Barlow problem

Dennis79 (Keith) · #1 19-01-2010, 11:51 AM

Hi Everyone,

I'm having a bit of trouble with my barlow, I can't get anything to focus properly. I can get nice sharp viewing without it but if I had the barlow to the same eyepiece, it all goes a bit fuzzy.

What is the problem?

mental4astro (Alexander) · #2 19-01-2010, 12:27 PM

Hi Dennis79,

Have you tried refocusing with the barlow in place? A barlow can significantly change the focal plane for an eyepiece.

If the focus lies further in than the draw tube can move, you will need to bring the primary mirror up closer to the secondary by adjusting the collimation screws, and then recollimate the optics. You would need a collimation tool to get the best results. Even if you don't raise the primary, properly collimated optics give the best possible image.

The only other alternative I see would be a new eyepiece to give you the higher power you are seeking. This is another reason I'm not a fan of barlows.

Other IISer's may come up with other suggestions.

Dennis79 (Keith) · #3 19-01-2010, 02:20 PM

yeah mate, I have tried to refocus but the best I can get is a bit blurry. My collimation is out a touch, normal viewing is 99% crisp.

the EP and barlow are supposed to be "in sync" (I don't know the technical term) with each other so refocussing is not supposed to be required. That is certainly true when I switch between the 13mm and 9 mm EP I have.

I do have an F4.9 scope so I suppose what you have described is the problem, perhaps the barlow is altering the F ratio too far for the focusser to handle.

Screwdriverone (Chris) · #4 19-01-2010, 02:34 PM

Hi Keith,

The barlow shouldnt affect the focus point that much, I have a LONG 5 x Powermate and it doesnt affect my SW12" collapsible much at all, sure I have to refocus, but when I do, its sharp.

You might find that the barlow lens may be filmy on the inside or outside, or perhaps you can unscrew the end lens element (mine does) and clean it with a soft lens cloth.

Either that or the 2 x mag is too much for the seeing?

Wait a sec, triple check your collimation because even a slight misalignment shows up badly when barlowed. If you havent already, invest in a laser collimator, using this on a collapsible is a doddle coz you can see the return dot on the secondary. This could be the issue.

Good luck.

Chris

wavelandscott (Scott) · #5 19-01-2010, 02:50 PM

What eyepiece are you trying to barlow? It just may be as Chris said that you are trying too much magnification for conditions.

Is everything cooled down and collimated?

Dennis79 (Keith) · #6 19-01-2010, 03:15 PM

Hi, it doesn't matter which EP I try to barlow, the view is always a bit blurry.

The max magnifcation I can get to is 266 and everything is cooled down.

Will have to check out the condition of the barlow when I get home, it is a new one so I expected it to be clean and never checked that.

wavelandscott (Scott) · #7 20-01-2010, 01:20 AM

Do you have anyone nearby that will allow you to test the barlow in another telescope?

If you reseat the eyepiece slightly outside of the barlow does it make a difference? I mean with the eyepiece not fully in the barlow...or with the barlow not sitting all of the way into the focuser...do this carefullly so it won't all tumble.

I'm at a loss...sorry.

dannat (Daniel) · #8 20-01-2010, 04:30 PM

have you got the barlow sitting properly in the focuser, try and take it out reseating it.
try it using the longest focal length eyepiece you have like a 20mm, you could try it in the daytime on some distant tree/powerpole

Fox · #9 20-01-2010, 06:57 PM

Dennis, as advised above, try daytime use on a distant object. You should of course get pure/sharply magnified images. If that is not the case, then something is definitely amiss, eg. the barlow sleeve length/design won't allow you to achieve focus with your optical train, or the barlow itself is faulty. Fox.

rider · #10 21-01-2010, 11:56 AM

How old is the scope? Does it have the new "white" SW focusser?

The new focuser has a shaft restriction that doesn't let some 2" barlows completely drop to the normal seating position, so the EP focus changes to outside the available range.

If this your situation, this may the reason.

bobson (Bob) · #11 21-01-2010, 04:31 PM

You might not have enough travel on your focuser for that Barlow. I have 2" Barlow that I have to pull out for about 2.5cm out and tighten it there in order to get focused.

cheers

Benno18 (Ben) · #12 21-01-2010, 11:38 PM

dont mean to sound like your mum, but have you extended the secondary part of the tube assembely all the way out? Could effect?

Dennis79 (Keith) · #13 22-01-2010, 02:41 PM

Thanks for all the replies.

I just went outside to test everything during the day.

Collimation is fine so that's not the issue.

I was looking at a hay shed about 1 kilometre away and got a crisp focus using the 25mm plossl. Then I put in the barlow and refocussed. I didn't get the same picture, not as crisp.

I went though this process with all of my eyepieces and the same thing happened.

I even tried moving the barlow along the focusser tube and the eyepiece within the barlow, that only made things worse. So the barlow moving the focal point is not the problem either.

I have to conclude that the barlow has a bit of a film on it and needs cleaning. Looking at my eyepieces during the day was a good idea, one of them had a big fat finger print in the middle of it!

Looks like I have a bit of cleaning to do on the weekend.

Dennis79 (Keith) · #14 25-01-2010, 10:18 AM

Hi everyone,

Thanks heaps for all the suggestions, turns out the problem was the novice viewer.

I took the scope out to an astronmer friend last night, turns out I was not following the collimation process correctly. I suppose that is what happens when you only read about things and never actually see them happen.

Anyway we collimated it and it all works fine now.

Bad collimation was the problem and is a bit of a problem with the collapsible versions, the scope moves a bit between each collapse and extension.

Screwdriverone (Chris) · #15 25-01-2010, 10:30 AM

Hi Dennis,

Yes, my 12" suffers a bit from collimation drift. I notice the secondary tilt changes from setup for about 2 hours, very strange, I think it has something to do with the thin secondary vanes cooling down and contracting.

I often find I have to check every hour to be sure, and more often than not, have to retilt the secondary, Bob's knobs are on the wishlist as the allen key dance is very annoying.

May just be my one (white, not BD black) which I suspect is an inferior version to the black ones from accounts I have heard / read in magazine reviews.

Because of the fast F5 mirror, if its not spot on, then the views suffer a lot.

Glad you sorted it out.

Cheers

Chris

wavelandscott (Scott) · #16 25-01-2010, 12:34 PM

Glad you got it sorted...no doubt collimation is an important skill to gain as a newtonian reflector owner.

Kevnool (Kev) · #17 25-01-2010, 04:51 PM

Try losing the Barlow as your only looking through extra glass.
Get the eyepiece to achieve the magnification you want.
But thats me.

Cheers Kev.