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Old 16-03-2019, 02:08 PM
glend (Glen)
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Best Budget Narrowband Imaging Scope Choices

The reason for this thread is to identify an adequate scope for pure narrowband imaging. As many would know, narrowband with its thin slices of spectrum is supposedly more forgiving of glass colour correction fidelity. Achromat optical designs have been used successfully for narrowband, and I have myself used a Bresser 152S for narrowband imaging with good results. The Bresser 152S is augmented with a rear mounted Petzval elements, hence it is a sort of modified Achromat.
There are plenty of threads, hundreds probably, that debate the optical design and glass types, best chosen for broadband imaging. People love to argue about that subject, but little discussion is dedicated to pure narrowband scope choices. And let's face it, for many people narrowband is the only choice if they want to image; given their light pollution situation, or preference for the billowing details that narrowband imaging provides on emission nebula.
So, let's have some discussion on this topic. Sure a great APO, or even your wonderful Newt can be used for narrowband, but that is not really what this is about.
Consideration to field correction should be given, so possible Petzval or corrector designs have some advantage, but add-on flattener can be an option. Speed is another attraction, as we all know narrowband can take some time to build.
Suggestions, recommendations, and remember keep the purchase price as low as possible, and keep emission nebula targets in mind.
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Old 16-03-2019, 03:32 PM
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Slawomir (Suavi)
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Hi Glen,

I'm one example of a guy who could only do narrowband for a number of years exactly because of light pollution, so such thread could potentially be quite useful for people starting off their adventure with (narrowband) astro

From my experience mount's capability to guide precisely is essential in narrowband, given that narrowband usually requires longer exposures than LRGB imaging, if we want to reveal finer details. This prerequisite perhaps can to a degree can be mitigated with low read noise CMOS cameras?

If we are on a very limited budget then we can assume a modestly performing mount that will put a cap of the length of useful exposure. Therefore photographic speed, perhaps some under-sampling to hide imperfect guiding and low RN are the key to good data. Speed can be achieved with a fast focal ratio (I'm taking extended object not stars), while low RN is a given with relatively affordable CMOS cameras.

If we assume that the sensor will be 3.8 microns as in ASI1600, then I propose a 500mm focal length or thereabouts, that will give some but not excessive under-sampling. If we would not like to go slower than say f/5-6, this in turn suggests aperture of around 100mm also meaning the scope could be carried by a modest mount. To sum up, one would like around 100mm f/5 telescopes to match with ASI1600 (or similiar). This leads us to FSQ 106...

But more seriously, given my very positive experience with TS ED doublet for narrowband, my suggestion is a TS-Optics ED 102mm f/7 doublet with a 0.8 TS reducer resulting in f/5.6 and 560mm focal length. This is what I used in the past with 3.69 micron pixels and got very satisfying results. The mounts were HEQ5 and later AZ-EQ6. The cost for this scope+reducer is about 1000 AUD plus shipping and import taxes.

A cheaper alternative would be 80mm TS ED f/7 doublet with the same 0.8 reducer for about 850 AUD plus shipping and import duties.

More saving could be made by buying a second-hand telescopes with similar parameters.
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Old 21-03-2019, 02:14 PM
tvandoore (Tim)
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I read at one point that although false colour in doublets isn't an issue with narrowband, the way triplets work will still give more sharpness through tighter focus. I'm not sure how much of a factor this is, but the narrowband imagers that put out those amazing pictures on astrobin all seem to use triplets.

Edit: autocorrect as usual

Last edited by tvandoore; 24-03-2019 at 12:29 PM.
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Old 21-03-2019, 04:40 PM
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Slawomir (Suavi)
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You are right Tim, a good triplet is a better choice for narrowband than the same aperture good doublet, mainly because the triplet can be faster than a doublet to produce the same or similar quality of data. However, we are discussing budget setup and doublets can be very capable of providing good data.

One example of decent narrowband data (and mediocre processing) captured with a doublet can be found here: https://www.spacetelescope.org/proje...r_lipinski_03/
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Old 21-03-2019, 07:32 PM
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If you are looking at just one wavelength, e.g. sub angstrom H-alpha, a well corrected doublet will do the trick, and provided it has no geometric vices
e.g. spherical aberration or astigmatism, you'll get filigree detail

The problem with shifting from red through to blue narrow band wavelengths
with refractors is, the focal length often changes, and has a different barrel distortion to boot.

In short: makes image registration very tricky.

Not sayin' you can't get good results, but pay the penny ride.....
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Old 21-03-2019, 08:12 PM
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Slawomir (Suavi)
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Good point Peter.

Luckily Ha and SII emission lines are both red and thus have similar wavelengths, and OIII is only about half way between red and blue, so we don't really need blue in a classic HaOIIISII narrowband imaging.

I had absolutely no registration issues with an affordable doublet when imaging at f/5.6 ;however, halos around bright stars were noticeable due to the out of band signal getting through.

Here is another example of a tricolour narrowband semi-decent image taken with the previously mentioned doublet: https://www.spacetelescope.org/proje...r_lipinski_06/
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Old 28-03-2019, 06:04 PM
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Echoing Suavi's comments - here is an image from a Pentax 67 300mm F4 lens. Its not the ED version.

I was surprised it worked so well. I think I paid around $400 for the lens.

That is a FLI Proline 16803 sensor here so 36 x 36mm or so.

These 67 lenses have something like a 60mm corrected circle at least.

Though Glen your 71SDQ would be hard to beat for this?

Greg.

Last edited by gregbradley; 30-03-2019 at 08:17 PM.
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