Maximum realistic focal length for Eastern USA? - Forums

Topic starter

Nov 28, 2023

I currently image with a ZWO ASI 533MC-Pro and have a 135mm f/2 Samyang, AT60ED+0.8 (288mm @ f/4.8) and an NP101 (540mm @ f/5.4).

There's a part of me that sees a lot of teeny, tiny galaxies and PN's and is thinking, "…it'd be nice to have more focal length."

Problem is, the jet stream's never that far from here in NE Ohio. If I got an 8" EdgeHD, even the 0.7X puts it at 1400mm focal length and I'd probably be oversampled at 0.55"/pixel…

…provided the atmosphere could even support that. It's one thing to have a lot of focal length to take advantage of those few clear nights where it's really steady. But I don't want to pay a premium for a focal length that the atmosphere might only support 10 - 20% of the time.

Realistically speaking, what's a good rough estimate for the most focal length the atmosphere will support in this part of the country at least 50% of the time?

Clear Skies,
Phil

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Dan_I

Nov 28, 2023 · Edited: Nov 28, 2023

One option, not too costly, would be to get an ASI183mm (pixel pitch 2.4µm) for those nights of good seeing. You could still use your 533mc for the color data (at the expense of some cropping).

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Jay Hovnanian

Nov 28, 2023

Hello Phil,

I hope I am understanding the gist of your question ... essentially as, What is the longest focal length I can image with (under my sky conditions) before I realize less-than-optimal data?

I live in eastern Massachusetts and image at home (Bortle 6), as well as three other sites in the region (Bortle 3-4). We rarely experience atmospheric conditions that render less than a FWHM of 3. Most nights (for me) are 3.5 - 3.9. There's nothing to write home about here ....

At best, we enjoy only 3-4 clear, moonless nights each month in our region. Nevertheless, if I had a permanent installation, I would add a C11 Edge to my arsenal. Presently, my longest Focal Length is 1016mm, and that is about to increase to about 1300mm. I'm not going to say, I don't care about atmospheric conditions; however, if I were to image at some focal length that would reap ultra exquisite data only 10 -20% of my available time, I might only be able to image 3 - 6 nights annually ....

What I'm getting to ~ and perhaps others might chide me ~ is, Don't overthink this. Just image. Take what you get. Do the best you can and love it.

Jay

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Phil Creed

Topic starter

Nov 28, 2023

Jay Hovnanian:
Hello Phil,

I hope I am understanding the gist of your question ... essentially as, What is the longest focal length I can image with (under my sky conditions) before I realize less-than-optimal data?

I live in eastern Massachusetts and image at home (Bortle 6), as well as three other sites in the region (Bortle 3-4). We rarely experience atmospheric conditions that render less than a FWHM of 3. Most nights (for me) are 3.5 - 3.9. There's nothing to write home about here ....

At best, we enjoy only 3-4 clear, moonless nights each month in our region. Nevertheless, if I had a permanent installation, I would add a C11 Edge to my arsenal. Presently, my longest Focal Length is 1016mm, and that is about to increase to about 1300mm. I'm not going to say, I don't care about atmospheric conditions; however, if I were to image at some focal length that would reap ultra exquisite data only 10 -20% of my available time, I might only be able to image 3 - 6 nights annually ....

What I'm getting to ~ and perhaps others might chide me ~ is, Don't overthink this. Just image. Take what you get. Do the best you can and love it.

Jay

Jay,

Yeah; you got the gist of my question. Your sky conditions are similar to mine. And I suppose I should have clarified what the focal length ceiling would be for deep-sky imaging.

Clear Skies,
Phil

Brian Puhl

IOTD Staff

Nov 28, 2023 · Edited: Nov 28, 2023

I have an acquaintance not far from you that's seeing FWHMs of around 2.5-3.0" in narrowband from his newt at 600mm. He's using an IMX 571 so should be around 1.3" sampling.

Personally, that's about where I'd stay… Especially if you're using a 533. This gives you the opportunity to drizzle your data as well.

If you really want to push for longer, I wouldn't exceed 1000mm, but personally I've seen very little benefit between the 550mm and 920mm scopes I've imaged with. FWHMs are about the same. I'm in NC and usually see very similar stats, if not maybe a tad better.

Also, keep in mind with smaller sampling, you need to factor in a quality mount that can properly guide at those image scales.

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Jared Willson

Nov 28, 2023

Here is the thing… Oversampling isn’t really an issue—you don’t get noisier images than if you “optimally” sample since you can always just reduce the resolution in software and get essentially the same result as if you had a shorter focal length to begin with. Except for one thing. Field of view. The problem with adding more focal length than you need is reduced field of view. It’s not oversampling.

So, if what you want to image will fit in the field of view of a reduced 8” Edge with a 533 based camera, then the aperture is worth it. If it won’t fit, then you need something with a shorter focal length or you need a bigger sensor (or both). I would recommend thinking of focal length as controlling field of view and just not worry about the oversampling.

- Jared

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Brian Puhl

IOTD Staff

Nov 29, 2023

Jared Willson:
Here is the thing… Oversampling isn’t really an issue—you don’t get noisier images than if you “optimally” sample since you can always just reduce the resolution in software and get essentially the same result as if you had a shorter focal length to begin with. Except for one thing. Field of view. The problem with adding more focal length than you need is reduced field of view. It’s not oversampling.

So, if what you want to image will fit in the field of view of a reduced 8” Edge with a 533 based camera, then the aperture is worth it. If it won’t fit, then you need something with a shorter focal length or you need a bigger sensor (or both). I would recommend thinking of focal length as controlling field of view and just not worry about the oversampling.

- Jared

This man hits it on the nail right here.

andrea tasselli

Frequent contributor

Nov 29, 2023

Jared Willson:
Here is the thing… Oversampling isn’t really an issue—you don’t get noisier images than if you “optimally” sample since you can always just reduce the resolution in software and get essentially the same result as if you had a shorter focal length to begin with

I shan't think so. You pay the price in read-out noise.

Scott Badger

Nov 29, 2023

I image at 2350 (C9.25 Edge) in typical NE conditions; 3+” seeing, as already mentioned. I’d like to go even longer, but I really should go shorter, like by half at least. With 3.76 um pixels I’m oversampled by more than double and life would be easier at a shorter focal length. A quality refractor with sharper optics (so they say) could even add a little resolution…. Anyhow, I could upsample the shorter FL image to the same Image scale as the 2350 image without any difference in resolution, but also have the wider fov option. For the moment, though, I have what I have gear-wise, and in this hobby especially, the devil you know…..

Cheers,
Scott

Jared Willson

Nov 29, 2023

andrea tasselli:
Jared Willson:
Here is the thing… Oversampling isn’t really an issue—you don’t get noisier images than if you “optimally” sample since you can always just reduce the resolution in software and get essentially the same result as if you had a shorter focal length to begin with

I shan't think so. You pay the price in read-out noise.

All current generation sensors have read noise low enough that taking shot noise limited exposures is easy, even with narrowband. So, no, the price you pay in read noise is, quite literally, negligible.

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andrea tasselli

Frequent contributor

Nov 29, 2023

Jared Willson:
All current generation sensors have read noise low enough that taking shot noise limited exposures is easy, even with narrowband. So, no, the price you pay in read noise is, quite literally, negligible.

I don't think so, not if your signal is very faint indeed. But do oversample please, it is your gig after all...

Phil Creed

Topic starter

Nov 30, 2023

I'm not so much concerned about oversampling, as I could always either bin or get a camera with larger pixels. I'm more concerned with the atmosphere. To use a (American…) football analogy, "take what the defense gives you."

If the atmosphere tops me out around 1,000mm I'd only get C8 / EdgeHD8 for things like portable visual aperture or when Starizona releases the Kraken–ERRRR–I mean, new 0.4X Night Owl. 800mm @ f/4 that's still mounted on the back end seems like a FANTASTIC combo of focal length, speed, and user-friendliness.

Clear Skies,
Phil

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Scott Badger

Nov 30, 2023

Phil Creed:
I'm not so much concerned about oversampling, as I could always either bin or get a camera with larger pixels. I'm more concerned with the atmosphere. To use a (American...) football analogy, "take what the defense gives you."

In addition to being generally poor, my seeing is super variable so I've taken to imaging Lum and RGB depending on the conditions. If the seeing is less than 2.75" or so, then only Lum, between that and 3.5", both Lum and RGB, and above only RGB. Basically, when resolution is best, put it where it matters most.

Cheers,
Scott

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Sean Mc

Nov 30, 2023

I’m in southern ontario. I’m a noob, but I’m pretty sure we saw some nights with seeing under 2” this summer. When i first got my edge 8, i had some shots that were far more detailed than i could get at my previous max FL of 700. It also gives you other options like solar and lucky seeing on the moon and planets.

That said, none of my rigs have been used for over a month due to cloud/wind/horrible seeing. :/

I might have to jump to solar/lunar if this keeps up. Shoot for a 5 minute break in the clouds.

Scott Badger

Nov 30, 2023

Sean Mc:
I might have to jump to solar/lunar if this keeps up. Shoot for a 5 minute break in the clouds.

I'm in mountainous terrain, so often the clouds above me are generated by air moving over the terrain as warmer ground level air moves into colder temps higher up, which means that clouds, and holes between them, often persist, even throughout the night. So, as long as my deep sky target happens to line up with a hole, I can shoot until the target moves out of it. More than a little frustrating, though, to see clear skies to the horizons, except for the single cloud bank directly above.......

Cheers,
Scott

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