What is a long focal length in imaging?

[swag72]

Many imagers have got various different focal length options that they work at for different targets and different times of the year. I have what I would consider a short focal length scope operating at 329mm.

Would you think of imaging at 1m a long focal length? Perhaps 2 or even 3m?

There are so many difficulties imaging at these sorts of focal lengths, all those things aside, what do you personally consider as a long focal length?

[oldpink]

I've seen Images from very low (Fast) F2.5 setups all the way to some shooting at above F40 (Normally Barlowed)

[swag72]

I'm thinking more of focal length with regarding DSO's really. Planetary and barlowing is something that is so far off my radar I didn't even consider it

[ChrisLX200]

Off-hand I'd say 'long' starts around 2000mm... maybe.    The longest I've tried is 2,500mm, but I suppose you should think more in terms of arcsec/pixel for demands on guiding accuracy?  What is the minimum acceptable guiding accuracy for, say, 11uM pixels?

ChrisH

[jambouk]

I heard ian king say (from memory):

Short <600-700

Medium up to 1500

Long >2000

Jd

[opticalpath]

I don't think there's a single answer to this, Sara.  I agree with Chris: a lot depends on arcsec per pixel rather than on F.L. alone.

1000mm at 1.1 arcsec/px seems 'medium' to me. But if I bin the camera 2x2, life becomes much easier and it feels 'short'; in fact, since the pixel scale of the final image is halved, it actually IS short to all intents and purposes!
Would I be able to distinguish between an image taken at 500mm F.L. binned 1x1 and another image of the same object taken at 1000mm F.L. binned 2x2 ..... or at 2000mm binned 4x4?  Would guiding accuracy (in pixels) be any different?

If we were talking about imaging Moon & planets, of course, that would be different again ..... I've gone to 25,000 mm in the past.  That was definitely long

Adrian

[ollypenrice]

Guiding and resolution do hinge on the arcsecs per pixel but still, as a rule of thumb, I suppose I'd agree with Ian King. There are no hard definitions here. Once you get to very long FLs by amateur standards you're chasing your tail unless you have the guiding and the seeing to support you. You're going to have to be seriously rich to have a three metre focal length at a fast F ratio so you are going to need a lot of clear nights as well...

So far I've spent a couple of years entertainingly working Yves' 2.4 FL metre scope and currently don't have a long FL here. I'm intrigued by seeing how much detail could be coaxed from the TEC (FL980) with a small pixel camera, though.

At heart I'm a widefield and refractor junkie, to be honest. Wipe 'em down with a damp cloth. (I feel the same about my hairstyle... )

Olly

[swag72]

Thanks all - interesting ideas. So probably 2m or over is generally considered to be a long focal length.

[steppenwolf]

Really is a bit of a generalisation but I'd say that Ian K has it about right but in my own 'stable' 1200mm is long to me!

[Earl]

700mm to 1000mm, thats getting to long and then beyond is long. most Short focal lenght scopes are around 450-600. im not really sure medium exsists or is relevant.

[lensman57]

Many imagers have got various different focal length options that they work at for different targets and different times of the year. I have what I would consider a short focal length scope operating at 329mm.

Would you think of imaging at 1m a long focal length? Perhaps 2 or even 3m?

There are so many difficulties imaging at these sorts of focal lengths, all those things aside, what do you personally consider as a long focal length?

A few weeks ago I managed a couple of captures of M51 and M82 using my Skywatcher 127 Mak @ F11.8, 1500mm of FL. I managed to guide reassonably well with my HEQ5 Pro but I don't think that with my sky and the present set up I would like to go any longer or slower.

Regards,

A.G

[MartinB]

For a 6-7 micron pixelled camera I reckon you don't need to worry too much about the seeing conditions up to 600-800mm.  At 1000mm it can become an issue.  Attempting to image at 1700mm over the last winter was a complete waste of time and I would have been much better off sticking to 1000mm, same resolution and less hassle. For most of us in the UK attempting to image at over 2000mm is an excercise in self delusion and frustration unless you are particularly fortunate with  seeing conditions. 

[ollypenrice]

I'm with Martin. What do I want first and foremost? I want signal. When you have that in the can you can start finessing. Until you do...  

I haven't yet had enough time with different setups to know what is worth chasing at my site in terms of resolution but for the moment, as I said earlier, an arcsecnd per pixel will do me. That's at 900 metres in the Alps.

Olly

[Epicycle]

For me 1.2m feels infinitely long already... The neighbor sneezes and my guiding goes to hell, but I would agree that in the UK

going much below 1" per pixel is a bit pointless,

E.

[swag72]

Interesting thoughts. I can certainly accept that pixel size contributes to image scale and can make things easier or not.

The smaller the pixel size the harder all round, right?

[ollypenrice]

Interesting thoughts. I can certainly accept that pixel size contributes to image scale and can make things easier or not.

The smaller the pixel size the harder all round, right?

Yes. To stay sub pixel in the guiding is harder because the pixels are smaller and details in your beam stray onto the next one more easily. You also share the incoming light between more pixels so each gets less. This means longer exposures (in the real world.)

Olly

[swag72]

Yes. To stay sub pixel in the guiding is harder because the pixels are smaller and details in your beam stray onto the next one more easily. You also share the incoming light between more pixels so each gets less. This means longer exposures (in the real world.)

Olly

So small pixels at more than 2m focal length is pretty mad then?

[opticalpath]

So small pixels at more than 2m focal length is pretty mad then?

Pain without much gain, imo.

 

Exceptions?

1. Planetary imaging (large numbers of very short exposures selected from the moments of best seeing) where over-sampling works.  

2. Brighter DSOs in very good seeing conditions, using active/ adaptive optics systems.

 

Adrian

[ollypenrice]

So small pixels at more than 2m focal length is pretty mad then?

Work out the arcseconds per pixel, first, or look them up here; http://www.12dstring.me.uk/fov.htm

What is a reasonable max resolution (smallest number of arcseconds per pixel) is a matter of debate. Nobody seriously argues for much less that 0.5 in DS imaging and these folks have excellent sites so far as seeing goes and they use use robotic focus between frames and have phenomenally accurate mounts. They don't consider 'round stars' to be measure of tracking! I'll let them get on with it. With Yves I imaged at about 0.6 and felt I wasn't gaining much that I couldn't have had at 1.0, but the signal is weaker per pixel, clearly. I'd rather have more signal myself, but I'm not a resolution nut. We can't take on the Hubble. My personal style is 'broad brush' if you like.

You can always bin a small pixel CCD at long FL. You will gain signal and not, in reality, lose resolution. Out in space, don't bin! 

Olly