Removing built-in Barlow from focuser

[mental4astro]

Hi all,

I'm considering getting a short tube reflector, but the one in question has one of these blasted barlow lenses built into the focuser. As it is the f/ratio is 7.7, giving an effective focal length of 1000mm. I'm suspecting a spherical mirror, but as this scope will be used for video, could focusing still be a problem at f/3.85 on the small area that is the GSTAR-EX chip?

Mental.

[mental4astro]

I should note that I have a Celestron FirstScope, being f/3.9, very similar to the scope I'm looking at. It has a spherical mirror, and other than a imperfect final figure, it does give a very nice image at low power using half decent eyepieces, such as a 25mm plossl.

From this I know that a spherical mirror does work with the human eye (at low power), but how would this be with the chip of a video camera considering it is not as adaptive as a human eye?

[michaellxv]

If you remove the barlow that would move your focus point in a bit. How much inward travel do you have before you remove the barlow?

I'm also wondering if the inbuilt barlow is providing any level of correction for the spherical mirror?

[mental4astro]

Michael, I hadn't forgotten your reply to this thread. Yes, the focal plane will be altered, and I had been expecting that, but it can be dealt with.

I had posed this question too in Cloudy Nights. It turns out that there is a Newtonian design that does incorporate a correcting lens system in a Newtonian that uses a spherical mirror - the Bird-Jones Newtonian. It sees a "barlow" corrector lens placed between the primary and the secondary. Fake Bird-Jones scopes actually just have a barlow lens stuffed into the focuser's drawtube and offers no correction at all for spherical aberration. In fact, a Newt using a spherical mirror with a natural f/8 figure shows less SA than a fake barlowed Bird-Jones with a synthesised f/8.8.

The best known true Bird-Jones Newtonian is the fabulous old Tasco 8V produced by Vixen. I believe Vixen produced other true Bird-Jones Newts.

One of the replies on CN gave a fantastic link to the sort of thing I have in mind in removing the barlow from the drawtube. The link also show how to force a spherical mirror into a parabolic shape by pulling down the centre of the mirror down from behind! Just ingenious:

How to convert a hybrid short tube Newtonian into an enjoyable RFT (http://www.cloudynights.com/documents/convert2.pdf)

Just the sort of thing that appeals to me too, :D :lol: ;)

[michaellxv]

I'll have to read that later so I don't miss anything.
One other thought. What if you turned the barlow around, would that make it a f/1.9 and give you a wider fov on the chip?