my new (old) skywatcher 250 px dob

[lunartic]

Hi all, just acquired a 2nd hand skyliner 250px dob, which I am very excited about getting to grips with. Although I have been reading quite a bit I am very new to the practice of observing. The scope didn't come with all the original kit and had a red dot finder, a 'sky's the limit branded 15 mm 2 inch EP, plus a 2 inch Barlow that won't fit all the way into the focuser tube and so I cannot use it. During my first times using the scope I noticed that the view of the stars is ok (but not perfectly sharp) and I don't get the whole field in focus at once, and when I roll the focus back and forth slowly, the focus seems to go from one side of the image to the other. Could the scope require some alignment?

In terms of first purchases I will be looking to get one relatively higher power EP and one lower power, to use with my one inch adapter. Probably look to spend between 40 and 60 per EP. ☺

Any comments and or advice much appreciated.

Thanks

Andy

[pipnina]

Hi all, just acquired a 2nd hand skyliner 250px dob, which I am very excited about getting to grips with. Although I have been reading quite a bit I am very new to the practice of observing. The scope didn't come with all the original kit and had a red dot finder, a 'sky's the limit branded 15 mm 2 inch EP, plus a 2 inch Barlow that won't fit all the way into the focuser tube and so I cannot use it. During my first times using the scope I noticed that the view of the stars is ok (but not perfectly sharp) and I don't get the whole field in focus at once, and when I roll the focus back and forth slowly, the focus seems to go from one side of the image to the other. Could the scope require some alignment?

In terms of first purchases I will be looking to get one relatively higher power EP and one lower power, to use with my one inch adapter. Probably look to spend between 40 and 60 per EP. ☺

Any comments and or advice much appreciated.

Thanks

Andy

Perhaps some images of the focusser with and without the eyepiece? It sounds like the secondary mirror or focusser is not aligned properly but images would help.

[Putaendo Patrick]

In general, 2 inch eyepieces are best for wide views at low magnification (20 to 40mm Eps). For higher magnifications, 1.25 inch EPs work well and are significantly cheaper. If you don't have the 2 to 1.25 converter, you should probably try to get one.

I don't understand why the 2 " Barlow won't fit. Maybe you could post a photo?

Reflector telescopes often do need periodic collimation (some more than others!) to get the mirrors perfectly alligned. Getting a Cheshire collimation EP will enable you to do this, and I suspect improve what you see very dramatically.

Good luck!

[Piero]

Welcome to SGL and congratulations for your new telescope!

For working properly, a telescope needs to be collimated and cooled down. 

A ) For collimation, check these two guides: 

1. http://stargazerslounge.com/topic/197640-collimation-and-star-hopping/

2. http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

If you don't want to use a bottle cap, you can buy a relatively cheap Cheshire collimator which works very well. 

B ) Cooling time. At least 30minutes before using medium / high magnifications (e.g. for planets, moon, double stars), but this can be longer if the difference in temperature between your storing and observing locations is large. If you use low power eyepieces, you can start observing earlier instead.

Of course, this assumes the seeing is good. If this isn't the views can look like as if the telescope is not properly collimated (see astrobaby end document).

At F4.7 (your telescope focal ratio), there is some demand in terms of eyepiece quality. Use the stock ones for now and try to learn how to collimate your telescope first, have confidence with your telescope and the sky. Choosing the right eyepieces is not easy. You need some reading and time.

Enjoy! 

Best wishes, Piero

[John]

Hello and welcome to SGL

Congratulations on the new-to-you scope - they are very good !

The focal ratio of the scope is F/4.7 (focal length of scope divided by main mirror diameter = focal ratio) which will show some distortions in the outer half of the field of view with relatively low cost wide field eyepieces. If the Skys the Limit 15mm 2" eyepiece is the one I think it is, it has an 80 degree field of view which is considered ultra-wide. They are also low cost (for an ultra wide) so the reality is that the field of view that they present will not be well corrected right across by any means.

I'd be blaming the eyepiece in this instance, not the scope.

If you invest in some better corrected eyepieces such as the BST Explorers or the Maxvision 68 degrees, you should get crisper focus across more of the field of view. The outer parts will still show a little distortion though. Some of that will be due to a thing called coma which the relatively fast newtonian optics produce and some will be astigmatism which will be present, albeit to a lesser extent, in the 60 and 68 degree BST and Maxvision eyepieces.

Getting a field of view which is wide and sharp all the way across in an F/4.7 newtonian can require some quite expensive eyepieces and a coma corrector.

[lunartic]

Thanks guys. Indeed I wasn't sure what the culprit might be - maybe could be both EP and a bit of alignment but a better quality EP I thought could help narrow it down a bit ☺ wish me luck - weather is awful!

In terms of the barlow not fitting I believe there is limit to how far things will go into the focuser tube; I read a similar account somewhere on the site but can't recall where...

[lunartic]

In general, 2 inch eyepieces are best for wide views at low magnification (20 to 40mm Eps). For higher magnifications, 1.25 inch EPs work well and are significantly cheaper. If you don't have the 2 to 1.25 converter, you should probably try to get one.

I don't understand why the 2 " Barlow won't fit. Maybe you could post a photo?

Reflector telescopes often do need periodic collimation (some more than others!) to get the mirrors perfectly alligned. Getting a Cheshire collimation EP will enable you to do this, and I suspect improve what you see very dramatically.

Good luck!