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TwoPi

Short extension tube for 2" EPs with filter thread & compression ring?

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Hi folks.

I've got a 1.5" focus extension tube (this one) but I find that when it's in use with my 50mm travel MoonLite CR2 focuser, I can't achieve focus with some of my eyepieces (which of course seem to have wildly different focus points across models and mixtures of 1.25" and 2" EPs).

I've tried taking the thinnest spacer out of the CR2 but then struggle to achieve focus when the extension tube isn't in place... basically the 1.5" extension tube I have is too long.

The reason for all of this faffing around is that I thought it would be great to have a short extension tube for use with 2" filters that I could quickly swap in and out across my whole EP range (i.e. be able to focus my EPs with and without the extension tube and also with and without the standard MoonLite 2" to 1.25" adapter - the latter doesn't really alter the length of the focus tube at all because of the design).

I'm beginning to think I should just give up and accept that I'll be screwing / unscrewing filters as required - it's not like I use them all the time so it's not the end of the world.

However... my goal might still be achievable if I can find an extension tube between say 0.5" and 1" long? Obviously it needs to have a 2" filter thread, and it would also be great if it was a compression ring design too, so as to not mark the EP barrels.

Anyone know of anything like this?

Thanks as always.

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Mine is 35mm, would that work? Asking b4 i go figuring what make it is and where i got it!

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Don't think he means barrel extender robin, Its a extender at the focuser he wants "Obviously it needs to have a 2" filter thread, and it would also be great if it was a compression ring design too, so as to not mark the EP barrels."

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It would be useful to know the issue that the OP has with each eyepiece specifically in terms of not enough inward travel, outward travel etc. Maybe then we could work out a tactic that would allow all to come to focus somewhere within the range of the focuser ?.

I use Baader FT rings on some of my eyepieces as rwilkey does and that helps them to reach focus at a point closer to other eyepieces that I use.

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I have just bought myself a baader 28mm ftr as the barrel is rather short for one thing and also to help with the focusing issue as described by john

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Hi all, thanks as always for all the helpful replies. :)

Steve, I think 35mm is still too long - my existing extension tube is 38mm (1.5") and although the 3mm of additional inwards travel might make the difference, I'm guessing it's not going to be enough.

Robin, Steve is right, I meant a focus extender rather than a barrel extender, although I'm intrigued how this would affect things. Am I right in thinking these would change the focal length of my eyepieces (and that presumably they would only fit certain models of eyepiece)? I'm not trying to change the focal length, just gain enough inward and outward travel to reach focus across the range, with and without the extension tube. Or have I misunderstood?

Danny, assuming that I've understood what the click-lock does, 47mm is definitely going to be too long.

John, my ideal goal would to be able to focus my 21mm Ethos EP (2" as you know), 13mm Ethos (in both 2" and 1.25" fittings), 8mm Delos (1.25") and 3-6mm Nagler (1.25") EPs both with and without the extension tube in - so I can quickly swap between a filtered and unfiltered view.

If I keep both base plates in the CR2 focuser (as designed) then I can achieve focus with everything bar the Nagler zoom, which I have to pull out of the focuser by a small amount due to lack of outward travel (no big deal as the MoonLite 2" to 1.25" adapter can sit slightly proud from the 2" EP holder if necessary or I can just not insert the Nagler in all of the way).

One thing to note is that the MoonLite 2" to 1.25" adapter doesn't add more than 1-2mm to the length of the focuser - it sits fairly flush against 2" EP holder.

If I then put the 1.5" extension tube in, I don't have enough inward travel - I need to recheck to remember exact which EP and barrel size combinations didn't work but I think it was the 13mm I was struggling with.

I tried taking out one of the CR2 spacers (the smallest - approx. 12mm) but then I was struggling with outward travel without the extender in quite a few combinations - so I don't think this is the answer and I've put the spacer back in...

So as to not waste everyone's time, I think the best thing for me to do would be to recheck and create a little chart so I can give you a clear description of what works and doesn't and whether more inward or outward travel is required in the various combinations.

Thanks again for replies.

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I have done a very in depth search and the shortest ext tube I can find is a 35mm like mine by revelation astro. Thats not to say shorter don't exist, but i can't find one

Edited by bomberbaz

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Telescope house have a short ext tube meant for schmidt casegraIne scopes but looking at it, it "might" be able to be modified for the use you are looking for. You will have to contact them first to see if its suitable by getting dimensions etc but it looks like it might work. If not they do stock the 35mm ext tube.

http://www.telescopehouse.com/acatalog/Revelation-2--Eyepiece-holder-for-Schmidt-Cassegrain.html

Out of curiosity, how many ep's fail to achieve sufficient back focus when you have the current ext tube in. Maybe the 35mm will allow a compromise to be reached?

http://www.telescopehouse.com/acatalog/Revelation-Self-Center-35mm-extension-tube-2-.html

ps, the 35mm is a fantastic piece of kit for the price you pay, don't know how they can afford to sell it that cheap.

Steve

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Its quite difficult to design extensions that are shorter than 35mm and still allow sufficient depth within the 2" fitting for an eyepiece or barlow to be fully inserted. Often there is an internal step or lip where the extension itself is machined to fit into the drawtube and that limits eyepiece insertion depth.

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Its quite difficult to design extensions that are shorter than 35mm and still allow sufficient depth within the 2" fitting for an eyepiece or barlow to be fully inserted. Often there is an internal step or lip where the extension itself is machined to fit into the drawtube and that limits eyepiece insertion depth.

+1 

 Some of my eyepieces "bottom out" on a 35mm extension tube.  Anything shorter than 35mm would likely result in all of them doing it. 

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Swap the moonlight 2"-1.25" adaptor for an Orion self-centring adaptor. This will add about 10mm to the 2" height and should allow you bring the Nag zoom to focus without being too long for the Delos 8mm.

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Hi Gav, I think you are right on my misunderstanding, however, I have never had any issues, but I understand about inward focus. Very difficult issue!

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Hi again everyone. I've finally found some time to do some rough measurements and the table below summarises the situation.

post-28925-0-76651500-1409863528.png

All measurements are from an arbitrary point on the focuser, at the base of the focus tube.

Having thought about the figures, they don't quite add up (my focus range seems to vary by 2-3mm in some situations - I'm putting that down to dodgy tape measuring in the dark!). But hopefully the figures are good enough for you to get the drift.

As you can see, without the extension tube I have to pull out the Ethos 13mm (in 2" mode), the Delos 8mm and Nagler zoom ever so slightly. No big deal.

However, with the 1.5" extension tube in place, the Ethos 21mm and 13mm (in 1.25" mode) can't be brought to focus; they would need to be about 5mm and 12mm further in respectively.

From this data, buying a 35mm extension tube isn't going to resolve the issue (and presumably the Ethos 21mm EP will lip out even further).

Two other points: my MoonLite 2" to 1.25" adapter when added to the extension tube actually lips out slightly too, adding a further 2mm to the minimum focus length.

And finally, as I said in an earlier post I can remove a 13mm spacer in the focuser, effectively adding 13mm to the focus distance required for each EP in my table above. As you can see, that solves the problem when the extension tube is in but without the tube, I can't pull 3 of the EPs out far enough to focus them.

So in summary I think I'm wasting my time with this (and yours :() and probably just need to accept screwing/unscrewing the filters as a fact of life. Unless I'm missing something?

Thanks as always for wading through all of this!

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Sorry if it is on your Sig (cannot see on my phone) but what scope are you using?

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Hi Shane, it's an f/6 200P with a MoonLite CR2 focuser.

Edited by TwoPi

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The solution seems to be to remove the 13mm spacer , always use the 35mm extension and use the ethos 13mm only in 2" mode?

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Cheers, but the goal was to be able to focus all the EPs with and without the ext tube present.

This is so I can have a filter screwed into the ext tube and quickly swap the tube in or out, regardless of which EP I'm viewing through.

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In that case remove the spacer and buy another 35mm extension to use with no filters?

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^^ what he said really. I have a 35mm ext tube and have to remove it for filter useage, screw in filter then put tube back in place. No big deal, takes less than 2 mins but if you want to quickly go from filter to non-filter then another ext tube is the answer.

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