Star Test to Fix Your Telescope
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- Опубліковано 30 вер 2024
- Improve your telescope! A star test is easy to perform and can tell you so much more than just your telescopes optical alignment. In this video we discuss some easy to spot optical problems, some of which are easily resolved.
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Dude!! Thanks for this video!!
I've been looking for a good introduction to star testing & interpretation. This is basically the only clip I've came across that showed me what I was looking for.
Man, you the man, man!!!
No problem :)
You're a wealth of information mate!! Superb work as always, looking forwards to the next one! 🙂Clear skies!
Very kind Luke! Cheers mate :) Everyday's a school day for all of us hey :) Catch you soon fella!
It is possible to correct spherical aberration with a lens, however the correction is a bit trick to do since the perfect one is a aspherical lenses.
Still, using a spherical one is possible to greatly reduce it, just very tricky to place in the optical train.
Just remember Polaris is a double. If you see a small dot of light near the star this is the small partner star.
Have you thought about doing a short video on the Sky-Watcher Evostar 120 achromatic refractor, but on imaging Mars? 🤔
I'm sure I can do that at some point but next up I'm reviewing a new smart telescope.
Too much talking, not enough showing...illustrate the discussion to make it meaningful.
Why do primary mirror clamps hook over the mirror??? Surely knotches or luggs or an external ring behind / outside of the mirror could provide exactly the same holding capabilities without impingement into the mirror area???
Thanks Chris for the excellent explanation on star testing in this tutorial, please keep sharing your expertise mate
Will do Anthony, there's plenty more in the pipeline! Cheers for watching and for your kind comments :)
Informative, straight forward and competent. Sub earned. Channel recomended!
Thank you!
"fre-nel" rings
The "s" is silent.
I can only use my telescope at weekends due to work and it`s always cloudy at weekends. So I thought I would get a baader solar filter. Guess what. The suns gone now for weeks. :(
Let me guess, night shifts at work right? I did 20 years of those in the NHS and it always seemed to be crystal clear when I was on nights, and cloudy on my nights off? Either my perception of things or sods law, I'm not sure which lol
@@AstrolavistaPermanent Late shifts. Finish at 11pm and then everyone is asleep. Night shift would be even worse lol. I would love to work days and be out every night. :)
One type of astigmatism comes from secondary mirror being deformed especially if tape or adhesive is inducing flex in it.
Yes! you can diagnose the secondary mirror by ruling out the primary. Take note of the star shape on the star test, then rotate the primary. If the star shape follows it's the primary mirror, and if not it's the secondary mirror.
I recall liking quite a few of your other videos, which were exceptional. This one is my favorite.
Wow, thanks!
Forgive me if this question is silly. If the secondary obstruction is in the center, how does a high power eyepiece work if the image plane is being examined in the middle where this shadow is? Are we just getting enough light around it to work? I find it hard to find any info on this topic.
thanks
Thanks for sharing Chris! I recently sold my 8" F4 newt because I just wasn't using it. The guy I sold it to did some star tests like the ones you described and told me the telescope had one of the best mirrors he'd seen. Now I'm sad I sold it :-D
Ah mate, he could have soften the blow by not telling you how good the mirror is, that's just mean lol All I can say is if you weren't using it you might as well have the money for something that you will use :) I should have posted this video a while ago by the sounds of it! :D
Man, this was very helpful, I’ve been struggling with my first Newtonian scope to get it sharp but the truth is I don’t know what are the limits of sharpeness I should be expecting, because I only see processed images that has a lot of software sharpening involved… so I always think my images are just not sharp, collimation is good, I think there is a bit of astigmatism, not much from what I can see, the center shifts a little from in and out focus, I corrected pinched optics, but instill see a little oval D shaped stars depending on the subs… very rarely I get really round stars… I think it is now more like coma, but since it’s not super sharp it’s hard to tell… I’ve heard from different people their Baader coma corrector also don’t correct coma as much as expected on 6” f4 newts… I would love to see a video that you go through some raw images and explain the possible defects on the optics, I could provide you some images if you’re interested, I have some of the pinched optics images and the current ones that I frankly don’t know if they are too bad or acceptable… I think they have a ton of issues but I don’t really know because of the lack of experience
I'm really glad it helped Andre! I currently use the Baader MPCC with a 6" f/4 Newt plus ZWO ASI585mc camera and the coma corrector does the job with a small sensors. I need to grab another old DLSR to remind myself how well it works with larger sensors though. One thing to note is that f/4 Newts are not as sharp as f/5 Newts. I've owned a number of both and can see the difference. I think there are a few things that contribute towards this - steeply curved f/4 mirrors are harder to grind and polish to a high accurately, f/4's have a larger secondary mirror increasing light scatter, the focal plane is finer at f/4 making it dip in and out of focus more easily, and finally the sweet spot for collimation is a lot smaller so even when a tiny bit out it can effect sharpness. Now this would make a controversial video topic, but I've seen it time and again so I stand by it. The sharpest Newtonian I've ever owned was the Sky-Watcher 150pl f/8. That thing was razor sharp! Thanks for the video idea, it's always good to hear what people would like a video about. I'm a little swamped to add any more videos at the moment though, I've borrowed two beginner refractors to compare and review, and Vaonis are sending me a Vespera smart scope to review next week. Plus I still have Dwarflabs Dwarf II smart scope to finish up with. Woe is me hey lol I'm not complaining, it's a lot of fun but it can be tricky finding the work/family/UA-cam life balance. Thanks for watching and try not to beat yourself up about the sharpness, especially with an f/4! :)
I am beginner to this hobby. Thank you so much for taking the time to explain this. It was a really comprehensive/well done video
Thanks!
New here. Love the detailed explanations. In Switzerland we have clouds for weeks too :(
Hi Chris and thanks for your informative videos. I've been seeing online commentary about the Skywatcher 200PDS - that it's very difficult to collimate and almost impossible regulate for astrophotography; this to the point that several people gave up and returned their instruments. Is there a solid basis for this or is it a case of finicky people not being able to deal with a slightly finicky instrument? I ask because there is one for sale in the area for what seems a reasonable price and I'm tempted.
Hi Pat, I've not noticed any issues with the Sky-Watcher Newtonian's I've owned over the years, and I've had quite a few! E.g. 130p, 130pds, 150p, 150pds, 200p (still have one of those), and a 200pds. Sky-Watcher make slightly scruffy telescopes (they can arrive with the odd cosmetic imperfection even when new), but the optics are very good for the price, especially second hand. I don't think you can go too far wrong. I've not had issues with collimating mine. Do you know how much experience these folks had with collimating telescopes? Newtonians are definitely well suited to the mechanically minded, who like to fiddle, tweak and mod their scopes, and refractors are there for those that just want a telescope that works out the box.
A video about how you may do this test on cheap store-bought fast achromat, (i am a sinner) of the f5.7 70mm types! Or similar! And for chrissake an explanation of what an airy disc is! And like Dezel Washington says, "Explain it to me like I'm a 3-year old" or something...well, mayne not so much goo-goo -gaa-gaa....but perhaps dumbed down!
Not to worry, Its the same test for any scope only you shouldn't need to worry about collimation with a refractor as they should stay collimated. An airy disk explained to a 3 year old (or Denzel lol) - When light from a star goes through your telescope it shows itself as a disk with a series of rings, this is called the airy disk.
It helps to use a green eyepiece filter when doing a star test with either achromats or fast ED refractors, this is because it just focuses on the green spectrum of light which we are most sensitive and where they mostly focus on when making these scopes.
Fyi, you can have pinched optics on a refractor too.
I must finish pimping up my newt, handy things to know about the optics though so I'll look out for any next run I get with it. A quick tip about the mirror cell clips is that I use a piece of paper like a feeler guage between the clip and the mirror when tightening the screws, that way the cell isn't overtightened if that makes sense. Great video as always mate clear skies.
Hey Tich, snap! I used the paper trick in my mirror cleaning video and it worked a treat so I second that advice :) Some worry about scratching the mirror however papers fairly soft and Silicon Oxide coatings on mirrors are relatively durable now days. Thanks for watching, cheers : )
Thanks for your explanation. I live in Brazil, I dont speak well, but let´s go. I have an Schmidt-Cassegrain Telescope LX90 Meade 12” (305mm), Auto Coma Free (ACF). I am encountering some difficulties while collimating the device. I can't collimate the telescope so that both the intra-focus and extra-focus are perfect (concentric circles). If one is perfect, the other is imperfect, and vice versa. Theoretically, the image (centered circles) from intra-focus and extra-focus should be the same. If one is correct the other should equally correct. But, In my case it is not. Do you know something that I could do? I don´t know if you understand my explanation. When the concentric circles are almost perfect in extrafocus, the intra focus are not perfect symetric.
It´s totally strange this result. I know that what I am saying is counterintuitive, but it is happening to me. Imagine that I'm looking at a star that is in focus. Now, I move the focus knob in a way that the primary mirror moves downwards and forms the image with concentric circles, and they are perfect. Now, imagine the star in focus again, and this time I move the mirror upwards, and the circles in this condition are no longer perfect concentric. How is it possible for this to happen, if the configuration is the same? If you have any tips, I'll be grateful.
Hi, you write very well :) Could this issue be caused by 'mirror-flop' (the primary mirror tilting whilst focusing). For example, if you collimate the telescope then adjust the focuser, the mirror can tilt slightly as it moves up or down the tube affecting the alignment between the primary and secondary mirror. I know mirror-flop can be an issue especially with the larger Schmidt-Cassegrain's and Maksutov telescopes, but collimation shouldn't go fully out.
If I'm right about this, locking the primary mirror in place and adding a SCT Crayford focuser or similar to the rear would solve it. Maybe pose the problem on Stargazers Lounge and Cloudy Nights forums for a second, third, fourth opinion to be sure before opting for locking the mirror and the additional focuser. I hope you solve the issue and clear skies!
@@Astrolavista I never thought of it that way, your idea is veryn good. However, the image never changes when I change the movement of the mirror. Suppose I have moved the mirror upwards and then start to lower the mirror, the concentric circles do not change because I change the movement and vice versa. By your logic, the primary mirror would suffer a slight misalignment as the movement occurs.
The fact is that when the circles are perfectly concentric at the extra focal, they will not be perfectly concentric at the intra focal point and vice versa. Throughout the extent of the extra focal point, the circles will remain the same regardless of whether the mirror moves up or down and vice versa. But thank you very much. I really apreciate your help. If you have any other tips ... Supose that the primary mirror would have a fixed misalignment, can it cause this? It seems to me that I can adjust the collimation to an intermideate point where the circles are not perfect concentric in the intra focus and the extra focus. It seems to me that´s the only way I can set the mirror to have a match between the intra and extra focus.
Very Interesting Chris! In My telescope the Donut star looks Pretty round, no problem! I'm Curious What would this be Like for Schmidt Cassegrains, Especially ones with Bigger apertures!
That's good to hear Vineeth : ) I've owned a few SCT's in my time, (up to 8" at least), and star test is similar to the Newtonian only there are no diffraction spikes present breaking up the rings, you just get the central obstruction shadow with surrounding complete Fresnel rings.
@@Astrolavista Lol, I'm avanteesh! I guess It's Urban Astronomer :), very cool to know btw. Modern SCTs are really good At Correction and Star patterns!
Thanks for the video.
Your definition of diffraction limited is a bit off.
All the light will never fall within the airy disk.
Struhl 0.98-0.99 probably doesn’t exist, if it is measured separately for R, G and B.
I believe optics are generally designed to perform at their best in the green to match the peak sensitivity of the human eye. They tend to measure in the green, and I agree high Strehl ratios of 0.98 might be too much to ask if measured across multiple wavelengths. It does seem to stand to reason.
@@Astrolavista High end companies, such as Tak, TEC, AP, measure Strehl in RGB. The best reported are for TAK TOA 130 and 150. TEC is not optimal in B.
Interestingly, 11” Edge still beats TAK TOA 130 in resolution. There are top notch comparison by Gary Imm. It tells how important aperture is.
If we go little bit up and put in mix PlaneWave 14-20” CDK, then it will be another level.
Thank you. 👍🏻
A star? Forgotten what they look like at this stage!
It's coming home
I must admit it's been a while here too! lots of clouds!
Excellent video, this may explain my problem, been fighting with my new 8” Orion, finding focus has been brutal. Just can’t focus at all with the same make coma corrector?
I had my 150mm refigured to .986 Strehl and 1/12th wave and the 96% reflectivity I ended up buying a new focuser. My $125 pawn shop C6-N (came with a Tele Vue 2x Barlow) ended up costing between $700-800. But I have the best 150mm f5 Newtonian in my club.🤣
Very useful information, thank you my friend. I've never heard anyone talk about this before. Thanks
My god everytime I look at something except the moon it looks like th white dots with the back likes HELP
Thanks for spreading good information. Hasta-la-Pasta Chef!
Thanks mate! And if I ever have a cooking channel I know what to call it :D
We could make one, I can cook Italian style 🍝
Awesome. I actually figured out some of these techniques but your tip to loosen the mirror clamps helped.
This is good content thanks for making it. I tried to do this with an artificial star using a torch and it nearly worked. I think I will repeat the test with a real star. I did find out I had an eyelash hair on the eyepiece so it wasn’t a complete waste of time!
Thanks for the info on under/over correction. Now I know how to test for that.
After watching quite a few vids, I don't understand what you need to do to see the dark round circle of the secondary mirror?? When I add an eye piece, it just shows me the star or object. How do I get the view of the mirror shadow?? Thanks.
Are you not able to de focus a bright star? If you de focus a bright star in a telescope with a secondary mirror, this should reveal the secondary mirror shadow.
Im receiving a doghnut shape through my newtonian, bang on in the centre but the image remains out of focus regardless of my focus settings
Hi Mark, if you are using an eyepiece you may need an extension tube to reach focus. If you are using a DLSR/Mirrors camera you may not have enough inwards focus travel available depending on which Newtonian telescope you have.
Any chance you can do some tips on using a Newtonian with a equitorial mount? I tried mine together and found the eyepeice ended up all over the shop and it was all round a bit of a mare.
It depends on the size of the Newtonian but for anything up to 6" I find it good to have the focuser inline but pointing away from the counterweight bar. In this position you should be able to reach the focuser either side of the meridian. Have your tripod set low to help with reaching. For larger Newtonians E.g. my 8" the focuser is starting to get a bit too high off the ground and I need to move the tube in the rings sometimes.
First thing I noted: Fresnel is French - pronounced Freynel.
I've been saying it wrong for years lol Someone let me know in the comments the other day, cheers!
Seems I see just about all the manufacturers claiming 0.95 strehl values now...
Are they, any manufacturers in particular Rat?
@@Astrolavista Let me restate it... seems any that will give you the strehl value never are below .95 in their claim.
@@tubedude54 if they claim it, they should offer certificate of the test.
Neat.
Thanks 👍
Keep making these types of videos, very nice work. You really need to look into "promosm"!!