Image Scale Question

[David Smith]

Ok, some of you will be familiar with my solar images, if not then have a look at the Flickr link in my sig. I want to get better resolution and I think the first step is to increase the image scale (see below single unprocessed frame).

To that end I bought a cheap Revelation 1.25" barlow that can be used as a traditional barlow and achieve 2x, or you can unscrew the front element and screw it into the T adaptor and achieve 1.5x.

Based on this site :-

http://www.12dstring.me.uk/fov.htm

I had the idea that even at 2x I should be able to get the full disc in the frame but if not then at 1.5x I should be safe. Not the case! Even at 1.5x the disc is too large for the frame.

What I actually get at 1.5x is

And you can imagine for yourself how it looks at 2x

So my questions are :-

1. Is the website I am looking at just wrong or am I doing something stupid (quite likely)!

2. Does anyone know of a barlow or similar that can give me a 1.3 or 1.4x focal length?

Thanks for your help

[Steve Ward]

I guess you'll need a shorter distance between the element and the chip .

I use a 2" Barlow element in a shorter adaptor that knocks it back to about 1.8 x with my set up ,

I found that a 1 1/4" gave me too much vignetting at top and bottom of the frame when pushing the image close to the edge but the 2" works fine.

Something like this . . . http://www.scopesnskies.com/prod/barlow/2x2inch.html and this . . . http://www.pulsar-optical.co.uk/prod/astro%20engineering/telescope/adaptors/45mm80mm.html

It's a case of popping to your nearest supplier on a sunny day and trying various combinations , well that's what I did . . .

[David Smith]

Thanks Steve. Will have to shop around but I am limited a bit by the fact my scope has only a 1.25" focuser.

[DrRobin]

Hi David,

I have one of those screw in barlow lens and it does depend on how far away it is from the ccd. I cut a nosepiece down from a DMK camera to get a lens very close to the ccd and therefore bring the magnification closer to the manufacturers claims. I used it for a focal reducer, but the same would be true for a Barlow. I measure the power of the Barlow by taking one capture without and then one with. Most barlows seem to be seldom near to the quoted gain when used with a camera. Interesting Televue publish charts to show the magnification for various distances from the top of the Barlow, pity the others don't.

I wrote a description of what I did in the following thread.

I hope this helps?

Robin

[Merlin66]

Which camera are you using?

With the PST the "native" focal length is 400mm, this would give a solar disk image close to 4mm...with a x2 barlow the diameter would be about 8mm.....

How does this compare with your chip size?

(EDIT)

I see you're using a DSLR with your 80/640 for white light!

OK the solar image would be around 6.5mm diameter and with the x2 about 13mm.

The DSLR has a chip height of 14.8mm.... looking at your image I'd say the magnification was just over x2...