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Skywatcher 10" collapsable dobsonian, poor image quality


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I have a 10" dobsonian from Skywatcher, but im not happy with the image quality. I am able to focus "passed" max-focus(so its not that the truss tubes arent extended all the way), but im not getting crisp focus.

Yesterday i was looking at Jupiter again, it was a clear night but i just saw jupiter as a bright blob, whit no bands or colors what so ever. I have tried this several different nights, with different eye-pieces, but cant get any detail out of the planets. I compared to my spotting scope(Vortex Razor HD 20-60x85), and got a much better image. Stars arent so easy to focus on, i cant get them to become just a spot without blurring out a bit.

I have collimated with a cheap laser collmiator, and its spot on(it depends a bit on how you fasten the thing, its a bit wobbly), and when i do the out of focus star, i think im seeing the mirror in the middle of the blurr.

Is my scope poorly collimated, or is there something wrong with it?

Edited by Aston
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The first thing that I would do is to check the collimation using a Cheshire. I'm not really a fan of lasers and generally speaking think a Cheshire will give an more reliable result. 

When it comes to poor images most problems are usually due to atmospheric thermals. The first and most important are those in the scope itself so make sure that you give it a good amounting time to cool. I would suggest at least an hour as you are trying to assess the image quality. 

Secondly, be careful about what you are looking over when you observe. Man made structures (buildings, roads) tend to hold and release heat much later into the night than fields and forests. If possible try to avoid observing over houses. Even waiting for an object to pass over the gap between two houses can have a dramatic difference if you're in an urban location. 

The amount of atmosphere you look through will also have a big effect, and larger telescopes suffer more from this than smaller ones. Observing an object when it is at its highest will minimise the amount of atmosphere you are looking through and hopefully improve the views. 

Beyond that, I would advise trying to contact your local astronomy group and see if there is any possibility that one of their experienced members could have a look at your scope and give their opinion. 

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Cheap lasers often need collimating themselves before using. There are threads here on SGL that explain how to do this.  It’s very much you get what you pay for with lasers. The Hotech laser is great and very accurate but not cheap.

As mentioned above a Cheshire eyepiece will be much better than a cheap laser.

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Have a look at http://www.stark-labs.com/craig/llcc/llcc.html for info on how to collimate the laser, it's hit and miss with the cheaper ones, sometimes they are spot on, other times, not so much...

Sometime the collimation screws are "hidden", so you may need to dig out the plastic to get at the screws

All the points raised above are valid though, letting the scope cool is very important, especially with larger newts, my 8 inch lives in a shed and still needs around half an hour cooling down before the views improve

Star testing after collimation is also a good idea, more info here https://www.skyatnightmagazine.com/advice/how-to-star-test-a-telescope/, Polaris is probably best to use for a dob as it's easier to keep it centered

 

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Thanx for the help, i will have a look at the links and try to change my viewing spot to somewhere with less housing.

I took a picture down the eyepiece holder, is it possible to say from that weather the scope is collimated correctly or not?

 

bild.jpg

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32 minutes ago, Aston said:

I took a picture down the eyepiece holder, is it possible to say from that weather the scope is collimated correctly or not?

 

Its difficult to assess as you really need to take the photo through a Cheshire or collimation cap to make sure the camera is properly centred, however I would say that the photo you have posted does look out. 

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I used my 3d-printer to print a collimation cap, that was a good fit. I took an image through it, and its a bit off. The white center circle on the main mirror is ofset a bit, and one of the screws can be seen at 5 o'clock on the secondary mirror. But since i havent got any experience, i have no idea if this is too much, or if it is not noticeble.

img2.jpg

img1.jpg

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After some tweeking with the secondary mirror i managed to get the dot on the main mirror to become more centered. The secondary mirror looks to be a bit off center(you can see the mounting screw) but i dont know if it affects viewing.

img3.jpg

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Waiting for the optics in the telescope to cool down is probably a good idea, i used the scope without any cooling down time. I did notice that when i used the "out of focus star" trick to look at the colimation, there was a boiling motion in the out of focus star, that looked like hot air turbulence. 

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I got a Cheshire tube, and i think i got it right, the cross is fixed at the spot on the main mirror, and everything is centered in the scope. Does it look OK?

 

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1 hour ago, Aston said:

I got a Cheshire tube, and i think i got it right, the cross is fixed at the spot on the main mirror, and everything is centered in the scope. Does it look OK?

 

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Is the camera not centred over the Cheshire properly? In the photo everything is offset from the cross hairs. If the cross hairs line up when you look down the Cheshire with your eye then the collimation might be ok. 

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That is all looking much better.
Go outside and use the scope. See just how well it performs.
Sometimes you can chase perfect collimation indefinitely and never use the scope🤨
If it is good enough for the way the sky seeing is tonight, it is good enough.

Having said that. I assume this scope has never given you good views?
If you don't get good collimation after trying nd trying again, ask at a local astro club for someone to take a look.
Collimation is about the mirrors pointing the right way and being reasonably centred in the tube.
But if the focus tube is off axis, the main mirror is off centre, or there other alignment isues, you can go round in circles.
In this case a more experienced person can save you hours of frustration by identifying other problems. It happened with my first newt.

Keep asking the questions,

David.

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Just one more thing to add. I owned a flextube 250 for a couple of years. I checked collimation after removing the mirror for cleaning. It was very near.
It was used by me and loaned out a club. Where it was moved on a trolley over doorsteps and rough paths.
Every time I looked at it, collimation was still good.
Once you get it straightened out, you will have very little to do.

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Seeing wasnt the best today, but i got out and took a look at Jupiter and was pleasently surprised, i could clearly see the color bands, and getting into focus was easy and distinct in comparison to earlier!

I dont know if it was the collimation or the fact that  let the telescope cool down for two hours, but im very happy with the results! Thanx for the help everyone :)

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The photos indicate you have really improved the alignment of your secondary. As for the primary, I wouldn’t trust a photo so let your eyes be the judge. My tool of choice is a Cheshire sight tube combination tool from Celestron.

Edited by Spile
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