Realistic maximum for usable focal length

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.

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

Phil Creed · Apr 6, 2026, 03:59 PM

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

I use an Apertura CarbonStar RC8 (not Newtonian) with a Touptek ATR533M at native focal length (f/8, 1624 mm), giving me an image scale of 0.48”/pixel and 5750 mm² arcsec² of pixel etendue in this configuration.

On the majority of nights here in Wisconsin, I am definitely limited by the atmosphere. But there have been some nights (primarily in the summer and fall) where the atmosphere has been extremely favorable and I could really take advantage of all of the rig’s resolving power. On most nights, I would probably benefit from a reducer or larger pixels, but on the best nights I’m very happy to have sampling that fine.

Here’s a counter thought for you…

How do you actually know you’re limited to 2” average seeing? You can’t really know until you get a scope that can properly sample beyond that. An 8” generic mirror tends to bottom out around 2 arc second. I’ve noticed that with my old GSO newt, best data I saw was around 1.8 arc second from it. Even a 100mm refractor, you’re realistically doing very good at 2 arc second. It wasn’t until I moved to my 6” refractor that I realized my skies were much much better. Even with the frac I have bottomed out at 1.3 arc second. I need a much higher quality scope with large aperture to really find out my sky quality.

To answer your main question of how good the eastern US is, here in North Carolina on the coast, I know with certainty that I have nights below 1 arc second during the summer. On average, as long as the winds come from the south, I’m in the 1.5-1.8 arc second range, however my scope just can’t resolve much lower, so the ‘average’ could be far better. Winter time is a much different story as the winds shift and laminar flow ceases.

I am the last advocate to push for longer focal lengths. You will have less disappointing nights when seeing is bad. Also, unless you’re upgrading that camera, I highly recommend against small sensors and long focal lengths. Ultimately, I think that newt is a pretty good decision. The paracorr is a great coma corrector, but you also must consider the focus plane for that corrector tends to shift outward, which means unless you have an adjustable primary mirror, you’ll need ~30mm of extension beyond the current focus point.

In other words, I probably should’ve just bit the bullet and gotten a cheaper SCT?

I just don’t like the speed disadvantage. Though as someone with a 533, a 0.4X Night Owl could’ve worked.

If the high-quality refractor advantage is as those say it is, also makes me wonder if the thing to do was get a used NP127 from the get-go and save the SCT for when I get a backyard observatory (in sunnier climes when I retire). The older (non-IS) version of the NP101 works great with my 533MC-Pro and the biggest chunk of my astrobin photos were taken through November-Papa-One-Zero-One.

An NP127 would bump the (native) focal length to 660mm and the pixel scale down to 1.17”/px. Probably noticeable, but not earth-shattering and not worth the marginal cost.

The Carbonstar 200mm RC f/8 with reducer is intriguing. And I can reduce it to a (more to my liking…) f/5.3 at 1,060mm in a reasonably-compact package and still faster than a reduced 8” SCT. And maybe I thought about this wrong, as the RC is available but the CS200 I ordered is, well, backordered.

How dew prone is the RC vs., say, an SCT or a Newt?

Ironically, I do have the means with a Paracorr to get 1,150mm @ f/5.7 — an 8” f/4.9 Orion Skyview Pro with a mirror refigured by Mike Lockwood that’s been my visual instrument going back to 2014 and…let’s say I’ve have no worries about the image quality. But the steel tube’s weight and length would be harder on my AM5 mount than a 200mm f/4 carbon-fiber newt and the typically-thin Orion spider vanes make me nervous about it holding imaging-worthy collimation all night. I will try to sell that scope later this year.

Paralysis by analysis. Gotta love this hobby.

Clear Skies,

Phil

Tony Gondola · Apr 7, 2026, 08:44 PM

The math says that doubling the focal length requires you to expose 4x longer and on paper, that’s absolutely true. But only if you are matching SNR.

Let’s say you have the same aperture and double your focal length (barlow), and I assume you can take individual exposures sufficiently long to swamp read noise with the latest low noise sensors.

In that case, if you view an object, say a galaxy at the same object size on print or screen, in both scenarios, the noise level would look the same, since it’s dependent on aperture.

So, the shorter focal length scope viewed at native resolution would look less noisy, but when you blow it up (or 2x drizzle it) to see more detail and match the object size of the longer focal length, it will look similarly noisy. Or, if you take the longer focal length image and resized it in half, it will look less noisy.

If you’re oversampled at the longer focal length and correctly sampled at the shorter, the level of detail in both will look about the same at the same viewing size, so the main difference is that you lose the context around the object due to the longer focal length for little gain.

Conversely, if you’re correctly sampled at the longer focal length and thus undersampled at the shorter, you’ve lost some amount of detail with the shorter focal length (some of which can be recovered by drizzling).

This is why it’s said not to worry as much about f ratio, more about aperture and field of view. And to think of reducers more as gaining FOV rather than increasing brightness. Yes, reducers increases sky background brightness and brightness of especially extended objects when viewed at the native 100% pixel scale. But normalize the viewing size of an object with an image taken without the reducer, it won’t be less noisy at all, just potentially less detailed. Again, a reducer trading FOV for detail

Now, If you doubled your focal length by doubling your aperture at the same focal ratio, that single object at the same viewing size will have far less noise with the larger aperture since you’ve collected 4x the photons from that object at double the aperture.

Phil Creed · Apr 7, 2026, 08:17 PM

e thing to do was get a used NP127 from the get-go and save the SCT for when I get a backyard observatory (in sunnier climes when I retire). The older (non-IS) version of the NP101 works great with my 533MC-Pro and the biggest chunk of my astrobin photos were taken through November-Papa-One-Zero-One.

High quality is not necessarily associated with the type of scope.

A scope like the 200 f8 could be a great “great seeing” scope and good for galaxy season where your targets are generally smaller. I’d probably only get a reducer if it improves star spot size or if you need that larger FOV for intended targets. If the spot size is about the same, then you just made your optics effectively worse since your FWHMs in “ will be higher. The reducer won’t enable you to collect more photons from a give object, just collect from a wider field.

I routinely use my TOA-130 at 990mm f7.7 and never have issues with dim objects. And, despite being a smaller aperture, it out-resolves my 8” f2.7 newt (with excellent optics) in seeing conditions better than 2.5-3” since it’s better sampled with my IMX455. Different tools for different purposes.

Tobiasz · Apr 6, 2026, 04:08 PM

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.

Depends on what you are trying to achieve. There are some beautiful images of the Andromeda galaxy on this site that were taken with 8 inch Newts and a 2 arcsec pixel scale.

You could shoot a picture of M94 at that same pixel scale with an Edge SCT, but then it will be only about 200 pixels across vs. about 5000 for Andromeda galaxy with the Newt. Same pixel scale but that of Andromeda galaxy is far more likely to be a “good” image.

“If you resample…” yes, but the key term is “if”.

Haha, clear skies, I wish. The last night I had any data was on the 6th and things look bad out to about the 18th and it looks like that will probably be one or two nights at best. It’s been a crazy warm later winter and spring here. It’s still hot for the season so things are really stirred up.