Eyepiece focus issue

[Outcast]

I seem to have an issue with a GSO Superview 20mm eyepiece but, I'm not sure. I cannot bring it to focus, the draw tube won't go in far enough.

Here is the whole story for context:

Finally, I have all the bits of my first serious telescope together & tonight I take it out for a bit of a muck around. I'm pretty new to this so, please excuse me if these seem like dumb questions.

Meade Series 5000 80mm APO (purchased secondhand in the IIS classifieds)
Draw tube has approx 120mm of travel
Meade 2" Diagonal (came with scope)
Eyepieces:
30mm 2" Superview (purchased new)
20mm 1.25" Superview (purchased new)
9mm 1.25" TMB Planetary II (purchased new)
6.7mm 1.25" Meade UWA (came with scope)
Televue 2.5x 1.25" Powermate (purchased secondhand thru ISS)

First thing I have noticed is that for all eyepieces the draw tube is in to the last of it's inwards travel. Approx 20mm out for 30mm eyepiece.

Every eyepiece except the 20mm Superview comes to focus fine on its' own.

Every eyepiece including the 20mm Superview comes to focus fine when combined with the Powermate (30mm obviously doesn't fit the powermate)

I do wear glasses but, can see well enough without them to use the scope. I tried it both with them on & off, same results.

Q1. Is it normal for the draw tube to be wound in close to limit of inwards travel when using eyepieces?

Q2. If this is normal, then why is there another 100mm of travel outwards?

Q3. Why won't the 20mm eyepiece come to focus on it's own but, comes to focus fine when paired with the 2.5x Powermate?

Q4. What (if anything) can be done about this?

Q5. Is it possible that there is a problem with this eyepiece?

Cheers for your help in advance

[ZeroID]

Seems a little unusual to say the least. Do you have a 1.25" diagonal you could try ?
Might be that the 2" diag provides too long a light path. (Just a suggestion, I don't have a 2" to check but I do have a 102 scope )

[Outcast]

Unfortunately I do not have a 1.25" diagonal to try.

[g__day]

Agree it sounds odd - what is the focal length written on the tube and what is the length of the tube from lens to end with the focuser racked all the way in?

In short term can you use say a 35mm 2" barrel with a 2" to 1.25" converter - about $39 at Bintel.

[Outcast]

Focal length written on the tube is 480mm
Length measured from lens to end of focusser tube (racked all the way in) is 385mm. I have not included the diagonal length in this measurement.

I looked at the 35mm 2" barrel on Bintel, isn't that for not enough back focus? My problem is not enough in travel on the focuser for this eyepiece. How will an extension tube assist?

Or have I completely misunderstood your advice?

[Outcast]

Maybe there is a better way to explain this & better questions to ask:

My problem is not having enough inwards travel on my draw tube for one eyepiece in particular, the 20mm Superview. It's almost in focus but, not quite & the draw tube won't go in any further.

Am I right in thinking that I need to somehow shorten the light path for this EP to come to focus?

Would a change to a 1.25" diagonal achieve this?

Would a recessed 1.25" adapter achieve the same thing?

Is my scope just not good with a 20mm 1.25" EP?

Why does my scope have 120mm of focusser tube travel if, when using EPs it only uses the first 20 - 30mm?



Is there potentially something wrong with the scope??

Are my eyes that screwed up??

[Robh]

Hi Carlton,

You are correct about shortening the light path to the EP.

The refractor may be designed to focus EPs without a diagonal. That's why you might have less in-travel with the diagonal attached. The 30mm probably comes to focus because it is 2" and goes further inside the drawtube.

If your 1.25 inch adapter has a 1cm flange find one with a narrow flange (2mm) for the 20mm EP if it nearly focuses. Or you could use it without the diagonal. Not as comfortable though.

If your other EPs work fine there is certainly nothing wrong with the scope.

Regards, Rob

[Pinwheel]

This wouldn't be another telescope shipped out with the 1.25"~2" EP adapter still in place? It was discussed in a post only last week.

Thread #4 & 5 http://www.iceinspace.com.au/forum/showthread.php?t=108647

[Varangian]

If you're set on using the 20mm eyepiece buy 1.25 inch (I.m assuming the 20mm is 1.25 inch?) extension tube ($20) and insert this into your focuser and then insert the 20mm into that. This will set the eyepiece back so to speak and give you focus without having to rack the focuser all the way in.

[Outcast]

Thanks for the confirmation Rob,

Tossing up whether to buy a low/negative profile adapter or simply look for a 17 or 18mm EP & see if that works.



No, I don't believe so. 2" diagonal straight into a 2" focuser. 2" EP holder is fixed to the top of the diagonal, 1.25" adapter straight into that.



Thanks but, not really the convenience I was looking for