Strange Barlow problem with Startravel 120

[Willi1972]

Hi all, I am relatively new and have a Startravel 120. I bought a 2" Revelation Barlow and was very pleased with my beautiful new piece of kit. However, I now have real issues in achieving focus. I can not get visual focus at all with the Barlow lens in place - with or without my 1.25" star diagonal. If I remove the Barlow lens, I can get visual focus using the 1.25" star diagonal if the focus is racked almost all the way in. If I remove the diagonal, it is near focus when racked all the way out - but not actually focussed. This seems a little strange, as I would not have thought that the diagonal would add that much to the total length. The total amount of focus racking is 57mm, by the way. I'm not sure if that is normal or not.

I bought a 2" Barlow in the first place, as I do more imaging than observing, and it made much more sense to work at 2" than 1.25".

Any ideas on what is going on here would be much appreciated.

Regards

Richard

[John]

I believe these short tube F/5 refractors are designed to require a diagonal to come to focus for visual use which is why you can't quite get an eyepiece to focus even with the focuser racked all the way out when there is no diagonal in place.

The diagonal does not add focal length but it does use quite a lot of inwards focuser movement up so when it is in place, eyepieces will come to focus with some, but often not lots, of inward focuser movement remaining.

The barlow lens, which I guess you will be using between the 1.25" diagonal and the scope becuase it's a 2" one, will also take up some of the inwards focuser movement which means you run out of inwards focuser travel when the barlow is in place. 

For visual use I suspect you will need to consider getting a shorty barlow that goes into the diagonal, rather than between the diagonal and the scope.

For imaging though things may well be quite different as CCD's, Webcams, DSLR's etc have different focal positions to eyepieces.

It is a bit messy and complicated I'm afraid

[Pig]

I have a 2" x 1.6 Barlow and do not get this issue, However, inward focus is still within a couple of mm of being used up.

Maybe this slight difference betwixt the two barlows helps.

[Willi1972]

Hi Pig,

do you have the Barlow in the Barlow assembly, or on the end of a T mount? I ask because the closest I can come to focussing is with the Barlow screwed into the filter threads on the T mount.

Regards

Richard

[Pig]

I simply put the Barlow in the diagonal.

Path = Eyepiece - Barlow - Diagonal - Focuser - Scope

[Willi1972]

From what I have gathered, this is the correct way to do it. My problem is that I have a 2" Barlow and then a 1.25" diagonal and eyepiece.

However, the observing does not bother me to much it is the imaging that is really giving me a headache. I think I need an 80mm spacer for everything to be able to focus.