A problem trying to focus using a Barlow ?

[alan g]

Hi all.

I've just been out looking at Saturn using my Heritage 130p with a BST 8MM and the view was very sharp and clear, but when I dropped in my new Revelation Astro 2.5x Barlow all I could see through the eyepiece was a fuzzy image of my primary mirror and a black image of the secondary mirror and its supporting arm.

I was unable to achieve focus until I actually shortened the length of the scope tube by about 0.5 inches and then the view became crystal clear.

Could someone tell me why this is ? and in plain English please as I'm a novice at this and still trying to get my head round things.

The scope has an aperture of 130mm and is 650mm long so even using the 8mm with a 2.5x Barlow gives me a mag of 203x which is within the scopes max of 260x.

I used the 8mm and Barlow last night with my Skywatcher ST80 which has an apeture of 80mm and is 400mm long and had great views that were far better than I expected with no trouble achieving focus ?

Any help in explaining why this is would be much appreciated, thanks guys.

[Merlin66]

First, IMHO using a x2.5 barlow with your 8mm eyepiece is pushing the limits with your scope..the seeing conditions would have to be pretty good.

The ST80 having a shorter focal length, can probably handle it better.

The barlow to function needs to be inside the converging light beam (from the primary mirror/ secondary to the prime focus) the distance varies from brand to brand, but it sounds like, in your case you need 50mm or so...if its not positioned correctly it can change the magnification and final focus position....

HTH

[John]

Barlows tend to move the at focus position of the eyepiece inwards a bit. The Heritage 130 does not have a lot of focuser travel so this can cause issues, as you have found out.

At least you can adjust the truss poles to shorten the length of the tube with the Heritage. With a solid tube scope the issue is much harder to remedy.

I tend to agree with Merlin66 that 203x is pushing things a bit with the scope as well though.