44deg.S).

#7 Pharquart

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Posted 12 December 2012 - 03:58 PM

I have to move my focuser inward about 1/2" when I add my 2x Barlow in the chain. That's pretty consistent with all of my eyepieces ranging from 40mm to 10mm, though some require an additional 1/16". My 3x Barlow requires about 1/4" movement inward from the unBarlowed state. I suspect each Barlow may produce different results.

If you can't reach inward focus, you either have to move your primary closer in (tightening the collimating screws as much as you can while still maintaining collimation will get you some without major tube modifications) or moving the focuser down closer to the primary. You also then need to move the secondary down to match.

Brian

#8 Starman1

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Posted 12 December 2012 - 06:35 PM

Standard Barlows require 1/2" to 3/4" additional inward focuser travel.

#9 Metalmanstan

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Posted 12 December 2012 - 06:54 PM

So what if I move the mirror up as much as I can and nothing changes? What could be the next step? And it is a shorty Barlow.

#10 Starman1

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Posted 12 December 2012 - 08:35 PM

So what if I move the mirror up as much as I can and nothing changes? What could be the next step? And it is a shorty Barlow.

1) You could use a lower-profile focuser (shorter).
2) You could use a TeleVue PowerMate instead of a standard Barlow (doesn't require the inward focuser travel)
3) you could remount the primary mirror higher in the tube (might necessitate changing the trunnions on the side of the tube since the balance point would change)
4) You could bring the secondary mirror a little closer to the focuser (say, by 1/4"), which would still allow you to collimate. It would tip the optical axis slightly toward the focuser, but that would only be of any importance if the scope had a set of digital setting circles (computer) attached to it, as it would reduce the accuracy of pointing.
5) You could try another eyepiece with a focal plane farther down in the eyepiece barrel. That would require additional out-travel of the focuser, which would be OK in your case.
6) You could use the barlow lens threaded onto the bottom of the eyepiece instead of in its barrel (if the barlow lens unscrews from the barrel of the Barlow). That lowers the magnification factor of the Barlow, but also requires less in-travel on the focuser.
7) You could take the focuser to a local machine shop to see if they could machine the focuser drawtube to allow more inward travel of the tube. If for example the eyepiece setscrew his the body of the focuser and limits the in-travel of the focuser, a slot in the focuser body could be machined to allow the focuser drawtube to move in even further.
8) You could use a combination of all of the above and achieve probably another 1/2" of in-travel. But I think either #5, #6, or #2 is simplest.

#11 Metalmanstan

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Posted 17 December 2012 - 07:07 PM

Thanks for the info guys!!! I moved the secondary closer and I also threaded the Barlow tobthe bottom of the eyepiece so I could get to the focal point. Thanks for the help!!

#12 Metalmanstan

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Posted 03 February 2013 - 12:25 PM

Ok, apparently I did not solve the problem entirely. What about a negative profile eyepiece adapter? I haven't seen any reviews on it and I don't see on the picture of it how it would clamp or hold the eyepiece in. Anybody got any info on this product?

http://www.scopestuff.com/ss_npea.htm

Stash

#13 Starman1

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Posted 03 February 2013 - 02:22 PM

Ok, apparently I did not solve the problem entirely. What about a negative profile eyepiece adapter? I haven't seen any reviews on it and I don't see on the picture of it how it would clamp or hold the eyepiece in. Anybody got any info on this product?

http://www.scopestuff.com/ss_npea.htm

Stash

This one has 2 small steel setscrews to tighten on the eyepiece barrel.
You could tighten them *just* enough to require a slight pressure to insert the eyepiece. That's typically how they are used if not a semi-permanent addition to an eyepiece.

That's a little easier with the similar adapter from Astrosystems.biz, (his website is incommunicado right this minute) which, IIRC, has a nylon setscrew to press against the eyepiece.

Either one will drop the eyepiece quite a bit IF the eyepiece is small enough to fit into the recess. Ironically, some larger eyepieces are, while some smaller eyepieces aren't. Trial and error prevails, I guess.

#14 Jim Rosenstock

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Posted 03 February 2013 - 02:24 PM

I have one and it works well. It allows you to "drop" a 1.25" eyepiece a little deeper into a 2" focuser...and provides that little bit of extra infocus that a couple of my eyepiece/scope combinations require.

It "clamps" onto the eyepiece (or barlow) with two allen setscrews. In practice, I rarly use the setsrews, I just drop the eyepiece in....in most observing positions, gravity holds it in just fine.

IMO it's a good product, and Scopestuff is a GREAT supplier! :bow:

Jim

#15 Metalmanstan

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Posted 03 February 2013 - 07:01 PM

So how does the eyepiece stay in the adapter without sliding out? Or is this something I would have to get for every eyepiece? I'm very interested in this product!! It might just solve my problem!!

Stash

#16 Starman1

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Posted 03 February 2013 - 07:12 PM

So how does the eyepiece stay in the adapter without sliding out? Or is this something I would have to get for every eyepiece? I'm very interested in this product!! It might just solve my problem!!

Stash

If you adjust the setscrews to touch the 1.25" barrel with *just* enough force to require you twist the eyepiece in or press it slightly, the fit will be tight enough. Maybe not tight enough to hold a heavy eyepiece upside down, but enough pressure to hold the eyepiece in.

#17 Metalmanstan

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Posted 03 February 2013 - 07:44 PM

Excellent. Most likely be ordering that this week. I thank you for all your help Jim and Don. Don, It seems like you are the one who answers every question I ask!! :-)






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