Problem with 2" barlow not reaching focus.

[Tiny Small]

I recently bought a couple of the Skywatcher SWA (70') 2" eyepieces and they are very good indeed. I bought the 2" 2x barlow along with them to double up, however, on the 22mm EP it doesn't quite reach focus in the 250p. The focuser bottoms out when the EP/barlow are about a mm away from focus. It works fine with the 1.25 inch adapter and a 6mm EP but with the 2" ones (which it is supposed to be designed for), no joy. Any ideas how I can rectify this?

This is the kit in question:

http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-ed-deluxe-2x-two-inch-barlow-lens.html

http://www.firstlightoptics.com/skywatcher-eyepieces/sky-watcher-swa-70-eyepieces.html

[ChrisLX200]

The usual fix is to move your primary mirror up the tube a little, and because it's only 1mm there may be enough slack available via the collimation screws. I'm not sure of the actual arrangement on your scope - but I would hazard a guess that if you unscrew all three collimation screws 2 turns each the mirror will move upwards about 1mm. If that isn't enough then it may be better to actually move the mirror cell, but for that you would need to drill 3 more holes in the tube.

ChrisH

[John]

I think a couple of things might be at play here. Firstly, the barlow might not be able to be fully inserted into the focuser - they have quite long barrels and some of the Skywatcher focusers have an internal ring or ridge that stops the long barrel from being fully inserted, which affects the focal point of an eyepiece used in it, moving it a bit further inwards than it would otherwise be. Secondly, I have some of the Skywatcher SWA 70 eyepieces to test at the moment, including the 32mm 2" one, and I've noticed that their focal point position requires quite a lot of inwards movement of the focuser to reach focus. A barlow adds to this (even without the insertion issue already mentioned) which means that you can run out of inward focuser travel.

Moving the primary mirror up the tube a little way will help with this but don't overdo it otherwise the light cone hitting the secondary mirror will be too large and you will be loosing some of your aperture.

I don't usually try barlowing long focal length eyepieces because the already long eye relief gets pushed out further by the barlow lens making finding the correct eye position difficult without the need to "hover" your eye off the top of the eyepiece.

[russ.will]

I second that. Two turns on each of the primary collimation screws and you'll have at least the extra millimetre to play with.

Assuming we're talking about the 250 Dob the only way to move the mirror cell up the tube, is going to be cutting off a few millimetres of the OTA tube, because the mirror cell is mounted in a casting that caps the end of the tube.

Russell

[houston]

Hi tiny small. I bought the skywatcher 2" ed barlow and had same problem wih 26mm panaview. To get to focus i have to play about with not putting barlow completely in focuser and eyepiece not completely in barlow.

[Tiny Small]

Thanks chaps. I'll give the collimation screws a couple of turns and see if that helps things. I do plan on getting the full range of EPs I'm likely to use eventually but they ain't cheap Might just get the 3.5mm version of the above EP's. That should finish the collection off nicely.

[rwilkey]

Hi Tiny, 2" widefield ep's do not Barlow well, I tried it and had to sell my 2" ED Barlow in the end, the other ting is, now I have more experience, is that I have learned there's no point in Barlowing widefiels ep's, it rather defeats the object, but as I say, I only learned this with experience.

[Tiny Small]

Ok... loosening the collimation screws a couple of turns did the trick. It gave me more than what I needed so now can go either side of focus. Just for the record, the 22mm EP barlows excellently. The 32mm not so well but then why would you barlow that anyway? These EPs really are very good indeed. The 32mm gives a fantastic field of view with great colour and contrast though does suffer from astigmatism from about ²/₃ the radius outwards. The 22mm shows no astigmatism at all. Both EPs resolve huge amounts of detail on Jupiter at their native focal lengths (32mm giving only 37.5x magnification but still clearly resolving the cloud bands with plenty of contrast) and barlowing the 22mm gives even more whilst still retaining a fairly wide field of view. The ED barlow is pretty decent as well, and combined with the 1.25" adapter means I can still use my plossls for really high magnification. All in all, an excellent buy for not huge amounts of money (certainly not cheap though).

[John]

I'm glad you have got it sorted and thanks for the interesting feedback on the eyepieces

I've had the 32mm, 13mm and 5mm from the Skywatcher SWA 70 range on loan from First Light Optics for a few weeks now and I'm hoping to post a review of their performance on the forum soon.

[Tiny Small]

I'll look forwards to reading that. The only EPs I can compare them against through experience are plossls. Build quality is fantastic though.