# Xilman

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1. ## Why don't mirrored telescopes have the light crossover before going to the eyepiece?

Indeed. But no more important than the many lens surfaces in a modern eyepiece. Mine is used solely for imaging, so I don't have the latter problem.
2. ## The tiniest of stars, can you see?

Total overkill (but you can never have too much overkill) Gaia goes down to roughly 21s magnitude. AladinLite with the DSS images, also goes down to roughly 21st magnitude and can overlay the Gaia catalogue. Filling in the details is left as an exercise. I have already given you easily enough information to fill in the gaps if you are sufficiently interested and know how to use a search engine.
3. ## Determining Pixel Size based on Focal Ratio

Here's what I do (and did). Calculate the theoretical resolution of a telescope with the aperture of yours. The Airy disc size is easily precise enough. Calculate the image scale in arcsec per pixel. Apply the Nyquist criterion: sampling should not be less than twice the size of the minimum resolution desired. Even with very poweful deconvolution software you are unlikely to exceed this recommendation by very much. All the above assumes perfect seeing. In practice, that will not happen and you will be seeing limited in almost all cases. Lucky imaging and speckle interferometry can often let you exceed this bound. To give an concrete example: The aperture of my telescope is 0.4m, leading to an Airy disk radius of 0.31arcsec My scope has a focal length of 2614mm, leading to an image scale of 12.7 microns per arcsec. So, the Airy disc is 0.31*12.7 = 3.9 microns in radius. Seeing is very, very rarely less than 1 arcsec at FWHM, unless you live high on an oceanic island. A more reasonable figure for a rather good night is 2 arcsec, which corresponds to 3.9 * 2/0.31 = 25 microns. On a bad night, and I speak from experience, it can reach 20 arcsec; such nights are useless IMAO. My camera has a native pixel size of 3.7 microns. The sampling over a 2 arcsec seeing disk is thus 25/3.7 = 6.8 --- way beyond the Nyquist criterion. In my case, the radius of the Airy disk is very close to the pixel size, and the disk itself is sampled at very close to the Nyquist criterion. Why do you think I chose that camera? Almost all the time I run at 2x binning for other reasons. Only when playing with Lucky and speckle imaging is 1x binning used. (All those numbers above can be checked with readily available sources. Finding and using them is left as an exercise for the reader.)
4. ## Why don't mirrored telescopes have the light crossover before going to the eyepiece?

The major difference is that in the Gregorian the light reaches its focus from the primary before reaching the secondary and is then brought to a second focus by the concave secondary beyond the position of the primary. That makes for a markedly longer OTA of a Gregorian than a Cassegrain. The Dilworth is more like a Cassegrain in that the first focus is located after the light reaches the secondary and is then brought to a second focus by a series of relay lenses. My 0.4m f/6.5 ( focal length of 2.614m) Dilworth is only about 1.5m long, as can be seen here: http://www.astropalma.com/equipment.html Further, all the surfaces are spherical, which makes construction markedly easier in some respects. The principal downside of the Dilworth in my experience is that the transfer lenses inevitably scatter some light. Alignment and collimation difficulties are only experienced once, when the telescope is first made, because it is a completely enclosed design and needs recollimation about as often as a properly built refractor. The flat window at the front keeps out all the dust, spiders, gecko droppings, etc. This image of Sycorax, Uranus XVII, shows a little of what I mean. Uranus was perhaps half a degree outside the field of view but, there again, the planet was about 15 magnitudes brighter -- a factor of about a million. http://www.astropalma.com/Projects/Satellites/sycorax.html Doubtless if all the relevant surfaces were scrupulously cleaned the scattered light could be reduced.
5. ## Looking for some FITS files fresh from your camera

It's not the fastest but I have found it to be very effective indeed. To see what I mean, try going to A shame you need to translate to JS. If you could use C it would be much faster.
6. ## Why don't mirrored telescopes have the light crossover before going to the eyepiece?

The Dilworth does just that and does not have a spider diffraction because it is essentially a Cassegrain with purely spherical optical surfaces. The relay lenses correct for spherical aberration. The reason I know this? My main telescope is a 0.4m Dilworth. The diagram at https://www.telescope-optics.net/miscellaneous_optics.htm is extremely close to that of my scope; I wonder if it was taken from Larossa's design drawings.
7. ## Looking for some FITS files fresh from your camera

I found FABADA to be very effective at noise removal. If you haven't yet tried it, a simple search will find it for you.
8. ## Eye piece suggestions for a 16” F4.5 Dobsonian

About 35 years ago I had an 18" Dob in Bortle 5/6 and I am now desperately trying to remember which eyepieces I used. 20mm and 8mm sounds vaguely familiar
9. ## Looking for some FITS files fresh from your camera

I can provide something over 38 thousand such. Unfortunately I am frantically busy preparing to return to La Palma tomorrow. Perhaps I'll send you a few when I get there.

Low-profile rockerboxes can be very small. Take a look at this one, for instance. https://www.rothervalleyoptics.co.uk/explore-scientific-ultra-light-16-dobsonian-telescope.html (I coulldn't find one under 18" at FLO.)

Dobs up to around 16" will go in the back of a hatchback in my experience, as long as the primary and secondary are separated by removable struts. Not a lot of room left over, though, for a family and their luggage. My 10" SW Dob fits very easily into my Nissan Pulsar, which is not a big vehicle by any stretch.
12. ## Slugs and Astronomy

If you don't want problems with slugs, I recommend you move to La Palma. It is far too dry for slugs there. OTOH, you will have problems with geckos leaving small black and white pellets almost everywhere, so you can't have it all.
13. ## Mini PC - recommendations and suggestions

Quite often done. A TCS does not need to run full-fat Windows. All it needs to do is control the hardware and put the images into storage. All the heavy lifting, including providing the disk storage, can be done elsewhere on a different machine --- which you are already planning on doing. What I am hinting at is that a Raspberry Pi is easily capable of driving a telescope, mount, filterwheel, cameras, autoguider, etc. It is dirt cheap, very small and weighs little (so it can be strapped to the mount thereby reducing cable clutter), and takes very little electrical power.
14. ## Anybody fancy a go at synchronous observations of Exoclock target TOI-1288b?

I would have been but my observatory is in La Palma and I was in the UK. I have a 0.4m (approximately 16") Dilworth there. If you wish to repeat the exercise in August/September count me in. Paul
15. ## Why invest in a hobby that doesn't return the investment. n

I have a spare pad equipped with ethernet and power at my place in La Palma. Another robotic observatory, run by a chap from Cheshire, is on my land. I am also building a roll-off roof observatory to house an old Meade Schmidt-Newtonian. That kit will be fully automated from the start. Paul
16. ## Show us your set up in action during the day.

Can't currently show you the telescope in daytime action (next month perhaps when I return to La Palma) but I can show you the results of some daytime imaging. These two FITS headers give the exposure time (60ms) and the time of observation (14:15 local solar time). DATE-OBS= '2024-02-16T15:31:36.00' /YYYY-MM-DDThh:mm:ss observation, UT EXPTIME = 6.000000000000E-02 /Exposure time in seconds In a fit of idiocy I thought I would try to image Messier objects in the day time, starting with the brightest - M45. It is far too big to fit in the 13 arcmin FOV of the telescope but individual stars ought to be visible.The images below, taken through a Sloan i' filter to reduce sky brightness show three of them. One image clearly shows a double and consultation with catalogues identified it as HD23964AC. The the known image scale and orientation are a perfect match for the 10.4arcsec in PA 235 degrees catalogued values. The components are i' ~7.4 and ~9.4. The catalogue position permitted a crude plate solve for the other images, which is why I could identify 26 Tau at around i'=6.0. Perhaps M35 through M38 may succumb this summer. HD 23964AC 26 Tau
17. ## M13 wide or crop?

Wide, undoubtedly. The zoomed version doesn't show the whole cluster.
18. ## Oh no ! Bad astronomy

g (in m/s/s) = pi^2 = 2^3 =10 is amazingly useful. c is the the resultant velocity of accelerating at 1g for a year (in the Galilean approximation of course). It is no coincidence that the radius of curvature of spacetime at the Earth's surface is one light year.
19. ## What have you lost lately?

Mojo rojo, mojo verde, or both? (See my profile if this one doesn't make sense at first reading.)
20. ## What have you lost lately?

Of all the things I have ever lost in my life, I miss my mind the most. Given that I have been interested in astronomy for ~60 years, this must count as astro-related.
21. ## Not looking good at Meade/Orion ... !

Correct. UBD.
22. ## Not looking good at Meade/Orion ... !

Right on the mark. Telescopes have a shelf-life which is not only longer than that of a Ratner's prawn sandwich but markedly greater than a Moggie Minor. I personally have made observations with two early 19th century refractors, one of which belonged to "Eagle-eyed" Dawes himself. Many other telescopes around the world are still in frequent use, despite having been made well over a hundred years ago. These are the 12" Northumberland and the 8" Thorrowgood refractors at the Cambridge IoA. Much older: I've peered through a 17th century refractor (at the roof of Blackwells' bookshop) and had serious discussions about getting one of William Herschel's 8" refractors back into working order. Its secondary is missing and the primary needs repolishing. Both these instruments are in the collection of the Museum of the History of Science in Oxford. Well worth a visit if you are in town.
23. ## Seestar S50 images

My idea was to use the telescope's mount, not the S50's which would just be dangling on the end of the large OTA. Treat it only as a (overly-complex) camera. I would hope that identical subs would still be stacked (identical because the large mount would be tracking reasonably well. Unfortunately I don't have a S50 with which to experiment.
24. ## Seestar S50 images

Crazy idea. A Seestar is just a camera focussed at infinity. What if you bolt it on to then end of a much bigger telescope, preferable one with a >=50mm image plane?
25. ## Seestar S50 images

Do I need to get my irony meter serviced again? I couldn't see any image in your post.
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