of Planets and barlows

[rfdesigner]

My scope has a 1500mm fl on 300mm aperature, to get decent resolution on planets what pixel size should I be aiming for?.. equivelent question to.. given camera has 8.3u pixels what barlow should I go for.

Derek

[brianb]

With 4.65 micron pixels f/20 is about optimal ... so with 8.3 micron pixels I think you should need f/35 to f/40 to get full resolution from the scope. That's x7 to x8 which you can get with the 5x Powermate and an extension tube. Being able to get a bit less than this may be useful on the nights when the seeing is poor (it usually is at my site!) as there is a tradeoff between resolution and short exposures beating the worst of the seeing.

[rfdesigner]

Great.. thanks

I've tried a 3x barlow but not with very much success... looking at others performance I was calculating something in the region of a 15x barlow.. so similar numbers.

are there any optical concerns when 'pushing' barlows from, say 3x to 15x?

my guess is 'it depends'.. is it a huge no-no?

Derek

[sixela]

Pushing a barlow from 2x to 15x will usually generate extra aberrations - mainly spherical aberration. Also, given you are already at a longish focal length f/75 is far too large a focal ratio; it's best to stick with f/20-f/35 or even lower and use the light to make the gain lower and the exposure time shorter.

5x or 6x is closer to what you want, and you will need stupendously good seeing to get better images than with a 4x barlow (you don't live in the Florida Keys, so being limited by the resolution of a 300mm scope isn't going to happen often, especially not if you don't make exposure time as short as possible).

But if you're trying a 3x barlow and you don't get good results (i.e. a fuzzy image), getting a stronger barlow is only going to make matters worse. Get sharp images at the 3x barlow image scale and then only do you know you can try to crank up the image scale...

[brianb]

are there any optical concerns when 'pushing' barlows from, say 3x to 15x?

Yes, if you're using a very long extension tube to get the extra mag then you're waaay outside the design parameters and serious spherical aberration is to be expected. If you're stacking barlows then the second is magnifying the aberrations of the first.

The TV 5x Powermate is designed to work well when "stretched" to about 8x. Of course it is not a normal barlow in design. If you really, really need more than about 8x then consider eyepiece projection, this tends to work better than overstretched / stacked barlows.

But nobody needs 15x these days ... f/70 was sensible with very high speed (coarse grained) film ... if you calculate the resolution of your scope & allow 2x for oversampling (Nyquist's Theorem) then it works out that the required focal ratio is approx. 4.5 times the pixel pitch (in microns) ... aperture & focal length cancel each other out in this calculation (but of course you get a larger image & more resolution with a bigger scope working at the same focal ratio).

Overmagnifying reiults in long exposures and/or very high gain, the first results in blurring due to seeing wobbles and the second in very noisy images. It also makes focusing almost impossible (it's never easy even at the best of times!)

Increasing the size of digital images is best done by resampling in post-processing. A 2x resampled image made at optimal focal ratio will look much better than a "straight" image made at twice the optimal focal ratio.

[dmahon]

I thought that the powermates didn't stretch - for example, unlike a Barlow, it makes no difference to the magnification whether you put your powermate before or after your diagonal.

[brianb]

I thought that the powermates didn't stretch

You're right for the others, the 5x is the exception that breaks the rule. Diagram of power vs. extension is included with the device.