More Image scale

[asimov (John)]

Hi All.

I'm trying to get bigger image scale (photographic purposes) I use a 2X barlow after the diagonal on the refractor with the toucam at present. Which supposedly gives me 3X...I think I remember reading somewhere in here to get slightly better image scale, one can add an extension tube, would this be correct? & If so....would one add this extension tube before the barlow or after? (sorry if I'm totally wrong with this 'concept')

Thanks in advance.

[h0ughy (David)]

I have no answer asimov, but you have asked a good question. i would like to know the result as well. It has been making me think how barlows and eyepiece projection would work for photography. I have a DSLRII eyepiece projection "toy" whereby I can change image scale by the eyepiece and the tube extension.

[asimov (John)]

Yeah, If I wasn't strapped for cash at present, I wouldn't bother asking.....just buy another barlow & stack em' or get a 5X power-mate..

[[1ponders] (Paul)]

Asimov, to get reeeeally long focal lengths (more than using a 2X barlow) and cheaper than a powermate, I use a meade tele-extender and a 20 or 15mm eyepiece in it and a "T" thread adapter for the ToUcam. For example if I use the 15mm with the tele-extender slid to about half way I effectively increase my FL from 2000 to 11000 and decrease the f ration from f/10 to f/55. Not that its really useful at that FL unless I lived in a vacuum. But because it slides I can vary my focal length with a 15mm eyepiece from 6000 upto about 17000 (again not a very useful FL )

Unless you're seeing is good to very good, even the 6000 is pushing is (f/30 ). The advantage I find is that the Tele-extender used in this configuration (Positive Eyepiece Projection) is that the Tele-extender will screw straight into a Camera "T" mount adapter. This again effectively increases your focal length which will increase your magnification.

If you can get hold of it, Michael Covingtons book "Astrophotography for the Amateur" has some great diagrams describing the process. If I get a chance over the next couple of days I'll scan the relevant diagrams and post them.

[iceman (Mike)]

Hi John.

The extension needs to go between the barlow and the CCD, not before the barlow.

Eyepiece projection will give you the biggest image scale, but if you're still working at prime focus then yes you can use an extension for your barlow, but only if the barlow supports it.

That is, not all barlows increase the focal length (image scale) by moving the CCD plane further out. In some, you'll simply have to rack further in to focus.

But in my 5x powermate, I can pull the Toucam out of the powermate as far as it will go to gain a little extra image scale. I will also get an extension tube to do it even more.

On your refractor i'm not sure how far you'll be able to push it, depends on your focal length and local seeing of course.

[Robert_T]

just adding to what Mike mentions below that to increase image scale with a barlow you have to add the extension between barlow and ccd - in effect if you simply shift your diagonal to that position it will function like an extension tube and give you an increased amplification factor from your barlow - there's a limit to how much you can push a barlow though, not sure of the maths or the effects but doubt you'd want to go much beyond say 3x from a 2x barlow

hope that helps,

[asimov (John)]

Thanks for the replies guys, that helps a lot. I shall do a bit of experimenting soon in this area. The refractor's an F8, so considering I'm imaging at F24 (if I'm getting 3X in the current configuration) them I'm pretty close to the supposed optimum of F30 anyway.

I feel the refractor could be pushed a little bit more but that remains to be seen.

BTW I really like this imaging at prime! No stuffing about with EPs & no adapters need to be made!