how do i increase focal length with out a barlow lens

[red dwalf]

i have a william optics gt 81 and a atik 314L+ ccd camera, i was reading about the best pixel per arc seconds ideal resolution for the focal length of the scope and using the reducer i`m about 3.5 pixel per arc seconds which isn`t ideal, so removing the reducer it comes down to about 2.9 which is getting towards the ideal range of around 2.4 or lower, i lower it even more i need to increase the focal length of the scope or buy a camera with smaller pixels, increasing the focal length of the scope looks the cheaper option, so my question is can i simply do this by adding a adapter onto the rear of the focuser ? this seems to be the logical thing to do but i`ve not seen anyone do it or talk about it.

i have a 2" william optics photo adapter that can add 50mm onto the rear of the focuser and the camera can attach to that.

is this method ok ? thinking logically it seems to be but wondering if there is another method people use.

many thanks. 

[Ant]

It isn't just a case of moving the camera further away from the lens cell.

The telescope will focus at a given point, but adding in the WO photo adapter doesn't change that. It just means that you will have to wind the focuser in by 50 (or not be able to reach focus). The F Ratio of your scope remains that same.

You need to increase the F Ratio - which really means a barlow of some description.

Antares do a 1.6x 2" barlow - this would get you to around 1.8...

Cheers

Ant

[ronin]

Being "sceptical" there is one other option. :eek:

Find an artical that gives the "right" arcsecond:pixel size that your scope delivers. :grin:

I have read these for some time, did debate a camera and I have read 1 arcsecond per pixel (SBIG) 2 arcsec per pixell (here on SGL about 10 days back, and up to 5 arcsec per pixel.

So depending on what you read I think you will get values from 1 to 5 arc per pixel, and when it comes to planets all the values go out the window.

Also WO do a flatener for the GT81 that if I recall will increase the focal length slightly

[steppenwolf]

I personally think that chasing the perfect match is a little futile unless you do it very seriously by buying whatever is required to achieve your goal as there are other important issues that need to be considered as well, for example:-

so removing the reducer it comes down to about 2.9 which is getting towards the ideal range of around 2.4 or lower,

Removing the reducer will also remove the field flattener so peripheral star shapes will suffer and that IMHO is worse than a sampling rate mismatch! This article on my website and its conclusions may be of some interest.

You could consider the purchase of a Hotech field flattener as this will keep the field fairly flat but will restore the original effective focal length as it isn't a reducer as well. But you now lose your wider field of view - compromises all the way!

[Nathan_Pembs]

Would a telextender work? can get them in x1.5 and x2 i think

[glappkaeft]

There is really no "best" resolution in astrophotography, only different compromises, so don't worry about too much. The perfect fit depends on the target and what you are trying to accomplish.

[m.tweedy]

would masking not increase the focal length 

[steppenwolf]

would masking not increase the focal length

Sadly not, the focal length of a telescope is a fixed function of the curvature of the various elements either lenses or mirrors.

[m.tweedy]

Sadly not, the focal length of a telescope is a fixed function of the curvature of the various elements either lenses or mirrors.

Thank you oh master.

[steppenwolf]

Thank you oh master.

I wish ...

[JamesF]

I agree with your calculated figure of 3.5 arcseconds per pixel for that OTA and camera.

This does technically mean you're undersampling, but if the image quality is good I think I'd be tempted to leave it at that and perhaps look at dithering the image if you would like higher resolution.

James