Why are quads charting?

[Grant Privett]

I’ve been thinking – yes, I know its a shock, but bear with me – about current trends in imaging.

A lot of people these days buy refractors of quite small aperture for deep sky narrow band imaging. I assume they are popular because the relatively short focal lengths they provide makes for some very nicer framing opportunities for larger nebulae and they are not overwhelming to use or require a permanent set up. You only have to look in Astronomy Now or on our own Gallery for evidence of the lovely results achieved.

Its true that you don’t see see as deep in star magnitude terms as you would with a larger aperture, but if what you really wanted was nebulosity, then you won’t be that fussed and it makes it easier to remove the stars for nebula-only processing.

So far, so good.

What I am not not certain of though, is why you need the refractor to be a triplet (or even a quad) involving ED glass. If you’re imaging using light constrained to a narrow range of wavelengths, then there is no chromatic aberration to worry about.

Obviously, coma is still an issue but if a coma corrector is employed with a doublet achromat, what is the added benefit of a triplet in narrow band imaging?

I suspect there might be a tiny difference in platescale for images captured in Oxygen, Sulphur or Hydrogen – especially with today’s big sensors – but thats nothing a fairly trivial piece of software couldn’t sort for you. Its easy, take your images, platesolve and resample to a fixed grid of 1 arc sec separation. Its only a few dozen lines of Python.

So, is the bottom line benefit that you can also image in RGB and not everyone wants narrowband or, is the issue the availability of the resampling software?

Whats the rationale?

Whats the great advantage I have missed?

Also, when wanting to shout at me, please remember, I have not used a refractor for serious observation in 20 years.

• This topic was modified 7 months, 3 weeks ago by Grant Privett. Reason: Typos

[Dr Paul Leyland]

The great advantages? Bragging rights to customers and profit margins to suppliers.

Call me a cynic, I don’t mind.

[David Arditti]

Capturing nebulosity requires low focal ratio. To get optimal correction of chromatic aberration and coma at low focal ratios requires more lens elements. Yes you can use an ED (or similar) doublet with a coma corrector, but that’s 4 lens elements in total, and in most cases it won’t be as optimal as it could be, and there’s likely to be spacing errors, so better really, if you know the telescope will be used for imaging at this focal ratio, to build it all into one optimised, correctly measured-up tube assembly. Many people wish to use the dual-band, tri-band and quad-band filters for one-shot imaging, so chromatic aberration will still be a moot point.

[Grant Privett]

Ah yes. Have never used one of the duo/tri/quad band pass nebula filters and don’t use OSCs. Agree: one of those filters would require properly corrected optics.

I was just looking at how much a Takahashi 106 costs these days and even a Borg 72mm is no change out of £2.5k – and lets not even mention the TB 203.

I’m probably just mean, but I always think “Ouch” if handing that much money over for something that isn’t necessary. Mortgages and such like taking precedence.

I am impressed that you now see refractors down to f/3.9 but a 72mm isn’t really giving that much light to filter. My approach would be a mono CCD/CMOS, single band filters and a SW 80 achromat and a focal reducer/coma corrector – but, yes, that wouldn’t be running at f/3.9. Can see why the 8″ V2 RASAs are so popular.

Taks and fast EDs are great for those who can afford them.

[Grant Privett]

Oops. For TB 203 read TMB 203. Sorry.

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