Planetary Imaging - What Barlow/Powermate?

[FarSide]

Something i've been trying to research for a while without success; using a C11 for imaging Jupiter or Saturn for example what is the optimal barlow or powermate to get a very good "close", detailed image? I've been debating for a while now whether to upgrade my standard 2x barlow and was wondering if any of you guys had any ideas or suggestions? I currently run a C11 riding on an NEQ6 and Flea3 camera.

Thanks in advance.

[JamesF]

I'd suggest that you may be thinking about things the wrong way. The question you should perhaps ask is more along the lines of: "What focal length should I be trying to achieve to maximise the resolution of the camera given the aperture of my telescope?" Once you've reached the point where the camera can't resolve any more detail than the scope can provide then I'd suggest there's no point trying to get "closer" optically. You might as well just do that in processing if you want it.

Irritatingly (from my point of view at least) there are multiple versions of the Flea3 camera with different sensors and different pixel sizes. These will have a bearing on the answer to the above question. Without knowing the pixel size the question can't actually be answered. As it turns out in fact, the telescope is largely irrelevant an the question actually becomes "what focal ratio should I be using with this camera pixel size?"

I wrote up my thoughts on this here:

http://www.tanstaafl...netary-imaging/

but if you find the maths scary I can tell you what my answers would be if you know the camera pixel size.

James

[FarSide]

Hi James,

My Flea3 is the model: FL3-FW-03S1M-C - Mono. I'm currently only imaging in mono purely for practice without the added job of filter switching.

As far as i know, James, the pixel size is 5.6 X 5.6 micrometers. With my old 250px the Celestron 2x barlow worked quite well (see attached image, baring in mind this was my first ever image of Jupiter) however the image is still 'distant'. The C11 obviously has an FL that is much larger and i wouldn't want to lose quality by over doing it. Plus i'm looking to upgrade to ether a Televue barlow or a Televue Powermate - money i'd rather put towards the right piece!

Thanks.

Mike

[JamesF]

That's a very nice image

One of the unexpected things I discovered when I started doing the maths was that the theoretically optimal configuration to match the resolution of a camera to the resolution of the telescope can be calculated completely independently of the telescope. With a 5.6um pixel camera it turns out that you want to be working at a focal ratio of up to f/40. This is the same whatever telescope you put the camera in. Obviously there are other constraints such as how much light the telescope can actually put onto the sensor and the quality of the seeing, but those are variables you probably have to work out for yourself on the day.

Your C11 is f/10, which means you could improve the image resolution by using anything up to a 4x power barlow or powermate to take you up to f/40. You could go beyond 4x, but you're unlikely to gain anything by it. If you can centre an image very well, or perhaps use a flip mirror, I might be tempted to go for a 5x powermate just because it has such a good reputation. Working at such long focal lengths is quite hairy though. The slightest misalignment means you can lose the target altogether. What I used to do when I was using a 5.6um pixel camera (the SPC900) was to start with a Revelation 2.5x barlow which is pretty good for the money, then add an extension to the back of that to increase the effective focal length further. By building up to the longer focal length in stages It was easier to keep the target on the camera sensor and I ended up with a multiplier factor of about 3.2x which worked out fairly nicely with the native f/11.8 focal ratio of my 127 Mak. You can only use extensions to increase the focal length multiplier with barlows though. They won't work with powermates.

So, the choice is yours, really Using a barlow or powermate of anything up to 4x should give you a more detailed image, but you have to balance that against the increasing difficulty of finding and keeping the target on the sensor as you increase the effective focal length. There may be a great deal of sense in building up to the longer focal lengths a step at a time and seeing how you get on. The Revelation 2.5x barlow and an extension of 40-50mm to go between it and the camera is probably not a bad way to do that. The Tele Vue 3x barlow may be an alternative. An illuminated reticle eyepiece may be a big help for checking alignment, too.

James

[FarSide]

Thanks James, this will help a great deal. I'll probably start with 2.5X and work up from there and see how it goes. A little experimentation doesn't hurt. With the colossal FL the C11 has anyway that should be helping a great deal regardless. Thanks again, James deffo a copy and paste jobby into word to keep.

Mike.

[SkyGibbon]

James is right about his calcs. On my f/8 newt I have 1.6x corrector screwed into a 2x barlow, then inserted in a 2x ES focal extender. Calculates to a tad over f/40. If I use a 3x TV barlow instead of the 2x ES, there is no more definition, just a larger pic. I get alot of different f ratios though, since I experiment alot.

The ES 2x and 3x focal extender and a 2x shorty barlow element would be my reccomendation. The ES' are fantastic and are way cheaper than the powermates.

[JamesF]

I'd forgotten the Meade teleextenders. Are the ES focal extenders pretty much the same product, by any chance?

James

[riklaunim]

Any good 3-element Barlow will do. TeleXtenders or mostly Powermates cost much more. They are handy when you need fixed magnification (or almost fixed) with distance from its lenses.