2x Barlow for planetary imaging with Celestron 127SLT — worth it?

SVBony SV305 Barlow Optics Buying Advice Help Needed Solar System
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Henrique avatar
Henrique avatar

Hi everyone,

I do planetary astrophotography with a Celestron NexStar 127SLT, and I’m considering getting a 2x Barlow — specifically the Celestron Omni 2x.

Right now I’m imaging without a Barlow and getting decent results, but I feel like I’m undersampling and not fully using the resolution of the scope. From what I understand, a Barlow could help reach a better image scale for planets.

However, I’m unsure if 2x is the right choice. The scope is f/12 (1500mm), so a 2x Barlow would push it to f/24. I’ve read that might be too much depending on seeing and camera specs.

I’m using an SV305, and my typical seeing conditions are average, occasionally good, but not consistently excellent.

My questions:

- Is f/24 too much for these seeing conditions?

- Would something like a 1.5x Barlow be a better match?

- Is the Celestron Omni 2x good enough for planetary imaging, or should I consider a higher-quality Barlow?

Any advice or experience with a similar setup would be really helpful.

Clear skies!

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Tony Gondola avatar
Tony Gondola avatar

You can effectively go to about F/14.5 with this combination without excessive oversampling. You’re already at F/12 so a 2X Barlow would be too much. At F/12 you wouldn’t gain much even if there was a way to make the small jump to F/14.5

In short, you’re already getting close to the maximum the scope can provide.

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andrea tasselli avatar
andrea tasselli avatar
You should sample your Airy disk with at least 3 pixels and assuming an Airy disk diameter of 0.84" you'd ned an image scale 0.28"/px (or smaller but not by much, depending on the seeing).  Given the scope a 1.5x barlow is all you need.
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Willem Jan Drijfhout avatar
Willem Jan Drijfhout avatar

For planetary, a simple rule of thumb is to image at an f/ratio that is 5-7 times the pixel-size of your camera. Your pixels are 2.9 micron, so you’re looking at something like f/15-f/20.

It just another way to come to the same answer that a 1.5x Barlow is probably all that’s needed.

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