Sensor FWHM readings versus actual sub FWHM values

Dark Site Remote Astrophotography Seeing Conditions Collimation
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Rabeea Alkuwari avatar
Rabeea Alkuwari avatar

Hello everyone,

I have a question about FWHM measured values through sensors Vs actual FWHM values measured in subs

I’m currently evaluating sites & options regarding establishing a remote observatory - the route I decided on pursuing was to look into the market for established setup to buy to potentially save on used equipment & shipping costs…this approach has the advantage of testing said skies and setup prior to any commitments. The idea for me to go remote is to access better data - i love traveling with my telescope and dont intend on stopping but the main idea is for me to access sub 1.5” fwhm data on the regular. In the desert its mostly 2.5-3.5” however we get lucky sometimes with chile calibre nights (lowest i got was 1.1” average on edge 14 - so i got addicted to it 😅)

Upon testing few setups available on the market (one CDK 17” in HCRO, one RC 16 on another location in new mexico) sub FWHM values were sort of high for the expectations. In nights were sensors detect “1-1.5 fwhm, sub data goes as far as 2.5-3.5 (verfied that data is in focus with high altitude and good tracking) measured through SubframSelector on pix with inputting the correct resolution in bin 1×1

So I was wondering if this is normal or if there are other limiting factors within the setups I have experimented with. Would love to hear from people with telescopes in sites that publish FWHM readings through sensors such as HCRO or El sauce

Thanks,

andrea tasselli avatar
andrea tasselli avatar

Not quite clear what you mean by “sensor” WHFM vs., instrumental (i.e., per sub) FWHM. In my experience (drawn from using remote setups for few years) Atacama, Namibia and southern Spain have an instrumental seeing between 1.5” and 2” on long integrations (300s to 600s) and a shade less for 30s to 60s (down to 1.1”-1.2” for narrowband on very good nights). That is on good nights, even Chile’s Atacama experience bouts of (relative) bad seeing but never registered worse than 2.5” across the board for several hundreds hours’ worth of integrations.

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Rabeea Alkuwari avatar
Rabeea Alkuwari avatar

andrea tasselli · Jun 7, 2026 at 12:46 PM

Not quite clear what you mean by “sensor” WHFM vs., instrumental (i.e., per sub) FWHM. In my experience (drawn from using remote setups for few years) Atacama, Namibia and southern Spain have an instrumental seeing between 1.5” and 2” on long integrations (300s to 600s) and a shade less for 30s to 60s (down to 1.1”-1.2” for narrowband on very good nights). That is on good nights, even Chile’s Atacama experience bouts of (relative) bad seeing but never registered worse than 2.5” across the board for several hundreds hours’ worth of integrations.

Hi Andrea,

I mean the discrepancy between advertised FWHM values through the night in HCRO/El sauce site (I believe they have specialized sensors for that, that calculate turbulence in arc sec) vs calculated FWHM through your subs.

Interesting to know that your highest FWHM value in El sauce was 2.5, can you please let me know your resolution?

Thanks

andrea tasselli avatar
andrea tasselli avatar

Advertised values of seeing should be taken with a large shovel of salt since they are based on scintillation values taken with a small lens pointing at a fixed point in the sky. At best they would tell the best expected seeing, all out, with a small aperture (say 4” or, quite possibly, less).

Not Obstech the site but nearby (ChileScope) so the same should apply. That scope was a 0.5 m astrograph with 0.95”/px but apart from that one the usual resolution is around 0.5”/px. The same would apply however.

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