Barlow for Small/Distant DSOs? - Forums

Topic starter

Feb 12, 2021 · Edited: Feb 12, 2021

Hi there:

Apologies if this has been covered before, but I'm wondering about the possibility of using a barlow lens to achieve better magnification on smaller/distant targets as a cheap & cheerful alternative to forking out for a telescope with a longer focal length. As a disclaimer, I know this isn't *ideal*, but I'm wondering if it would be better than either ignoring these targets or trying to image them without a barlow.

My set-up is:
- William Optics Zenithstar 61 APO (360mm focal length, f/5.9)
- Canon T3i/600D APS-C DSLR camera
- Skyguider Pro tracker

I do have a flattener (out-of-spec, that's another story…

but I'd read that it was pointless to use a flattener and a barlow together. I'm thinking that for smaller galaxies and planetary nebulae this might be an improvement even after the reduction from f/5.9 to f/11.8 is factored in, though that might make faint planetary nebulae difficult to image with my CLS filter. Still, that leaves lots of galaxies I could image without my CLS.

From what I've read about the Skyguider Pro, the consensus seems to be that it could potentially track well up to 600mm, and 720mm isn't pushing that envelope too far…

If this *is* an approach worth considering, could I get away with a $75 Celestron threaded barlow for my t-adaper, or would I need to fork out for something fancier?

Thanks for any/all advice, even if you all think I'm crazy. Just thinking how to maximize my time under the stars as the "larger" targets slip under the horizon.

Cheers,
Mark

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framoro

IOTD Staff

Feb 12, 2021

Hi Mark,

Not expert enough for a definitive suggestion. However, my knowledge is that Barlow lenses would not be that useful for DSO, as they will lead to a significant increase in time required to get enough signal from the magnified DSO.
Look here:

https://www.astrobin.com/forum/c/astrophotography/deep-sky/barlow-and-dso/

and here

https://stargazerslounge.com/topic/270751-barlow-with-dso/

CS!

Jérémie

Feb 12, 2021

Hi @Mark Germani ,

I think there is no right or wrong answer to that, it all depends on what you are shooting and the Barlow you will use.

I have a slightly larger aperture than your setup (71mm for a focal length of 347mm) and I already tried to shoot a DSO (the Bubble Nebula) with a very very very cheap 2x barlow (a Walimex teleconverter that cost me less than 40 euros).

Indeed, you will need 4x more integration time to get the same signal to noise ratio than without the 2x barlow, but if time is not an issue and you want to spend several nights on a DSO without buying a new scope, why not ??

I let you judge the result :
https://www.astrobin.com/u8xmhp/?nc=user

In my opinion : my image suffers from lots of aberrations introduced by my cheap Barlow (I should have used a TeleVue, now I bought a 4x one but for planetary), and 71mm of aperture for small objects don’t bring sufficient resolution to get nice details with the barlow (due to diffraction).

But using a Barlow on larger aperture scope for bright planetary nebulas for example is something that can gives good results, just check the page of @Exaxe : https://www.astrobin.com/users/exaxe/

Hope this helps !

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dkamen

Feb 12, 2021

Hi Mark,

It is true that apart from the reduction in SNR there are optical considerations.

The Barlow will alter spacing requirements, interact with the filter and cause relatively strong aberrations, not just in star shape but in chroma too.

I think the easiest, cheapest and cleanest way to achieve the same result for what you want to do is 2x Drizzle. SNR reduction will be about the same as with the Barlow (perhaps a little better), but optical quality will be completely unaltered since you are not really changing anything in your optical train.

That said, the Barlow is a cheap accessory. It can always prove useful for visual observation, especially for planets. And will fit on (almost) any telescope you might have in the future. So why not give it a try? There are worse ways to spend 75 dollars.

Cheers,
Dimitris

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Jérémie

Feb 12, 2021

I would nuance a bit the use of Drizzle though : it works well when you have undersampled images.
If, like me, you use very small pixels (ASI183MM Pros has pixels of 2.4microns large), then it is just like upscaling an image in Photoshop (without the fancy upscaling AI filters they now provide or that made Topaz famous :-) ). I would only use it with a full frame DSLR that has pixels twice as big than on my astrocamera, or with larger aperture scopes of course.

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Mark Germani

Topic starter

Feb 13, 2021

Thank you guys!

This is exactly the feedback I was hoping for - I really appreciate it!

@Jérémy, your spectacular Bubble Nebula definitely suggests that I should give this a shot. It's SO helpful to see that you were also using the Skyguider, though I see that you were also guiding so perhaps I should wait until I start doing likewise.

@framoro, those forum links were sobering. I'll have to keep my expectations low. Thanks!

@dkamen, I hadn't considered the interaction with the filter. I have a hard enough time with my Svbony CLS, so perhaps if I do go the barlow route, I'll ditch it and focus on broadband targets. I just completed a pretty rough attempt at using drizzle on M101 (https://www.astrobin.com/qvflmz/). I'd been waiting on an update to Siril so that I could use drizzle, and I'm happy with the detail, though I think I'll need to double my exposure time to account for the reduction in SNR.

Thanks all!

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Jérémie

Feb 13, 2021

@Mark Germani thanks for your kind words ! I am happy with my bubble... given my cheap setup :-) But I have seen splendid images of it.
For the skyguider, I think that your 61mm will be lighter than my setup, so maybe there is no need for guiding if you keep short exposures, less than a minute (but then you need more HDD space and will have to spend more time on integration in Siril :-) ), even with a 2x Barlow : once again a few tests would be welcomed, and check when the stars start to lose their roudness.

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