Trouble with eyepiece focus

[garryblueboy]

Don't know if any body can help or had same problem I've recently purchased a 400 p Flextube picked up some great Maxivision eyepieces recommended by you guys the 2"inch 28 I can't seem to get to focus I'm using the supplied skywatcher 2inch adaptor extension but run out of focus travel it's ok if I pull the eyepiece out a little and then secure with thumb screws I've also tried with out extension and still not enought travel the 1.25 work fine with the extension etc for them well just about any thoughts guys.

[russ.will]

You've answered your own problem - The SW extension isn't long enough, so you need to move the primary mirror up the tube to push the focus point further out. Two or three turns on the collimation thumb-screws may be all you need to push the focus point outward to where the 28mm will function and it's unlikely you'll run out of in-travel in doing so.

I really got hacked off with the need for the SW extension tubes. I *may* have a compression ring adapter that will screw into the silver focuser tube and a Altair Astro 2" self centering extension tube that, with a normal 2-1.25" adapter, will save the need to swap the blessed SW adapters in and out. Can you measure the thread on the inside of the silver tube? If the thread size matches up, you can have the compression ring adapter FOC.

Russell

[alan potts]

I have a similar issue with the M/N 190mm there are a few of my TeleVues that will not reach focus without an extension tube, as luck has it I had one but I find it annoying as well. You end up playing the dual tube focuser and extension a bit like a trombone.

Alan

[garryblueboy]

Ok will do Russ thanks don't feel to confident doing the other moving the mirror . Thanks

[garryblueboy]

Hi Alan I can see it being I need to stand in the neibours garden to look down the eyepiece with all the extension needed lol

[Dude_with_the_tube]

its the outward travel what is too short for you right? Issue is that skywatcher put they focusers so that they are in some sort of middle way for both photography and visual.Well thats they excuse lol what i found a bit of a pants as i doubt you will be doing a serious AP on azimuthal mounted dobsonian anyway,its more like money saving.

Options you have:original skywatcher 2" extension is only 50mm,you will need to buy a 80mm extension and you will be able to reach focus with your 2" Ep`s. but this could mean that you will need to work with 2 extension tubes:original + 1,25 for your 1.25" EPs and 80mm for your 2".

Sounds a bit of a pain.

Option number 2. Manufacture a raiser plate for your original skywatcher focuser and put it under the focuser to raise it off the base by an 1" that should be sufficient for you to use original 2" extension to reach focus.Could be complicated and costly if you cant do it yourself.(but can be done at home)

Option number 3 what most people have done with skywatcher dobs: replace original focuser with either Moonlite or Feather touch.Moonlite comes with various installation kits and raiser plates and you can play around to find how many of them you need to install to reach focus without any 2" adapters or extensions.

All depends on what are you prepared to spend. 

[ronin]

You've answered your own problem - The SW extension isn't long enough, so you need to move the primary mirror up the tube to push the focus point further out. Two or three turns on the collimation thumb-screws may be all you need to push the focus point outward to where the 28mm will function and it's unlikely you'll run out of in-travel in doing so.

Moving the mirror up the tube would mean you need even more outward focus. and as you do not have sufficent at present it makes the situation worse.

Get a longer extension tube.

[russ.will]

Moving the mirror up the tube would mean you need even more outward focus. and as you do not have sufficent at present it makes the situation worse.

Get a longer extension tube.

Quite. I meant move the mirror down the tube. Thou shalt not post when cream crackered.....

@Garry - To move the mirror down just turn all the collimation bolts until they won't turn any more because the springs are fully compressed. Primary mirror collimation will now likely be way out, but you'll only need to turn two of the bolts to get it back in collimation.

Hopefully, there is enough adjustment left to do this. If not, it's back to extension tubes. Let me know that thread diametre.

Russell

[alan potts]

Russell,

I think as annoying as it is the extension tube is the best way to go, I have just looked at the amount of space there is behind my SW scope and it is only about 5mm but I accept that other scopes may vary.

As for the length of the tube in the focuser, some time back one of the Moderators posted a picture of a  TV Powermate with 21mm Ethos therein, with this you may have to be in the garden nextdoor to your neighbours.

Alan.

[russ.will]

The reason I hate the SW extension tubes, is that you're stuck with a single metal screw and no compression ring to secure your EPs - Not nice.

There is a reason I have this compression ring screw in adaptor for SW and I'm pretty sure it was for my 300p. I've got a dim memory that I had one of those 2" ED Barlows we all succumb to at some point. Because of the way the SW extensions fit using a dovetail, there is a step in the bore that prevents longer 2"accessories like that barlow from inserting all the way. I think I bought the compression ring adaptor and then unscrewed the barlow nose, using the body as the extension tube. This meant I also had a 2" compression ring, plus the 2-1.25" adaptor it came with and it all operated like a normal focuser. Because the 2-1.25" adaptor it comes with is low profile, all of the EPs came to focus with less racking in and out between EP changes too.

If it does fit, then it will open up some tidier solutions for Garry.

Russell

[alan potts]

I think my 35mm extension came from TS and has a brass fitting with two screws. The type you are talking about I guess is much the same as the fitting on the back of the Mak 180mm. This is a horrible cheap and very nasty piece of kit that actually gave up on the loading of a SWA Meade 40mm almost spilling it to the concrete, it's now in the bin.

Alan

[garryblueboy]

Hi Russ I think is 50mm or 52 mm sorry poor tape measure

[John]

You could try putting a 14mm or 28mm Hyperion Fine Tuning ring on the 2" barrel of the 28mm Maxvision. That will mean that you won't be able to fully insert the whole barrel + FT ring and it will stick out of the focuser a little but there will be the security of the longer barrel to hold it. This is the approach that I use with my 6mm and 8mm Ethos eyepieces to avoid having to put a longer extension tube in my Orion Optics 12" dobsonian. The Hyperion FT rings will take 2" filters as well.

[russ.will]

Definitely not 54mm? I do recall something about there being 2 sizes of focuser, or a point at which SW changed the size, but I'm not sure from what to what.

The one I've got is 53-54mm to the outside of the threads, so it's close. I can stick it in the post to you FOC so you can just try it. PM me your address.

It should be noted that this won't solve your in/out focus problem by itself (if it works) but it will make things easier. There's a 2" ED barlow on Astroboot at the moment for £40. That would be cheap for an extension tube and 2-1.25" adaptor, never mind the possible use as an actual barlow.

[triton1]

I have the same ep which works fine in mine but the Bst 8mm did not,same problem I had to pull it out of the holder slightly to attain focus.I then noticed the following morning that the locking screws for the primary where screwed in near enough all the way so I screwed them out and wound down the collimating screws and then recollimated.The three mill or so made all the difference as basically what Russ said.

[garryblueboy]

I don't have a collimator At the moment and not very experienced yet with Doing that .

[Dave In Vermont]

I don't have a collimator At the moment and not very experienced yet with Doing that .

This is one of the most clear and lucid guides to collimation I've found. And the author is a member here:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Hopefully this will take the fear of collimation out of you. Then you can happily move mirrors about without panic.

Enjoy!

Dave

[Saganite]

Hi Garryblueboy

I am not a million miles away, at St Neots, and glad to render assisstance.

[Saganite]

OOPS! hit the S key too many times

[garryblueboy]

Thanks Saganite I need all the help I can get .

[Saganite]

No problem, I will be in touch