68 replies to this topic

[Spaceman 56]

Hello Imagers. Perhaps a light hearted and Friendly Thread.  

 

Here in the beginners section we mostly battle trying to come to terms with Short Focal Length Imaging.

At some point we master the basics, and many start trying their hand at Longer Focal Length Imaging.

 

I wonder if YOU if you have discovered YOUR practical Focal Length limits, beyond which things become too difficult to be worth bothering with.

 

Thoughts and comments about your own experiences, and what do You think the practical limits might be ?

 

I am aware that people image at long Focal Lengths using SCTs, but notice its mainly for Planetary, and very bright targets.

For Deep Sky we shoot long exposures of very dim targets, so my question relates more to practical Deep Sky acquisition.

 

thanks Spaceman   

Edited by Spaceman 56, 25 June 2025 - 01:28 AM.

[ayadai]

I seldom take the path well-trodden. As a result, my journey with long focal length imaging has been an exceedingly fun one, albeit lengthy. I do feel, however, that it's been a progressive one with satisfactory end results.

 

Galaxy season with its attendant tiny and frequently dim targets sees the C8 getting quite a bit of imaging time at full resolution with what I consider satisfactory results. I think my only practical limit is budgetary. I'm pretty sure that I could put a C14, EQ8R and automated dome to good use, but I'd prefer to keep my kidneys where they are.

 

M63 -Sunflower Galaxy
April 18- 28th, 2025
Celestron C8, ASI294MC
Skywatcher EQ6R
11 hours/60-second exp /UV/IR filter

 

Edited by ayadai, 25 June 2025 - 01:51 AM.

[Tapio]

My practical limit is about 1100mm.
That is C8 reduced to about f/5.5.
I'm not interested in small PN or galaxies so don't need longer fl.

[Zambiadarkskies]

I haven't discovered it, but I know exactly where it is without having actually gone there.  Around 800mm with an absolute max of 1000mm

I prefer to do things well and I value efficiency and I have a budget limit and live in a place where gear is difficult to acquire and import.  The breakdown of that statement: 

- Around the limit of a refractor that is no more than $2000 

- I see way too many pretty poor long FL images of blurry, noisy galaxies that all look pretty similar to me.  To get excited about super long FLs and do it well would mean gear that I am not prepared to buy.

- I see some truly amazing pictures from people with more budget options such as newtonians that are obviously super dialed-in.  Trying to get something like that into Zambia would be a nightmare.  

- Oh, and it is not necessarily a question of taking a few half decent shots at a short FL and declaring that one has "mastered the basics" and moving ever up the focal length scale.  One can spend a lifetime with a 50mm lens and iterative improvement.  

 

Ultimately, like Tapio I am just not that interested in such objects.  I get far more excited about dark and dusty IFN than some faint fuzzy.  Just like how planetary holds no appeal other than a random moon shot.  Although solar does kind of appeal.  

 

Genres are important to me.  One cannot do everything (or at least do it well).  To me quality and the satisfaction that comes from iterative improvement in imaging and processing is far more important than aperture fever.  Just like how I like taking pictures of nature and people.  And yet street photography I find about as interesting as watching paint dry.  Thankfully we are all different!  

[900SL]

My focal length limit is determined by budget and seeing.

[Hicks]

I image at 1271 roughly f6.3 on my 8" LX90, although at some point I'll probably give native f10 a go if the right target comes along as I think the mount will manage it. Saying that, there's also just as much chance I get a short focal length scope to open up the number of targets before going longer.

 

Really it's going to depend so heavily on exactly what gear you have, experience and how picky you may be over the result, that there isn't really an answer. Shorter focal length provides an easier time all around and some will cut off around 800-1000 because of that, even if their gear can be pushed further but I bet if you look far enough, someone is imaging at 2000-3000

[BQ Octantis]

For DSOs, 2700mm is my limit—but only because it's f/15.

 

 

 

 

For planetary and lunar, it's more like 50,000mm…

 

BQ

Edited by BQ Octantis, 25 June 2025 - 04:43 AM.

[UKnewbie1729]

Personal record with my star tracker thats so old iOptron dont make them any more is 790mm with a C5 and 0.63 reducer on the Cigar Galaxy. 

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Edited by UKnewbie1729, 25 June 2025 - 09:01 AM.

[imtl]

For DSOs, 2700mm is my limit—but only because it's f/15.

 

For planetary and lunar, it's more like 50,000mm…

 

BQ

If you go 300,000 km away and setup you can do Terra imaging with a 50,000mm.

[BQ Octantis]

If you go 300,000 km away and setup you can do Terra imaging with a 50,000mm.

Wait, wouldn't that put me almost at the Earth-moon L1 point?

[Wjeremy 15]

I asked a similar question to this recently about what is the optimal focal length for amateur imaging. But considering more I think the right question is about pixel scale and resolution. If you are imaging at 0.5 arcsec/pixel then all else equal a shorter focal length is better because you get both the resolution (ability to zoom into any focal length digitally) and a wider field of view. In this sense a large aperture RASA or similar may be theoretically best. Of course all else is not equal bc this comes with such difficulties of tight focus and collimation tolerances.

But the bottom line is I think the right question is one of the smallest practical resolution. I see many people say we are usually limited by seeing conditions, which is true. But occasionally seeing is excellent even in usually tough places like New England. I would argue to build a primary imaging rig around the best possible seeing conditions to take advantage of those rare nights. (Had one yesterday actually). For a mass produced mount this is probably limited by guiding, so maybe 0.5 arcsec/pixel?

Edited by Wjeremy 15, 25 June 2025 - 08:49 AM.

[dhaval]

To be honest, I have found my Baby-Q at 329mm FL a bit challenging - just because there is so much going on in a wide swath of the sky. You have to ensure the gradients are decent (meaning, you have to shoot mostly in regions of the sky where there is consistency in light pollution), even with an APS-C camera, you have to ensure there is no tilt (BlurXT helps, but the raw data can be disheartening). If I was imaging with an Epsilon or a fast Newtonian, collimation would be an issue. On the other hand, a long FL scope, with the right mount (a money problem), is way more manageable and sometimes forgiving. If you end up with a 10Micron mount, you don't even have to worry about guiding. And if you can set up in a remote observatory, you don't have to worry about weight. Admittedly with the CMOS cameras, the resolution can get challenging, but a good mount should solve for that. And yes, you may be over-sampled for your seeing conditions, but I'd much rather be over-sampled than under-sampled. 

 

Thanks and CS!

[Wjeremy 15]

 If you end up with a 10Micron mount, you don't even have to worry about guiding. And if you can set up in a remote observatory, you don't have to worry about weight. 

Two things I would love but not in my immediate future!

[PIEJr]

Well, I started with an 80mm Triplet, 480mm FL. And I enjoyed it for almost 10 years doing AP.

But I wanted more reach. So I moved up to my second telescope, an AT130mm EDT at 910mm FL. F6.0

Wow, the old familiar stuff got bigger, because it got closer!

Then I got curious about a FR/FF. So I got one.

Dumb me didn't do the math. 0.8 FR took my giant 5" telescope down from 910mm to 728mm FL    But made it faster at F5.6

But it did improve the picture, Albeit smaller. A step backwards. But still moving forwards.

So then I got really curious about framing my shots, because I've always strived to "get it right in the camera".

For a decade I always thought "Oh, I can rotate it in processing." Or I can rotate the camera. Though I never did rotate my camera after getting the sensor squared to my Parked Position.

The itch got so bad I had to scratch it. In setting up the rotator, I decided to make everything ridged to the focuser tube.

And that was a lesson in exacting the Backfocus. I got it within 1/10 of a mm of 55mm. Nope, that wasn't right for my equipment.

So I kept adjusting the Backfocus until I liked what was coming out of the camera.

Final answer: Backfocus at 53.x mm, FL= 728mm, F5.6, and I'm happy.

Probably for the rest of my life. All the old friends in the sky are calling me in the bigger pictures, my processing is still very minimal, and things are still right in the camera for what I want.  

 

As long as you are happy with what you are trying to do, that's all that really matters. I don't think I miss the 80mm, I'm too enthralled with the 130mm.

Edited by PIEJr, 25 June 2025 - 03:52 PM.

[psienide]

1000mm seems to be about the practical limit for me. That's the point where scopes get too large/heavy, less ergonomic, suffer more from seeing and/or wind, become more challenging to guide, etc.

Below that, there's no real sweet spot for me. It's all determined by object framing/pixel scale and i've got plenty of choices. Probably the easiest to use out of my selection of scopes is the 65PHQ. 

[72Nova]

I live a few miles from the leeward side of an 11,000' mountain so my seeing is average at best.  My unreduced RC6 at 1377mm focal length was an exercise in frustration but I've had good success with it reduced to 950mm, so I'd estimate that 1000mm focal length is about the maximum for me.

 

I have four imaging scopes that cover focal lengths of 300mm, 432mm, 570mm and 950mm.

[dswtan]

For my sky, cameras, mounts, and patience, I'm finding 2000mm is my upper limit.

 

I'm actively imaging with the EdgeHD 11" reduced to 1960mm for galaxy season just ending, and will continue for more nebula close-ups and planetaries. I don't see me ever tearing it down now, except for maintenance. 

 

I have enjoyed ~1000mm for a lot of the popular targets.

 

Wider is better for some targets (though not always easier in my experience, at least once you have the basics learned), including the most famous. I currently run a 300mm wide field rig in parallel with the 11" on a separate mount (and usually different targets).  

[jdupton]

Spaceman,

 

Here in the beginners section we mostly battle trying to come to terms with Short Focal Length Imaging.

At some point we master the basics, and many start trying their hand at Longer Focal Length Imaging.

 

I wonder if YOU if you have discovered YOUR practical Focal Length limits, beyond which things become too difficult to be worth bothering with.

 

Thoughts and comments about your own experiences, and what do You think the practical limits might be ?

 

I am aware that people image at long Focal Lengths using SCTs, but notice its mainly for Planetary, and very bright targets.

For Deep Sky we shoot long exposures of very dim targets, so my question relates more to practical Deep Sky acquisition.   

   I am not sure I have reached my limit just yet but I am probably close. I am currently imaging at about 1975mm focal length with my focal reduced EdgeHD 11" setup. When I first started out many years ago, I shot at 700mm focal length and struggled. With equipment upgrades ($$$) and lots of experimentation and tuning of capture techniques, I finally got to this point.

 

   I know a few others here regularly image at even longer focal lengths with very nice results. Since I do like to challenge myself, I may some day attempt to use my SCT at its native 2800mm focal length. First, I will need to relearn what I knew three years ago when I had to step away from astrophotography when real life intervened. I am just now starting to get back into the game.

 

   Below is one of my last single night images from 2022. I have been recently playing with the data again to relearn PixInsight with all its recent updates.

 

 

   This is a 1:1 very tight crop from the center of the unbinned integrated frame. My image scale is 0.391 "/px. I image from a Bortle 7/8 area just outside a large city and have a lot of light pollution. My seeing is sort of average at best here in the city.

 

   I shoot at this long focal length primarily because my preferred targets are generally smaller galaxies and galaxy groupings. My goal is to get as many pixels across the galaxy as possible to be able to see hints of what lies within.

 

 

John

Edited by jdupton, 25 June 2025 - 06:46 PM.

[fewayne]

Concur that it's much more about what you're willing to accept for IQ than any math. Certainly the RC at 1625mm and the 183 at 2.4μ (0.3"/px) is really a challenge for me. My new acquisition (910mm and a 2600MM, so 0.85"/px) feels like a refreshing vacation!

[dx_ron]

It's all about money. My sense is that for "affordable" amateur equipment, 2000m is about it, going down to 1000mm depending on the exact gear. 

Have a look at https://www.cloudyni...m51/?p=14149768 for an example of what can be accomplished with good reachable gear (C11, AP Mach2 - well, not reachable by me, but by plenty of folks on the forum). Imagine taking their custom software and pairing it with a PW1000 on a big PlaneWave mount. Not "reachable" for most - but for the particularly wealthy amateur? Sure. In fact, I have met someone with a similar setup, though not with the custom software.

[Alex McConahay]

Practical for whom?

 

Hanging around in beginner's corner, I would suggest people stick to 1200-1500 mm or so (as a top limit----less as you are just getting going).

 

With a little experience, 2200-2450 is not hard. 

 

Alex

 

[jml79]

Focal length is rather imprecise so I'll go with pixel scale. I quickly jumped on the 294M with it's little 2.315um pixels early on. It has it's disadvantages but resolution isn't one of them. I ran a scope at 0.83 arcseconds for 2 years and loved it except for the speed. It was SLOW. I have imaged as low as a native 0.66 arcseconds and have processed my own data to even smaller pixel scales (My Newt drizzled is 0.48*). I haven't done what I would call a proper image yet with my C11 but it is coming along and it's at 1780mm with a 294M camera (Bin2) which gives a native image scale of 0.53. I wouldn't want the C11/EQ8 combo without a permanent observatory though. The 8" Newt/EQ6R is as big as I ever want to transport and even then, the 4" refractor is a much easier setup to travel with. I would only take the 8" to a star party or something where there was going to be a few nights of amazing conditions.

[luxo II]

It's possible, with some patience. I'm imaging with a 10" f/12 MCT, yes at the native 3000mm focal length, with an ASI2600MC DUO. The image scale is 0.25 arcsec/pixel. No reducer, no flattener, no dithering or drizzling.

 

The downside is that to make this work you need a beast of a mount that can guide reliably at 0.5 arcsec rms or better - mine is a CQ350 - and you have to be prepared to wait a while. The next challenge is guiding this.

 

But it’s worth it IMHO, as it can reach many interesting nebulae, galaxies and even quasars that are hopelessly small for the little refractor crowd.

 

Here's what can do with an hour on M83, I've downscaled this by 50%.

 

FWIW 9 years ago I bought this scope as the “ultimate visual planetary scope”, but with the planets heading north for several years I’ve upgraded it for imaging instead. I’ll confess that journey has been costly and was not trivial.

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Edited by luxo II, 26 June 2025 - 04:09 AM.

[Rydeen 98]

I don't see a limit.  If your mount can track/guide for the exposure time you're going for it will work.  You might encounter oversampling due to modern cameras having tiny pixels that makes the image look a bit soft but that's about it.  Binning is a possibility to remedy this.  I tried my hand at doing deep sky with a 2x Barlow on my SCT just to see if I could recreate the Hubble image of the Pillars of Creation.  Like I said it came out a bit soft due to oversampling but otherwise looks pretty good for shooting at 4000mm F20.

 

Crop

 

Full image

 

Edited by Rydeen 98, 26 June 2025 - 04:56 PM.

[Sacred Heart]

My practical limit is 130mm at 1040mm in focal length.  That's as far as I want to go.  Driving force as to why a 130mm refractor, #1 Seeing conditions, #2 My set up is on wheels in the garage, so storage space is number 2.  

 

Joe