Realistic maximum for usable focal length

ZWO ASI533MC Pro Seeing Conditions
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Phil Creed avatar
Phil Creed avatar

I live in Ohio and while I occasionally travel to other states my imaging is strictly in the Eastern U.S. I’ve got a 533MC-Pro and an NP101 as my main imaging OTA. The 540mm focal length and 3.76-micron pixels put my image scale at ~1.44”/px.

I’ve ordered a Carbonstar 200mm f/4 Newt whose 1.0X coma corrector puts me ~0.97”/px. If I throw a Paracorr on there to boost it to 920mm @ f/4.6, I’m down to ~0.84”/px.

I had thought about getting an 8” SCT and particularly an EdgeHD8. A few things dissuaded me:

(1) unlike SCTs, I DO know how to quickly collimate a Newt with either a laser or Cheshire, having been a longtime visual observer.

(2) As few clear nights as I get around here, speed matters. Both the focal length AND the speed scale up with the CS200 while most SCT reducers bring things down to f/6.3 or f/7.0, and that’s just not fast enough for my tastes given my lousy weather.

(3) Even with a 0.63X reducer on an 8” SCT, given my ~2” average seeing am I at the point where the atmosphere is the limiting factor? Would I really see a difference in fine detail at 1260mm vs., say, 800mm or 920mm on the average night?

Put another way, given 3.76-micron pixels and my image scale in arc-sec/pixel being 775mm/(FL), what’s realistically the max. usable focal length in the eastern U.S. on a typical clear night?

Thoughts?

Clear Skies,

Phil

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Tobiasz avatar
Tobiasz avatar

Phil Creed · Apr 6, 2026, 03:59 PM

(2) As few clear nights as I get around here, speed matters. Both the focal length AND the speed scale up with the CS200 while most SCT reducers bring things down to f/6.3 or f/7.0, and that’s just not fast enough for my tastes given my lousy weather.

The newton is not “faster” than the EdgeHD, both have 200mm of aperture. If you resample the EdgeHD image to the same pixel sampling as the Newton you will have a similiar signal to noise ratio. You compare the FoVs of both scopes and take the one which you like more.

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Rick Krejci avatar
Rick Krejci avatar

There’s of course this site that will give you some guidance https://astronomy.tools/calculators/ccd_suitability

As was mentioned, aperture will be a larger determinant of detail captured (given the same optical quality) of a given object than focal length, which determines FOV. But, at a given aperture, you need to have sufficient sampling such that you are capturing that detail. That said, drizzling can recapture some of that detail if you are undersampled.

If you have your focal length/sampling for the best possible conditions for your area, then you are compromising your imaging for normal conditions by being oversampled and having a narrower FOV.

To me, a FL of 800-1000mm with the largest aperture you can get with great optics is the sweet spot of capturing all the detail that can be gotten under normal conditions and still taking some advantage of good seeing nights.

I’ve found optical quality matters a lot as well…I’d rather have excellent optics at 600mm than average optics at 1000mm. My TOA-130 (130 f7.7, 990mm) consistently outresolves (lower arcsec FWHM) what I could get out of my old Quattro 250 (250 f4, 1000mm) at my location (2-3” seeing normally) despite being half the aperture and similar focal length. Even my OCAL H2 (200 f2.7 565mm) can normally outresolve the Quattro despite being a much shorter focal length. Both of those have incredible pinpoint across the field optics.

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Tony Gondola avatar
Tony Gondola avatar

I would 2nd Rich’s comment about optical quality. This matters more and more as you start push out in focal length.

Just to put some of this in perspective. My 6” Newt. runs at F/6 with a focal length of 914mm giving 0.66” per pixel with my 585. I consider that my wide field view. Lately I’ve been using an ED Barlow to push out to a focal length of 1580mm, 0.38” per pixel. On most nights there is a gain in detail and smoothness of detail at the longer FL. Personally, From my point of view, you’re really operating at the low end of the focal length scale with plenty of room to expand IMO.

Here’s an example of what I’m getting at 1580mm:

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

I’m in Oklahoma under B8 skies. The bigger limitation for me is light pollution, not focal length.

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Phil Creed avatar
Phil Creed avatar

Tony Gondola · Apr 6, 2026, 07:24 PM

I would 2nd Rich’s comment about optical quality. This matters more and more as you start push out in focal length.

Just to put some of this in perspective. My 6” Newt. runs at F/6 with a focal length of 914mm giving 0.66” per pixel with my 585. I consider that my wide field view. Lately I’ve been using an ED Barlow to push out to a focal length of 1580mm, 0.38” per pixel. On most nights there is a gain in detail and smoothness of detail at the longer FL. Personally, From my point of view, you’re really operating at the low end of the focal length scale with plenty of room to expand IMO.

Here’s an example of what I’m getting at 1580mm:

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

I’m in Oklahoma under B8 skies. The bigger limitation for me is light pollution, not focal length.

My NP101 is a high-quality scope. It does pretty good at 540mm — better than I thought it would. I just don’t want to burn the money scaling up from that focal length with a refractor while trying to maintain fast speed.

How do you get 1.73X out of a 2X barlow? And is it a case where coma just isn’t noticeable at f/10.4? I’m guessing I couldn’t do that with an f/4 Newt as I’d only get f/7. Then again, I’m using a small 533 instead of a 2600, so…?

I heard APM is reintroducing a 1.5X coma-correcting barlow. 1,200mm @ f/6.0 doesn’t seem so bad if I needed it for something special.

Clear Skies,

Phil

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Tony Gondola avatar
Tony Gondola avatar

It’s a 2x Barlow but if you adjust the spacing from the Barlow to the sensor you can get different degrees of magnification. I’m using a 585 sensor which is even smaller than a 533, 3840×2160 with 2.9 micron pixels.

With that sensor, coma really isn’t a problem even at F/6. It’s a non-issue at 10.4. Whatever little there is corrects perfectly with BlurX. I’m really very happy not to have to use a reducer.

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