which barlow for planetary

[badabing82]

Hi All,
Just a quick one.

I'm looking at getting more scale out of my scope for planetary imaging
8inch f/6 on eq6pro
spc900 cam

what is the best barlow / powermate?

i've currently got a 2x barlow

would a 2.5 powermate or 4x powermate be a next logical choice for this setup or will 5x powermate be useable ?
will this simply make the image too dim for this aperture / camera combo?

at some stage i do plan on upgrading the camera

any suggestions would be great!

Dan

[PRejto (Peter)]

You could probably go to f24 with a 4X. Why not just try an extension tube with the barlow first... the further from the barlow that you put the camera the more it will magnify. You might not need another barlow.

In very good seeing you might be able to use a 5X powermate. The one I have gets little use because the atmosphere usually isn't steady enough so I find that f21 (my scope is f7) works well most of the time.

[Shiraz (Ray)]

rule of thumb - with a quality webcam, multiply the pixel size in micron of your camera by 4x and then get a Barlow that takes your scope up to about that as an f number (or maybe a little more if your camera is very sensitive). This rule automatically takes care of limiting resolution and scale issues.

I think an SP900 has 5.6 micron pixels, so you should aim for about f22. A 4x Barlow would do the job, bringing your f6 up to f24. Recommend a 3x TV Barlow with an extender to get it up to 4x (the TeleVue site tells you how much extension you will need).

if you later get one of the new high performance cameras with 3.5 micron or smaller pixels, the 3x without extender should be about right

Edit- Peter posted before mine was completed - we agree.

[Screwdriverone (Chris)]

Hey Dan,

My 5x powermate/DBK works fine at F25 with my F5 scope. I have attached a JPG of Jupiter, taken on 6th Dec with this combo.

Cheers

Chris

[badabing82]

HI Guys,

Interesting!

I guess the question is how much of a jump is going from f/25 to f/30 ?
Will this cause the general image most nights to be unstable?

nice idea on the extension tube i've read this but though the extra focal length gain would be quite limited?

whats everyone's favorite ? powermate or barlow?

[Screwdriverone (Chris)]

Powermate.

My 5x TV Powermate craps all over my 2.5x ED Barlow. Not just the image scale, but the quality.

Enough said.

Chris

[Shiraz (Ray)]

Originally Posted by badabing82

HI Guys,

Interesting!

I guess the question is how much of a jump is going from f/25 to f/30 ?
Will this cause the general image most nights to be unstable?

nice idea on the extension tube i've read this but though the extra focal length gain would be quite limited?

whats everyone's favorite ? powermate or barlow?

The jump is a 25 squared over 30 squared reduction in signal with no better resolution. With 5.6micron pixels, f30 will condemn you to slow frame rates, particularly on dim objects (like Saturn). Only consider high magnification if you have a very sensitive camera and the seeing is exceptional. Otherwise, you will lose resolution as seeing degrades the images more at slow framerates. If you get more than a handful of nights a year with stable enough seeing to take full advantage of f25 with 5.6 micron pixels, you will be doing well.

extensions will allow you to go out to about 5x with the TV 3x Barlow. Extensions do not generally do anything with a Powermate (exception is the 5x).

Favourite? - don't care which - Powermates and Barlows both do the same job in this application. Just get a good quality one as Chris
points out.

Also, just edited the earlier rule of thumb to better reflect the capabilities of lower sensitivity cams

[badabing82]

Hi Guys,

Thanks for the advice.
I've decided to go with a Televue 3x Barlow and use an extension tube / tubes to give a range of magnifications

anyone want to sell me 1?

[Paul Haese]

My recommendation for the aperture is 2.5 powermate. No more than that. You just don't have the aperture to create a good histogram. Just speaking from experience.

[badabing82]

Paul! ur killing me here lol!
only issue i have with the powermate 2.5 is your essentially stuck at the power, and if it's not enough then your stuck at that point.

i'm looking at upgrading the cam to an asi120mc (seems to be a really good performer at the price point) so i'm hoping that being more sensitive i can make do for now till i upgrade the cam,

thoughts?

[asimov (John)]

With a 100mm's of extension tube, a 2X Barlow will turn into a 3X barlow - You can now obviously do the math & have any length extensions to get almost any FR above F20.

[badabing82]

Ahhh thanks john!
That made me go take look at extension tubes and how they work in realtion to distance of barlow to imaging chip and what effect it has on the effective f ratio of the complete system. I thought this was somewhat fixed to how far yo can push it.

but with a 2x barlow you can effectively vary the distance to achieve from x2 to x5 with different length exentsion tubes.

all said i'll go the TV x2 Barlow and a few extension tubes.

Does anyone recommend a supplier for the extension tubes?

[asimov (John)]

If you have a look here Dan http://www.iceinspace.com.au/forum/s...d.php?t=103750 You can see the exact difference between 2X & 3X. The camera was dragged out of the barlow as far as it could go hence the F22 rather than straight F20 & the same goes when I used my extension tube to add the further F10 ratio. In theory the extension needs to be twice the length to get to F40-42 however I wouldn't like to be working with that length extension!! Sag/miscollimation of the system would start to factor in..

[asimov (John)]

This might come in handy too...

[Paul Haese]

The trouble with all this is that light fall off with increased magnification is a real problem. You just don't have enough diameter at 8" to push the magnification too far and have a well exposed histogram. Anyway I am sure you will find out one way or another.

[badabing82]

cheers guys,

I just bought a TV x2 barlow from colin since it came up.

Does anyone have a link or 9the calculation) so i can calculate extension tube to magnification increase. I've got conflicting reports that 100mm extension on a 2x barlow goes to x3 rather then x3.5

Regards

Daniel

[Shiraz (Ray)]

Depends on the focal length of the specific Barlow Daniel. However, I think that the graphs that Asi posted come from the TV site - maybe check on the site to be sure, and use the graph that applies to whichever TV Barlow you have.

[asimov (John)]

Originally Posted by Paul Haese

The trouble with all this is that light fall off with increased magnification is a real problem. You just don't have enough diameter at 8" to push the magnification too far and have a well exposed histogram. Anyway I am sure you will find out one way or another.

Well that depends Paul. When I was using Lesters 8" SCT a few years ago I could quite easily get to F30 on Saturn because as we know, you don't need a full histo for it. Jupiter & the others are different kettle's of fish of course.

Yes that's correct, I picked that graph off the TV site Daniel. With my Meade APO 2X barlow 100mm (give or take a few mil) = 3X or F30 in an F10 system.

[2stroke (Jay)]

I would have just went with a 2.5x powermate with that combo. Planetary is a hard game and unless you get the perfect night and have the a good geography everything is wasted, thankgod winter isn't far off

[badabing82]

Thanks for all the comments guys!

I once i get all the bits and pieces come in i can give it a good run through.
Always helps having a few different total focal lengths to play with to cope with conditions.