› Fora › ASTRO-FORUM › ER DU NYBEGYNDER? › Billede i refraktorteleskop ikke skarpt nok (?)
- Dette emne har 8 svar og 4 stemmer, og blev senest opdateret for 1 år, 9 måneder siden af clauskaf. This post has been viewed 593 times
- 13. juni 2021 kl. 16:15 #327423
Jeg vil beskrive mit problem på engelsk, fordi det er for svært for mig at oversætte alt til dansk. Men i må godt skrive jeres svar på dansk. Tak på forhand!
I have bought a second-hand refractor Meade DS-2080AT-LNT (aperture 80 mm, focal length 800 mm, simple rack&pinion focuser) together with eyepieces (Bresser Plössl, 1.25″) and Barlow lens (TZ-Optics 2x) and use it to look at Jupiter and Saturn. I have discovered that I cannot get sharp images of any of the two planets – no matter how accurately I try to adjust the focuser, the best result I can get is that where Jupiter looks like a fuzzy circle with 2-3 colored stripes, and Saturn looks like a fuzzy circle inside a fuzzy ellipse. I have tried to connect a mirrorless camera to the telescope (without eyepieces, but with the Barlow lens), apply digital zoom to see Jupiter or Saturn in more detail and then adjust focus and take pictures – same result. Moreover, the photos actually seem less sharp than what I can see with my eye through the eyepieces (I have tried to compare the image quality by looking at distant objects on Earth through the eyepieces and through the camera).
I understand that 80 mm is not a super large aperture, but the telescope’s ideal resolution seems to be ~2 angular seconds, while the angular size of Jupiter is 30-40 seconds, so I expected to see up to 15-20 different elements on the diameter of Jupiter, when in fact I can see 3-4. I have never used any other telescopes, so I have no benchmark against which to compare my telescope. It would be great if somebody could answer the following questions:
– is it normal to only get fuzzy images of Jupiter and Saturn on a 80/800 telescope like mine?
– how do I identify the main cause of insufficient sharpness? Could it be the main optics of the telescope, the eyepieces, the focuser or the atmosphere? (both Jupiter and Saturn only rise to ~10 degrees above the horizon before the dawn)14. juni 2021 kl. 09:05 #327424
- Neutron star
Der er delte meninger om dette teleskop.
Som udgangspunkt skal en defokuseret stjerne se ens ud på begge sider af fokus.
Det lyder dog som om der er et grundlæggende problem med optikken.
Jeg har været ude for et tilsvarende problem med en 75 mm refraktor. Det viste sig, at objektivet var samlet forkert. Samlelinsen havde den forkerte side ind mod spredelinsen.
Efterfølgende tegnede teleskopet fint.
TOC Observatory - "http://tocobs.org -14.5″ – f:4,2 Newt - Atik383 - ZWO2600-mono – SXV H9 - QHY8L-color - SkyWatcher 80 mm ED refraktor - 60 mm F:6 apocromat - TAL Apolar 125 f : 7,5.14. juni 2021 kl. 09:10 #327425
Citat: “or the atmosphere? (both Jupiter and Saturn only rise to ~10 degrees above the horizon before the dawn)”
Her rammer du lige netop problemet. Jeg tror at med Jupiter og Saturn så lavt på himlen, må du ikke forvente det bedre lufturo spiller utroligt meget ind
prøv at lægge mærke til stjerner i samme højde, flimrer de så har du forklaringen. Observerer du henover/igennem en lokal varmekilde (Skorsten)
så kan det også være forklaringen. Prøv at observere/fotografere nogle stjerner højt på himlen og se på deres billede14. juni 2021 kl. 11:31 #327428
- Neutron star
Jeg er enig med Claus. Under de forhold, og efter en varm sommerdag, hvor planeterne ikke står højere, så er det næsten umuligt at få skarpe billeder.
En mere sikker måde at trste optikken på er ved at observere stjerner højt på himlen, men heller ikke her er der garanti for skarpe billeder her om sommeren.
TOC Observatory - "http://tocobs.org -14.5″ – f:4,2 Newt - Atik383 - ZWO2600-mono – SXV H9 - QHY8L-color - SkyWatcher 80 mm ED refraktor - 60 mm F:6 apocromat - TAL Apolar 125 f : 7,5.14. juni 2021 kl. 12:20 #327429
Vladimir, several effects at play here.
1) The theoretical resolution in the telecope may be 2 arcseconds, but you are looking through the atmosphere. Normal seeing in Denmark is 2-4 arc seconds, 2 arcseconds being a rare phenomenon. For this reason alone, your images are more blurry than you would expect.
2) Secondly, as noted by others, you are vieweing close to the horozon. This causes the light rays from the objects to travel through more of the Earth’s atmosphere, causing even more blurriness.
3) Your mount is probably slightly wobling. It is not totally stable, even a small gust of wind, your steps on the ground or just you breathing close by, will cause motion of the tube, resulting in blurred pictures
4) The actual photograph-action (the mirror inside the camera moving) will result in a shake of the whole system.
5) You are taking shots of, say, 1-2 seconds? During that time hot air cells in the atmosphere will shift, causing further blurriness.
6) Your camera may be slightly out of focus, no matter how accurately you try to focus
7) Inside the tube, the air will be warmer or colder that the surrounding air, and will be cooling down or heating up accordingly, causing air turbulence inside the tube.
These effects all combine to produce less than ideal images.
The shots you see online are typically made using a technique called “Lucky imaging” – the idea being that a video is shot. Say, 10.000 images each of 1/10 second. Each individual frame is then analyzed by the computer and, say, the 50% sharpest are selected, These are then stacked by the program, i.e. shifted sideways in one or two dimensions, to produce optimally sharp images. Further, the image is treated with wavelet transforms, and various sw magic to enhance contrast etc.
Single shots are likely to be exactly as you describe.
Try googling Lucky Imaging on youtube and see how it is done.14. juni 2021 kl. 20:54 #327430
Hi Torben, thanks a lot for your feedback. I have no doubt that there must be “split opinions” about this telescope, but this is the only one I have 🙂
I have tried to do the “star test” on Arcturus and Vega (which are quite high in the sky at 0-2 a.m. this time of year, especially Vega). The diffraction patterns on the two sides of the focus are symmetrical and do look roughly the same, but I don’t think I have enough experience to judge the quality of these patterns properly. I am waiting for an eyepiece adapter to take photos through the eyepiece, so that I can take photos of those diffraction patterns and then show them to others.14. juni 2021 kl. 21:12 #327431
Thanks for your comments. I know about the “lucky imaging” technique, but I am not there yet. My main problem for now is that I don’t like the quality of the picture even when I do visual observations of bright objects (i.e. Jupiter) 🙁
My mount is definitely wobbly, and I am aware that the telescope needs to reach a thermal equilibrium with the environment, but these factors do not seem to be the cause of the problem – the picture is blurry even after the telescope has been outside for >30 min and after all the visible wobbling in the field of view has gone away.
Could you please elaborate a bit on point 6 (the camera slightly out of focus)? What do you mean by that? Note that when I use the camera, I connect it to the telescope without the lens (through a T2 adapter ring), so the the image is formed on the matrix by the telescope’s optics.15. juni 2021 kl. 08:41 #327432
Hi again Vladimir
For visual observations, you should be able to get reasonably sharp images. In this case I have to agree with Torben. Try focusing on one of the brighter stars high in the sky. That should give a quite sharp image, with the diffraction pattern symmetrical on either side of the focus point. Being a relatively low-cost telescope, you should, however, expect that color fringing could occur, since the achromatic lenses are unable to focus red and blue light in the same point. And again, if you try Jupiter and Saturn low in the horison, don’t expect perfect images for the reasons discussed above.
For photographic imaging, my point about focusing was just that it can be really hard to focus a DSLR precisely (I speak from experience). In this situation, normally one would either look through the viewfinder of the camera and try to focus, or if the DSLR have “live view” on its screen, zoom in on a star and focus until it becomes a sharp as possible. The latter is recommended, because the zoom effect makes it easier to focus compared to just looking through the viewfinder. To focus a camera on a telescope, I normally use a Bahtinov mask, which makes it easier to get optimal focus.
So – welcome to the wonderful world of astronomy and astrophotography! You have just begun on a long journey of ever bigger scopes, of ever sturdier mounts, of dedicated astro-cameras, guide telescopes, lot & lots of software and countless long nights in order to produce those wonderful images one sees online 🙂15. juni 2021 kl. 15:13 #327433
Her er et link til en anden side, hvor der er en rigtig god video der forklarer meget: https://www.astronet.dk/forum/viewtopic.php?f=24&t=4079&sid=219045cc396f5e79edf31072918d5d22&p=30053#p30053
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