which barlow lens for planetery imaging?

[lookus]

i have just ordered a dbk21 colour camera for my 8" mak/cass (f/l 2500mm) to do some planetery imaging. what setup should i use to project the image onto the chip?

i have been told i can use a barlow lens(2x or 3x). if so which one is better and why.

or i would need to buy an eyepiece adaptor. the thing is that my best eyepiece meade 5000 series 14mm i think will be too big to fit, so i would need to use my 10mm generic plossl which is nowhere near as clear as my meade 14mm. i also have a meade 5000 9mm, but conditions are usually not quite good enough to get a really clear image.

so which is the better option for imaging? adaptor or barlow?

thanks for any help.

[iceman]

Barlow is generally best, less glass I guess. You can get a huge focal length with an eyepiece but I haven't seen too many people doing it well using that method (there are some exceptions).

A native FL of 2500mm is pretty long already, you'll likely use a 3x barlow to give you around 7500mm FL with an 8" scope. Big enough image scale.

You'll just have to judge it based on the object and the focal ratio and whether you're getting enough light into the histogram.

[citivolus]

Warning, the following is based entirely on dry theory, rather than practical experience ;)

I just ran the numbers, and with that sensor you would be imaging at 0.46" per pixel without any reducer or extender in place. The diffraction limit of your scope is 0.58", so even under the best conditions you likely would not gain much by exceeding f/20, with an effective focal length of 4000mm.

In perfect seeing conditions, a 2x barlow would result in imaging at f/25, 0.23" per pixel, and just over the Nyquist sampling resolution of 0.29". In more typical seeing, you would likely be over-sampling already.

I'd personally suggest a 2x Powermate if you have the budget, but bear in mind that it will likely just result in muddy images under less than perfect conditions.

As a practical example of what you can expect with that camera and scope, imaging Jupiter tonight would result in a disk diameter of 89 pixels.

[lookus]

thanks for the help.

so if i went say with a 2x barlow just to reduce the risk of overmagnifying. ( ive seen some people have a 1.5x - 1.8 x barlow but i can't seem to find any online)

should it be a powermate or another barlow . it seems that most people choose the powermate as being the best barlow. is this right?

and should i get a 2" or a 1.25". i understand that a 2" will fit a 1.25" aswell. so why would i not get a 2". 2 for the price of 1 so to speak, no? i plan to use it mainly for imaging but i have a 2" 36mm eyepiece that would work on a 2" barlow.

i believe that the 2x powermate is a 2" piece?

any additional help would be very helpful to me. thanks.

[mental4astro]

Another Q following on this theme.

I'm looking at completing the webcam project listed in the 'Projects & Articles' using a Logitec webcam.

My intention being to photograph the moon & planets during 'show & tell' at schools to add another dimension to the experience.

I'll be using it mainly with a 17.5" f/5 (2000mm) scope & unguided. What size barlow would be the most practical under the circumstances, 2X, 3X or 5X?

Mental.