34 replies to this topic

[Wjeremy 15]

I've dabbled in many different focal lengths for a few years.  Now I want to pick one OTA to focus on for a while and tune it up to perform reliably at its highest potential, full sensor illumination, perfect calibration, perfect corner stars, no odd optical artifacts, etc.  I have some thoughts but want to hear some other opinions.

[afd33]

Something around 300mm depending on the sensor size.

[Andros246]

Askar SQA 85

 

super sharp basically gives you seeing limited detail while retaining the low FL of 400 which gives a super wide FOV.

 

 

Cannot really pick a specific FL by its self because the quality of the optics is more important IMO

Edited by Andros246, 16 June 2025 - 08:21 PM.

[WadeH237]

Let me answer from a slightly different point of view.

 

Specifically, I have lots of experience and have worked with many different configurations and a pretty wide sampling of available imaging gear.

 

If I could have only one focal length from now on, it would be 1200mm.  The image scale when matched with the Sony IMX533/571/455 sensors is a very good match for typical seeing (which I characterize as 2 arc seconds).  Also, these three sensors give you a good choice of fields of view and price.  If you had a scope with a large enough imaging circle, the IMX455 sensor would get you a legitimately wide field, while extracting most of the detail available in typical seeing.

 

If I had a scope with 1200mm focal length at something like F/4, with an IMX455 based camera and well corrected field, I think that it would cover just about everything that I would want to do.

[CalifDan]

I started with 61mm x 350mm with a .8 Reducer.  It works well for Nebula, but for Galaxy season it was lacking.  I replaced it with 80mm x 480mm, no reducer and a significant increase in quality.  That made galaxy season more enjoyable as some things that weren't good options at least became acceptable options.  I am considering getting the .8 reducer for this scope before the nebulas start coming around.  That would give me a 384mm option.

[Spaceman 56]

Hard question.

 

imaging at short focal lengths is enticing, but you are limited to very large targets, of which there are not so many.

thats why we see hundreds and hundreds of imagers in the beginner section shooting the same objects again and again. 

 

if you have a long focal length OTA there are vastly more targets that you can image. almost endless possibilities.

 

I have a few scopes now, but for me honestly the shorter the focal length the less likely I am to use it.

 

also it completely depends on if your happy shooting Nebulas or if you want to shoot Galaxies. 

having said all this, if I had to choose one scope, it would be my 10 inch RC. focal length 1484mm. 

[joshman]

I've dabbled in many different focal lengths for a few years.  Now I want to pick one OTA to focus on for a while and tune it up to perform reliably at its highest potential, full sensor illumination, perfect calibration, perfect corner stars, no odd optical artifacts, etc.  I have some thoughts but want to hear some other opinions.

Thats a tough question to answer, because there are so many other factors outside the focal length that are more important.

 

Most importantly, what camera will you be using?

 

The aspect ratio, pixel size, and sensor technology all play a very significant role in meeting your criteria. For example, I see you have a 1600MM listed in your signature. The sensor in this camera is an older technology that is prone to "odd optical artifacts" (micro-lensing).

 

But to answer the question as presented, IMHO, about 500mm range is a perfect focal length. I've exclusively used the Takahashi fsq-106ED (530mm) for the last 2.5 years with a 2600MM, and it's been fantastic. I've found that focal length to be a great balance between wide and long, and under the right conditions, you can easily drizzle your integrations to gain a bit more, and mosaic to go a bit wider.

 

I eventually plan to have 2x rigs running, one with the Takahashi and a FF camera, the other at longer focal length (~1,300mm, hopefully) with an APSC camera)

Edited by joshman, 16 June 2025 - 09:30 PM.

[rollomonk]

Over the last decade or so, I’ve been working on imaging virtually ALL popular, northern hemisphere stargazing (visual) targets — close to 250 — using only full frame cameras.  This includes, emission/reflection nebulae, open clusters, globulars, supernova remnants, planetary nebulae and galaxies. I divided them into 400mm, 1000mm and 3000mm focal length imaging setups. This was all based on target size (in arc minutes) and target type (I.e. open clusters tend to look better with lots of space around them while galaxies and planetaries look best zoomed in, etc). 

 

I found that only  a handful of these targets are best imaged at 400mm. Of the rest, about one-third are best imaged at 1000mm. Two-thirds at 3000mm. So, if I had to choose one focal length, it would be the 3000mm.

 

But what is best for you or any individual imager comes down to the targets that YOU are most interested in and the size of the sensor on your camera(s).

[Mark Lovik]

Depends on the focal length for the scope and the camera pixel size.

I like the 533 and 2600 camera lines with 3.76um pixels

 

==> 850mm is a great focal length for these cameras

 

 

I have a couple of cameras with 2.4um pixel size

 

==> 520mm is a great focal length for these cameras

 

Interesting ... about the same pixel scale!

 

Wide field ... go for shorter focal lengths (duh) where resolution is compromised for fov

 

Long focal lengths ... I don't have large pixel cameras for my scopes, and I tend to use OSC cameras.  Long focal length systems are a waste of time with these constraints.  Exception: lucky imaging

Edited by Mark Lovik, 16 June 2025 - 10:26 PM.

[Drothgeb]

If I had one OTA to pick, it would be the GTX 110 (660mm) that I just declined due to failing eyesight. Good enough to do wide with a large chip, and sharp enough to shoot galaxies with smaller pixels.

[Zambiadarkskies]

Askar SQA 85

 

super sharp basically gives you seeing limited detail while retaining the low FL of 400 which gives a super wide FOV.

 

 

Cannot really pick a specific FL by its self because the quality of the optics is more important IMO

I would probably vote for the same. 

[mayhem13]

I've dabbled in many different focal lengths for a few years.  Now I want to pick one OTA to focus on for a while and tune it up to perform reliably at its highest potential, full sensor illumination, perfect calibration, perfect corner stars, no odd optical artifacts, etc.  I have some thoughts but want to hear some other opinions.

Your inquiry appears to be based around your sensor…….asi1600mm? Please correct me if your direction will include a new scope AND camera for the perfect pairing?

 

Some thoughts…….fully illuminated circle with perfect stars to the corners is limited to a 4:3 size sensor with any Newtonian and a 2” focuser tube….if you’re planning on APS C, you’d have to customize or build a newt with a 3” focuser and coma corrector and larger secondary mirror, creating more of a central obstruction and reducing resolution in the process…..not a worthwhile venture at all

 

I’ve settled this discussion a few years back for myself and that’s an 8” native F4 Newtonian and an ASI533 sensor along with three different coma correctors

 

The Starizona Nexus gives me 600mm of FL

 

The Explore Scientific HRCC gives me 835mm of FL

 

The Televue Paracorr 2 produces 920mm of FL

 

All consistently illuminate the 533 sensor with perfect stars to all four corners and the square sensor allows for less required framing rotation and what not. The square sensor also makes creating mosaics for larger targets much easier to stitch. The pixel size is a good match to the FL and my typical not so great seeing.

 

This wasn’t easy to accomplish though….the Explore Scientific 8” scope needed A LOT of work to get where it’s stable and consistently able to collimate to the threshold required at F3……new focuser, spider, three tube rings and a reworked primary mirror cell. But it’s there now.

 

In all honesty, if or when I can find/afford a 130-140mm F5 four element refractor, that’s where you’ll find me. The clear aperture of a refractor combined with good optics and a stable tube with temperature and dew makes things a lot easier IMO.

[Ranger Tim]

Something between 1000-1300mm. Enough reach to pull in most small objects with some cropping - larger fields can be achieved by merging stacks.

[jml79]

My previous favorite rig was a 4" refractor at 571mm paired with a small pixel camera for 0.8 arcseconds of sampling. My current favorite rig is a 8" f/4 Newt at 800mm paired with a 2600 for a sampling of 0.96 arcseconds. My small wide field refractor is a reliable workhorse and always on one of my mounts. I like to think in sampling and FOV over focal length because I have multiple cameras with different pixel sizes to choose from. I really like my main scope to be in the range of 0.8-0.95 arcseconds which isn't perfect for everything but gives a nice blend of detail and FOV depending on the camera size. It also happens to be the sweet spot for the average seeing where I live. I get some nights where a smaller sampling could be used but only for a couple of months per year.

 

With that in mind, a scope between 800-1000mm, as fast as you can afford and 2 cameras (3.76um and a smaller pixel one) would hit most of the points. If the scope happened to also have different reducers available to tweak the focal length, that would be a bonus too.

[GiffS]

Pretty happy with my 0.8 reduced 120mm APO. It gives me 672mm @ f 5.6 which suits most of my available targets pretty well.

[Spaceman 56]

In all honesty, if or when I can find/afford a 130-140mm F5 four element refractor, that’s where you’ll find me. 

I think perhaps 130 or 140mm is the cross point where mirrors start to make more sense ?

 

for an equivalent budget you get more focal length and aperture from a mirror based scope.

 

what do you think Mayhem13 ?  

[mayhem13]

I think perhaps 130 or 140mm is the cross point where mirrors start to make more sense ?

 

for an equivalent budget you get more focal length and aperture from a mirror based scope.

 

what do you think Mayhem13 ?  

 Currently, i agree……a quality 130-140mm F5 quad doesn’t exist as far as i know…….and if it did, it would likely be $6k or more…….but with advances in manufacturing and tech, we’ll see one soon enough……Askar is getting close with the SQA series

 

For folks that live under clear and dark skies that don’t really need the speed, F7 would do just fine…..not me though…..I’m Bortle 9 white zone.

[MeteorBoy]

I'm in Bortle 9 skies.

 

I need a focal length that can lower my background sky brightness.  So in my situation about 2,500mm.  My washed out skies already greatly limit the number of targets, I don't need it limited even more by short focal lengths. 

[WadeH237]

…a quality 130-140mm F5 quad doesn’t exist as far as i know…….and if it did, it would likely be $6k or more.

You can get pretty close.  A TEC 140 with Astro-Physics QuadTCC reducer/flattener gives you a 140mm F/5.5 refractor with a well corrected field large enough to cover a full frame sensor.

 

As you suggest, it's not a cheap solution, but it's also essentially a no-compromise solution.

[Patrick]

I've dabbled in many different focal lengths for a few years.  Now I want to pick one OTA to focus on for a while and tune it up to perform reliably at its highest potential, full sensor illumination, perfect calibration, perfect corner stars, no odd optical artifacts, etc.  I have some thoughts but want to hear some other opinions.

That's like asking a pro photographer to only use one lens!  Probably not going to happen...maybe 2 or 3.  

[Sacred Heart]

I've dabbled in many different focal lengths for a few years.  Now I want to pick one OTA to focus on for a while and tune it up to perform reliably at its highest potential, full sensor illumination, perfect calibration, perfect corner stars, no odd optical artifacts, etc.  I have some thoughts but want to hear some other opinions.

My answer to your title

 

If you could choose only one focal length for AP what would it be?

 

would be none, I would stay with binoculars and do visual.

 

In answer to your post,  I'd say my Agema Optics SD130, at 1040mm.  For all the reasons listed above.  With a 2400 camera I have somewhat of a wide field of view, wide enough for me.  With the 533 and 571 I can narrow it down some and with a 2X Powermate and a 462 color camera I can have some fun with the planets.

 

When I put an eyepiece in, it is pure enjoyment, planets / Moon or deep sky. 

 

Lets just put it this way. I rarely use my 7" maksutov any more.

 

Joe

[Wjeremy 15]

Appreciate all the replies.  It's interesting there doesn't seem to be a consensus, with everything from 300 mm to 3000 mm suggested. 

 

As some said the question is incomplete without specifying other features of the optical tube and sensor, but I sort of left that open on purpose to see what ppl came up with.

 

Let me answer from a slightly different point of view.

 

Specifically, I have lots of experience and have worked with many different configurations and a pretty wide sampling of available imaging gear.

 

If I could have only one focal length from now on, it would be 1200mm.  The image scale when matched with the Sony IMX533/571/455 sensors is a very good match for typical seeing (which I characterize as 2 arc seconds).  Also, these three sensors give you a good choice of fields of view and price.  If you had a scope with a large enough imaging circle, the IMX455 sensor would get you a legitimately wide field, while extracting most of the detail available in typical seeing.

 

If I had a scope with 1200mm focal length at something like F/4, with an IMX455 based camera and well corrected field, I think that it would cover just about everything that I would want to do.

I think this is where I am, but not gonna spend the $$$ to build that rig.  An APS-c or 4/3" rig getting close to these criteria seems like a more affordable target, and I think I'll try to do this around my ONTC 8" f/5.

 

 

Hard question.

 

imaging at short focal lengths is enticing, but you are limited to very large targets, of which there are not so many.

thats why we see hundreds and hundreds of imagers in the beginner section shooting the same objects again and again. 

 

if you have a long focal length OTA there are vastly more targets that you can image. almost endless possibilities.

 

I have a few scopes now, but for me honestly the shorter the focal length the less likely I am to use it.

 

also it completely depends on if your happy shooting Nebulas or if you want to shoot Galaxies. 

having said all this, if I had to choose one scope, it would be my 10 inch RC. focal length 1484mm. 

This makes sense to me and I am surprised to see so many advocate for shorter focal lengths due to the limited targets.  I agree that most of the time the larger aperture-- longer FL are seeing limited, but those special nights do happen sometimes and I think it pays to be prepared.

 

 

Your inquiry appears to be based around your sensor…….asi1600mm? Please correct me if your direction will include a new scope AND camera for the perfect pairing?

 

Some thoughts…….fully illuminated circle with perfect stars to the corners is limited to a 4:3 size sensor with any Newtonian and a 2” focuser tube….if you’re planning on APS C, you’d have to customize or build a newt with a 3” focuser and coma corrector and larger secondary mirror, creating more of a central obstruction and reducing resolution in the process…..not a worthwhile venture at all

 

Not sure 100% illumination is necessary if vignetting is not too severe and flats out well.  Would like to see perfect stars in the corners, which is one of the reasons I backed down to f/5.

 

Something between 1000-1300mm. Enough reach to pull in most small objects with some cropping - larger fields can be achieved by merging stacks

Think I agree on the FL, but the larger fields is what gives me pause, I have found I don't love processing mosaics, esp trying to match gradients, stretch, colors, etc.

[jpengstrom]

The focal length I most want is 131,400mm. Of course it would be best if it was orbiting around Lagrange 2. 
 

But I’ve been using my 1000mm scope much more frequently than my 550mm scope since I got it. And that’s with an APSC sensor in my QHY268. 

[rgsalinger]

I consider my AP155EDFS with the AP flattener coupled to a full frame ASI2400/6200 camera to be the best compromise between size, focal length and field of view. With the FF chip, the FOV is wide enough for most targets, yet the scope can pull in almost anything with excellent details in the B3/4 where I image.

 

So, while these are only available used, I'd go with a 5-6" refractor of around one meter that will cover a full frame camera with small round stars. That would be a TEC140 or an TOA130. 

 

These days though I do find myself wondering about a 10 F4 Newtonian with coma corrector. Costs less, is faster and has the same, in the end, image scale/resolution. Do these cover a full frame chip would be my only concern. 

[kevinkiller]

If I had a scope with 1200mm focal length at something like F/4, with an IMX455 based camera and well corrected field, I think that it would cover just about everything that I would want to do.

Here you go: https://artesky.it/i...2278161336.html

Now if I could find that scope without diffraction spikes, I'd buy it too.
 

Edited by kevinkiller, 17 June 2025 - 10:34 PM.