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Eyepiece dilemma - what now?

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Well, I did it. Spent ages building my eyepiece collection to suit my C9.25 at 2350mm focal length. It was perfect. But now I have a 1200mm focal length Dob! Just use a x2 Barlow then... but... what other choices are there? My x2 Barlow is a 2" and this scope doesn't like heavy kit at the eyepiece end, especially if I stick an LVW in.

I have a 12mm, 10mm and 9mm Vixen NLV set which is now sort of obsolete. I could swap them for a 6mm, 5mm and 4mm SLV set. That would give x200, x240 and x300. Very nice but expensive.

I do like using my orthos though. So how about a x3 TV Barlow? It has quite a reputation. I could get, with just 3 eyepieces:
18mm                 x67
12.5mm              x96
9mm                 x133
18mm + x3       x200
12.5mm + x3    x288
9mm + x3         x400
Add in the 7mm at x171 and that's a really nice spread, with the x400 reserved for close doubles.

Yesterday on the moon I was using a Meade 140 nosepiece screwed in to my 9mm and 7mm orthos. That gives x208 and x300. The 7mm on it's own gave some wonderfully contrasty views at x171; the other combos were too strong for the conditions. A bit fiddly in the cold though.
Using the Meade 140 is an option, but I don't like the lack of compression rings, it's a screw type. I do have a 6mm TMB but it seems to lack 'crispness' compared to the orthos.

You can never have too many eyepieces of course. But I would like to be realistic. Suggestions welcome :smile:

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Michael you already have some fantastic eyepieces which will work great in your new 10" GoTo Dob. I use very heavy 100 degree EPs in my 12" Dob and its just a matter of using a weight to balance the scope. I have a weight with a large magnet underneath and I just move it up or down to get fine balance.



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Posted (edited)

You could move somewhere with worse seeing that would limit your magnification! 

I say that in jealousy as I consider myself lucky if I can go higher than ~150x most nights.

Your Barlow plan seems reasonable. Did you get rid of the C9.25?

I'm in the process of buying a 10" dob myself; I plan to use an APM 24 UFF and Morphei 17.5, 12.5, 9, and 6.5. I've got a 2x Barlow if the jet stream decides to take the night off.

Edited by Virtus
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I also used a large magnet at the bottom of the tube, opposite to the focuser. No balance issues even when I the ES 30mm 82 deg was in the focuser.

10" is a nice aperture. Televue and Pentax XW?

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I do like a three eyepiece plan - one in the scope, one in each pocket 😛

2 hours ago, Virtus said:

Did you get rid of the C9.25?

Yes, the mount was too heavy for me to lift. I've already used the Dob more since it arrived on Monday than the C9.25 in the last two years :ohmy: On the rare occasions I got outside, after lugging the EQ6 around my back was already destroyed and observing time was measured in minutes. So It had to go. The Dob I can lift easily in its two parts.

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Tonight I've been using the Meade 140 Barlow. Ancient though it is, the optical quality is outstanding. It's supposed to be x2 but measures a lot more than that.

Here's a list of the magnifications using the measured factor of the 140 rather than the nominal x2. I've included the ratio between each as this suggests this is a selection close to being perfect.

18mm                    67    Ratio
12.5mm                 96    1.4
9mm                    133    1.4
7mm                    171    1.3
12.5mm + 140     225    1.3
9mm + 140          318    1.4
7mm +140           415    1.3

Seeing conditions tonight weren't good enough to support high powers - I was mostly enjoying the 18mm + 140 Barlow giving x144. But I did compare the x171, x225 and x318 and it seems a good progression.

My only concern with the 140 was the fact it uses a screw to hold the eyepiece and might damage them. However, in practice, the Barlow/eyepiece fit tolerance is so small the eyepieces are not easy to pull out due to suction. I've therefore being using them without the screw being tightened. It's in a Dob so the eyepieces are never going to fall out as they are always pointing downwards.

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Replace the thumbscrew with a nylon screw and tighten it down without any scratches.

I've never understood the reluctance to tighten down a thumbscrew.  A slight mark will not damage the eyepiece in any way that affects it.

Do you really only ever buy eyepieces expecting to re-sell them in "new" condition?

All the eyepieces I've resold (hundreds of them) had marks on the barrels and they sold instantly anyway.

As for the weight in the focuser and scope balance, as others have said, a simple counterweight on the tube will suffice to balance it with a heavier eyepiece.

But, you already have quite a set, so go ahead and use the LVWs with a Barlow and they'll work quite well.  Adding a counterweight is just part of using a dob under 20".

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Another trick to try is to round off the end of the set screw with a fine file so it creates a dimple instead of a circular scratch.  Either way, you've got to really crank it down to make a mark.  Just lightly thumb tighten it, and it generally won't leave a mark.

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  • 4 weeks later...

We are back on track. Yesterday I managed to pick up a 4mm Circle-T Ortho to match the rest of my collection :biggrin: That gives x300, so, just about right. I need the 5mm now, x240, and maybe slip in the 6mm, x200.

I have a vintage 0.965" 4mm ortho so I'm aware of the eye relief. It's tight but usable.

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