Dose a barlow make sense? - Forums

Topic starter

29 days ago

I have a possible rig purchase in mind, one being advertised on CN.

It’s for the WO UltraCat / ZWO ASI2600MC Pro combo at Star Front Observatory.

If I decide to buy the rig (it’s basically a ready-to-go system) and also pay the SF pier take over, it’s going to come down to understanding how the combo might work by adding a 2x barlow to the image train.

Why am I considering adding a barlow?

I ran the CCD Suitability analyzer at astronomy.tools, plugged in the scope focal length (518mm) and selected the camera; no barlow.

For OK seeing (2-4” FWHM), the combination was solidly in the green.

For Good or Exceptional seeing, it was not. The analyzer claimed the combo would under sample due to the mismatch between the seeing values and the camera “/px value.

So… I added a 2x barlow.

That improves the combo IF the seeing quality at Star Front supports it.

So my questions are a general one and a two specific one:

Are barlows a useful way of making a short focal length scope match up better to a camera when under good to excellent seeing conditions?

Would this be a good idea for the package noted?

There’s a number of people herein with gear hosted at Star Front. Would you PLEASE chime in with anecdotal information on your opinions (good or bad) about sky quality there?

As this is my first rig I’m considering and I’m not particularly wealthy by any stretch, this would be a big step for me in my AP journey. Any assistance with these questions (or the ones I don’t know to ask that YOU know) would be greatly appreciated.

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Tony Gondola

Frequent contributor

29 days ago

Yes, adding a barlow will give you better resolution if and when the seeing supports it. That of course is going to be at the cost of FOV and F number. It’s a useful option for certain objects that can benefit from it, I’ve done that many times. The problem is, I don’t think you’re going to want to run the system with a barlow 100% of the time but because the rig is at a remote observatory you’re going to be stuck with it so in that case, I wouldn’t do it.

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Brian Puhl

IOTD Staff

29 days ago

If I’m reading the specs right, the WO Ultracat is a 4” frac. With 3.7um pixels, you’re already very well sampled. You’re at a very appealing speed for a refractor as well. I would highly advise against putting a barlow on that scope, assuming it would even be possible.

Just sit and do the math for a second. By adding a barlow, you’re taking an F/4.8 scope thats well sampled and turning it into an oversampled nearly F/10 system. You’re wasting pixels at that point. Realistically I wouldn’t expect to exceed 2 arc second FWHM’s with that scope, just because of size and speed. Adding a barlow will gain you nothing and lose almost every advantage you have.

The beautiful thing about scopes of this size though, is if you encounter good seeing while using them, they drizzle extremely well.

Barlows are for visual and planetary work. Not for deep space objects.

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Ashraf AbuSara

29 days ago

To add to what Brian said, most Barlows are not going to be optically sound to be well corrected for a large sensor like your IMX-571. They will also add significant chromatic aberration. You will lose the benefit of Petzval like design on the Ultra-Cat and have to figure out a good backfocus for whatever barlow you will use.

Some scopes however have specially designed extenders that are meant to be used for DSO imaging to cover large sensors and maintain a well corrected field. An example is the Takahashi Epsilon 160ED. In fact I paired it with a 1.5 extender and used it at f/5 as opposed to f/3.3 for quite some time. It worked well.

But in the case of your UltraCat, I would strongly recommend you stick to its native focal length. If you want to image smaller objects, just drizzle 2x or even 3x and you will find that you will get some great details out of it with good seeing.

Edit:

Here is an example of a really small galaxy that I used a Drizzle 3X with my UltraCat 108 to process:

📷 NGC 6951

https://app.astrobin.com/i/o9ttb1/

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Gordon Pegue

Topic starter

29 days ago

Thanks all for the responses. You’ve all made perfect sense WRT adding a barlow to an image train. Kinda thought the idea didn’t make sense but since I’m coming from the visual arena (where barlows ARE useful), I wanted to get a better idea of if it made good sense from an AP perspective.

Tony Gondola

Frequent contributor

29 days ago

It doesn’t apply to your case Gordon but I don’t think that the use of a Barlow in deep sky imaging should not be totally rejected. Given a good quality ED Barlow, there are situations where it can be useful. Small, high surface brightness under good seeing and with a bit of FWHM based culling, can benefit.

Rick Krejci

29 days ago

Like others have said, drizzle would be a much better option before considering a barlow. The optics of the Ultracat are great and well corrected across the frame even on full frame and it’s reasonably sampled with the imx571. I saw FWHMs in the mid 2”s with good seeing. For that aperture, I was impressed.

My only comment would be to look at the tilt from some raw samples. I had an issue of inconsistent tilt with mine with the STX (ironically) of the scope. If you don’t plan to rotate at all, then they may be able to adjust the STX to correct any tilt and just leave it there. But if you put an Electric rotator between the STX and the camera, there may be issues. For some it’s been fine, especially with APSC, but for me with FF it was problematic.

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SonnyE

Frequent contributor

27 days ago

I wouldn’t. I’ve messed with Barlows in the past, but for visual, which is what they are for. Every piece of glass added to an imaging stream degrades the result.

And I have never seen where a Barlow was recommended for AP.

Tony Gondola

Frequent contributor

27 days ago

https://app.astrobin.com/i/it8kws

D. Jung

27 days ago

Just for the sake of clarity, adding a Barlow will not increase your resolution. The resolution is purely a function of the wavelength and aperture.

What you will change is the sampling of your system.

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andrea tasselli

Frequent contributor

27 days ago

What would really limit the resolution for extended objects is the seeing foremost.

Tony Gondola

Frequent contributor

27 days ago

D. Jung · Dec 15, 2025, 05:47 PM
Just for the sake of clarity, adding a Barlow will not increase your resolution. The resolution is purely a function of the wavelength and aperture.
What you will change is the sampling of your system.

Of course. Under sampling reduces resolution in the image and over sampling can gain resolution in the image if there’s any headroom available after seeing and all the others things that can degrade it have done their work. It’s a fine line that going to be different for every system, setup and night.

D. Jung

27 days ago

Tony Gondola · Dec 15, 2025, 07:00 PM
D. Jung · Dec 15, 2025, 05:47 PM
Just for the sake of clarity, adding a Barlow will not increase your resolution. The resolution is purely a function of the wavelength and aperture.
What you will change is the sampling of your system.
Of course. Under sampling reduces resolution in the image and over sampling can gain resolution in the image if there’s any headroom available after seeing and all the others things that can degrade it have done their work. It’s a fine line that going to be different for every system, setup and night.

That's an interesting thought. I can see how under sampling will “throw away” information and hence reduce the resolution, but how would over sampling overcome the Shannon-Nyquist theorem in your opinion and increase the resolution?

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Tony Gondola

Frequent contributor

27 days ago

I think that would be hard to define as that point of balance, if you will, is affected by seeing, guiding, filtering as well as culling of the data for best FWHM, if that’s the goal.

With a given system and conditions it may be that there’s no benefit in going beyond say 0.66” per pixel. The same system under better conditions might be able to push to 0.44” or 0.33” per pixel. You see this demonstrated very well in the Astronomy tools CCD Suitability Calculator. The optimum pixel size (sampling) very much depends on the seeing conditions.

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