Are all refractors like this? Have to add or remove extension tubes to reach focus with different EPs

[Saturninus]

Forgive me - I was raised in SCT land where backfocus is something that is taken for granted...

So I got my first refractor - a 102mm f6. I love the views, but I didn't realize how much fiddling around I have to do to get different EPs to come to focus. I put in a short focal length EP - I have to put in an extension tube to bring it to focus. I switch to a long focal length plossl - I have to take out the extension tube. I use this same EP with a 1.25 prism diagonal to control scatter, I have to put the extension tube back in. The scope actually came with two extensions tubes...

Is this something I just have to expect to deal with when using most refractors, or are some refractors more prone to this than others? I don't see this kind of thing usually discussed in product descriptions, but if one scope said "can reach focus for most eyepieces without having to use extension tubes", I would consider that a huge plus... 

[Joves]

Which scope are you using?

My fracs have no issue coming to focus with any of my EP's without a need for any extension tubes and I use anywhere from 2.5mm to 31mm EP's.

[Saturninus]

Its an Explore Scientific 102mm carbon fiber triplet. It came with 2x 120mm extension tubes, so I think it was designed this way. Why they would design it this way I have no idea! Then again, i am new to refractors<br />

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[Stu]

I'm with Joves, I don't have any issues with reaching focus with any of my refractors with a similar range of eyepieces and a mixture of types.

Does seem strange but as you say, it was supplied with them so is presumably designed that way. Don't know why though!! Is the focuser very short travel? That's all I can think of.

Stu

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Edited April 26, 2014 by BigMakStutov

[Saturninus]

According to the scale on the focuser, there is 45mm of travel. Is that short by normal standards?

[Dude_with_the_tube]

your triplet with short focus travel is meant to be for astro photography where you do not require long focus travels and only one camera sits there pritty much permanently attached.For visual use focuser travel on refractors is normally much longer and rarely will require any extension tubes.You have 2 options there if you like your triplet:

1.Continue using it with a bunch of ext.tubes

2.Change focuser to one with longer travel what is meant for visual use. 

[Stu]

According to the scale on the focuser, there is 45mm of travel. Is that short by normal standards?

Yes, that's very short I would say, mine all have around double that, more in some cases.

The answer above explains it all very well. I have a triplet too though, and the focus travel on that was still around 90mm with the original focuser. That was bought some years ago and I think triplets have very much become focused towards imaging now though.

Stu

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[Owmuchonomy]

I've just added an AA 102 triplet to my 9.25SCT. The refractor is, marvellous to say the least. All my EPs fit and focus out the box. It has the new AA 3" focusser with over 100mm travel. As mentioned it sounds like your scope is more of an imaging device.

[Joves]

I think therein lies the answer. As suggested by Dude, if the extension tubes are too much a pain in the backside (which, to me, they would be), you'll want to change the focuser. If it's the one I'm thinking it is, then I'm sure it's a nice scope, so you'll no doubt be very pleased with it once you get it sorted.

Best of luck.

[ronin]

45mm of travel is small, and there is no real reason why it should be just 45mm although the scope is good for imaging it is still a scope and you should expect to use it visually.

I have a Megrez 90 which has 84mm travel and a WO GT-81 that has 79mm of travel and the GT-81 is certainly aimed at the imaging fraternity.

I would consider contacting ES and asking if 45mm of travel is normal/expected. Just wondering if there is a problem in the focuser and it is sticking badly at one position and this is being taken as the extent of travel.

I presume you are using a selection of different eyepieces. It is one reason why when people ask I tend to suggest those from the same set, they tend to be reasonably parfocal.

[lensman57]

Forgive me - I was raised in SCT land where backfocus is something that is taken for granted...

So I got my first refractor - a 102mm f6. I love the views, but I didn't realize how much fiddling around I have to do to get different EPs to come to focus. I put in a short focal length EP - I have to put in an extension tube to bring it to focus. I switch to a long focal length plossl - I have to take out the extension tube. I use this same EP with a 1.25 prism diagonal to control scatter, I have to put the extension tube back in. The scope actually came with two extensions tubes...

Is this something I just have to expect to deal with when using most refractors, or are some refractors more prone to this than others? I don't see this kind of thing usually discussed in product descriptions, but if one scope said "can reach focus for most eyepieces without having to use extension tubes", I would consider that a huge plus... 

Hi,

This one is an Astrograph and a very good one at that, the short focus travel is to ensure that the combined weight of the CCD, the FW and the FF/FR  does not cause the focuser tube to sag or deflect under gravity. Short of using extension tubes the only other thing you can do is changing the focuser to a Feathertouch or something similar but would be expensive.

Regards,

A.G

[John]

The focuser travel on my refractors ranges between 125mm and 200mm and, like Joves, I use eyepieces from 3mm to 31mm and they all come to focus without the need for extension tubes.

So, to answer your original question - no, most refactors are not like that.

[ronin]

The present specification say 102mm f/7 so not particularily an imaging scope. ES also says visula/imaging in equal amounts in the application summary.

They all seem to have a 2" focuser and 2 draw tubes as standard.

I cannot see a reason for so short a travel length, still think that an email or call to ES would be useful.

Being honest they are a refractor I have looked at and would like to know the situation.

If I go the the Astro show here in June I will certainly be taking a close look at them - curious now.

[lensman57]

The present specification say 102mm f/7 so not particularily an imaging scope. ES also says visula/imaging in equal amounts in the application summary.

They all seem to have a 2" focuser and 2 draw tubes as standard.

I cannot see a reason for so short a travel length, still think that an email or call to ES would be useful.

Being honest they are a refractor I have looked at and would like to know the situation.

If I go the the Astro show here in June I will certainly be taking a close look at them - curious now.

Mine comes to focus with the diagonal with no problem, although I do not do much visual, so perhaps as you have suggested the OP should contact ES.

A.G

[Moonshane]

surely all you need to do is establish the mid point for the most inward focusing eyepiece and most outward focusing eyepiece and then buy or use an extension tube that allows for this difference within the 45mm of travel?

e.g. the difference between the most inward position (7mm BGO) and outward position (26mm Nagler) is about 18mm. in your shoes, I'd set the focuser at say 10mm then establish how much extension your most inward focusing eyepiece needs. then do the same with your most outward focusing eyepiece (this will be easier with someone's help, for the measuring, possibly). if the differece is less then 35mm (the difference between 10mm out already and max outward position) then the length of extension for the most inward focusing eyepiece to achieve focus will be fine for all eyepieces. you'll then just need the one extension which can always stay in the scope.

hopefully this will be a standard extension distance (35mm, 50mm or 80mm, or even your 120mm supplied with the scope).

another option might be to do the above process but use a baader fine tuning ring (28mm perhaps) to allow some additional small extension to your 120mmtube to achieve the same result.

hope this is clear but feel free to seek clarification.

[Saturninus]

I guess I'll just keep the extension tubes in. It's just kind of a pain because when the extension is in, there is one EP that won't come to focus. It would be really nice if the tube had been short enough to allow me to use a binoviewer, but alas it seems Iike the worst case scenarios. It's too short to allow EPs to come to focus without and extension tube, but not short enough to allow a binoviewer without a glasspath corrector. I guess it really is optimized for imaging. 

I'd probably want to upgrade the focuser at some point anyway. Thanks for the guidance all

[Davey-T]

I came across this problem when researching for a new refractor, was going to get WO 102 GTF but after reading posts on CN  decided against it as I wanted a dual imaging, visual scope so got WO 110 FLT and WO F/R 1V  and really pleased with it.

Dave

[Nakedgun]

Saturninus,

You did not mention which diagonal was in use, and they do not all have the same focal length. Some experimentation at, say, the next star party may find you one that will accommodate your ep collection. 

Good luck.

BTW, if you think that ES refractor has a short-travel drawtube, try the Tak FC76 or FC100. All of one inch!

[StarryEyed]

It might also be worth noting that eyepieces from the same line all have the same focal point. So its always a good idea to stick to the same brand and line even though this can be restrictive on options and price. I do beluve that all televues have the same focal point regardless of the line. (Correct me if I am wrong here)

[michael.h.f.wilkinson]

It might also be worth noting that eyepieces from the same line all have the same focal point. So its always a good idea to stick to the same brand and line even though this can be restrictive on options and price. I do beluve that all televues have the same focal point regardless of the line. (Correct me if I am wrong here)

Not all series of EPs are parfocal. The Televues have a few series that line up quite well (like the Radians), but there are quite large differences between the series, and I need to refocus a lot. Having said that, my APM 80mm F/6 handles all the ones I have well.

[StarryEyed]

Depending on the prices of your ep's a new focuser might be a nice idea however a longer draw tube might cause a problem with the light path when racked all the way in. I would add that my WO 80FD came with extensions not needed with a two inch diagonal but as the over all weight on the end of the draw tube goes up the focuser starts to struggle both in holding the weight and movement. I wish it was better but I have learnt to live with it as it works for any combination I can think off.

[cs1cjc]

To my mind, this a problem of your choice of eyepieces rather than just the restricted focus travel of the scope.  Bringing a scope to focus for viewing is actually bringing the eyepiece focus and the prime focus of the scope to a common point. Clearly the prime focus is fixed for any given scope so any variation in focal point is solely down to the eyepieces used. 

Personally I have selected eyepieces with a fairly small range of focus at least in part because it is really inconvenient to focus a long way when changing magnification on a very faint field of view.  Apart from Teleview, few suppliers give the focal point so you have to try, However it was a major reason for my selling a Hyperion - its focus point was too far from my other eyepieces.

[Pingster]

I have the 127mm of this scope. Explsore designed them this way so that theu are as short as possible for transprtation.

The positive thing is that without any extention tubes you can use binoviewers with them at native magnification. Or errect inage 45deg diaginals for daytime viewing without correctors or barlows. If you get into binoviewing. Its gonna be a real treat for you.

[Commanderfish]

My Explore Scientific ED127 had the same problem, I couldn't get all my EPs to focus with just one of the two 40mm extension tubes supplied with the scope, some needed both 40mm extensions.  I looked at how far out the focuser was with each eyepiece and did a bit of maths. It looked like they'd all come to focus with one 50mm extension tube. I got one from Altair Astro (2" self centring twist kind) and sure enough all EPs come to focus with that one extension. If you still have issues then you need a focuser with longer travel as well as a single extension tube. 

Edited August 30, 2016 by Commanderfish

[kirkster501]

Using a diagonal in visual then no extension tubes ever required.  When doing AP and a camera at Prime Focus then extension tubes often required.