scope, eyepiece or collimation?

[tyler_jrb]

hey, just had my new scope out lastnight skywatcher 150p dobsonian and was looking at stars through my super eye releif 25mm eyepiece at various stars, clusters etc. All seemed very well perfect pin points of light when focused and I was blown away by the views. doing a star test also with the 25mm(supplied with scope) seemed well a circle of light with the secondary centred in the middle. needed to zero in finderscope and so did by looking at what I think was arctus. managed to get it centred and looked great a nice pinpoint of light with the 25mm(48x). Thought what it would look like with the 10mm(120x) supplied with the scope but the star was not a pinpoint when doing the star test with the 10mm again all was well circle with secondry centred in the middle.but upon focus the star was like a pinpoing with all multicoloured bits around the star:icon_scratch:(it wasnt other stars around it). but then looking at other dimmer stars it seemed ok. this is what star test looked like

.

but when focused it didnt look like this.

why is it like this is it because the mirror of the scope is dodgy which i really hope it isnt:icon_eek:. or is it something simple like the eyepiece being faulty or the scope not being collimated properly. I am also wondering if it because of turbulence in the air it was quite warm around 13 degrees c. I really hope it isnt the mirrors being poorly done and hope its something simple like collimation or eyepiece or even turbulence. Any help appreciated.

[brantuk]

The eyepieces aren't par focal so you will need to refocus when you change them - this is quite normal and what you are seeing before focusing is correct.

To check collimation, get sharp focus on a star, then take it just slightly out of focus by turning the knob one way. You should see perfectly concentric circles. Then defocus slightly the other way and you should see the same. This is called a "star test".

If the circles are all concentric both ways, then the collimation is good

[tyler_jrb]

hey, but when i focus the star with the 10mm it has like other multicoloured points of light around the star, its strange. I will give it a proper star test when im next out. i dont know why it almost wont focus properly its not with like the 25mm(pinpoints of light). with the 10mm when focused it has little multicoloured lights around the star when focused. is this normal an why does it happen? thankyou

[brantuk]

The eyepieces supplied aren't the best, and indeed the 25mm is usually better than the 10mm. The multicolours you see demonstrate this because the lens is refracting the light rays and colours all over the place rather than focusing them all together sharply in the focal plane.

This is the difference between a good lens and a not so good one. If you look at the moon you'll see a yellow'ish tinge round the limb (edge). Google "Chromatic Abberation" (or CA) for a full description.

Hope that helps

[tyler_jrb]

hey again brantuk I thought chromatic abberation only happened in refracting telescopes.my telescope is a newtonian reflector telescope skywatcher 150p dobsonian. Also if it does occur in reflector telescopes is it something wrong with my scope or just the eyepiece? if so can you reccomend me a eyepiece around 9mm. up to 30 pounds.

[brantuk]

Refraction, and CA, happens to a greater or lesser degree in all glass. It all depends on what that glass is designed to do. Better eyepieces use two or more lenses together to correct the colours and how they focus the light. The 10mm supplied ep's imho are quite poor and the 25mm only marginally better.

In fact the best place to see refraction is in droplets of water when the suns light is scattered in all colours to form a rainbow. But your mirrors sound fine if you are seeing concentric circles in a star test

(As you rightly say - colour correction is a big issue in refracting telescopes)

[brantuk]

I use Televue 5mm, 8mm, 16mm and Hyperion 8mm eyepieces plus a Hyperion Zoom lens. But they're a lot more than £30. Under £50 I undestand the TMB eyepieces are good vfm but I've never looked through one.

Meade, Skywatcher and Celestron do some nice eyepieces one or two steps up from the supplied ones in the £35-£80 range. And be careful when choosing a barlow - Tal 2x at £35'ish is good vfm. I allways advise against the cheaper eyepiece kits.

I'd recommend you join an astro soc or obs group and try a few loaners to learn about what suits you, so you can decide what to get (it's very much a personal choice thing with ep's)

[tyler_jrb]

hey thanks for the advice brantuk what do you think to the skywatcher super plossl's. i was going to buy a 10mm seeing as they are relatively cheap at 20 pounds but it says focal ratio of f/6 or slower. My scope is f8 will they not work well in my scope? I will also look at the televue and a tal x2 which will buy in the next few weeks as the 10mm included is very poor . Glad to know it's not my scope was getting worried there but the 25mm is much better than the 10mm but will proberly invest in some high quality 9mm and another 25 or 32mm. I am willing to go for around 20-35 pounds for a decent eyepiece then i will proberly go for the tal x2 or celestron omni x2. for around the same price. The links below can you tell me which one you think is of better quality. the omni i want is 9mm or the skywatcher 10mm below.

What do you think to these eyepieces:-

First Light Optics - Celestron Omni Plossl Eyepiece

First Light Optics - Skywatcher SP Plossl eyepieces

Barlow here:-again which do you think is better celestron omni x2 or tal x2?

http://www.amazon.co.uk/Celestron-Omni-X2-Barlow-Lens/dp/B00008Y0TM

I am must also say that the video included earlier looks like my scope when i did the star test and un-focused. is this ok?. still havent been able to see saturn yet but i have been able to see what i think is the m102 galaxy and a few open clusters which was very nice too look at. when i get some better eyepieces should hopefully have crisp clear view at high magnification of stars etc but the 25mm so far is fine, its just the poor 10mm for some reason :/. Again thanks alot for the huge amount of helpful advice since I joined SGL.

[RikM]

Get a Cheshire eyepiece to do your collimation properly.

Defocussing on any old star and seeing a white circle with the dark circle sort-of in the middle is NOT a star test.

A real star test is only possible when the atmospheric seeing is good. You use a very high magnification (250x) on a not too bright star (Polaris is the normal choice) and the star needs to be in the exact centre of the eyepiece. You are looking for a series of fine concentric rings. This is the diffraction pattern.

Arcturus is not a good choice for star testing. It is too bright and too low at the moment so you are certain to get issues with seeing and will have a tough time keeping it centred in the eyepiece.

[brantuk]

Ok - first your scope - it's f/8 and 1200mm focal length. f7 is usually regarded as a "mid range" focal ratio - anything lower (f6, f5, etc) are "fast" scopes, higher focal ratios (f8, f9, f10, etc) are "slow" scopes. This relates mostly to imaging and camera film speeds (as in the old 35mm format film).

In relation to ep choices it's useful to remember that "slower" scopes (higher focal ratios) are more forgiving of poorer quality ep's and "faster" scopes less so, favouring better quality ep's.

Due to UK seeing conditions, moments of excellent seeing are few and far between and higher magnifications of 300x plus are rarely possible. So picking ep's you're best advised to aim at a maximum of 200x to 250x - most folks operate around the 200x mark.

To calculate f-ratio = focal length / aperture (1200/150=f8 for your scope)

To calculate magnification = focal length / eyepiece size (1200/5mm ep= 240x mag for a 5mm ep in your scope)

Remember also a 2x Barlow doubles the magnification (eg a 5mm ep and 2x barlow will yield 480x mag - far too much). At that magnification you'll have trouble tracking an object and all you'll magnify is the atmospheric imperfections making the view very dim and grainy. More to follow....

[tbird]

Hello Tyler, I see from your signature you're from North Lincs, the North Lincs Astro Group meet at Far Ings, Barton on Humber on the 1st Monday of the month, the next being 1st August. http://stargazerslounge.com/astro-events-local-meetings/127268-north-lincolnshire-astronomy-society.html

Everyone welcome, a couple of us use SW 150p's and have a variety of EP's you can have a look through.

[PETENAUD]

Can only comment on the SW EP

I have 2 SW scopes and the 25mm is ok on both, but the 10mm is not so.

I use the 25mm alot, but dont use the 10s at all.

[brantuk]

I had a look at the 3 ep's you linked above. The nearest I come to any of them is my Celestron Ultima barlow wich is very good and popular amongst a lot of folks. You can get them s/h in the for sale section for around £45'ish in great condition but I don't know the Omni series at all.

The SP's might be ok but I'd personally prefer the WO Super Planetarys. It's hard to advise on your links with only the paper spec to look at. And it is very much a personal thing - they might possibly all be ok for you.

The best thing to do is take up Tbird's very nice offer next time they all meet up. Obs groups are where I learned most about eyepieces - that and I also read this primer:

http://stargazerslounge.com/primers-tutorials/63184-primer-understanding-choosing-eyepieces.html

One other thing - I only buy s/h ep's now - saves a lot of money and regular astronomers look after their kit so you usually get very good to excellent condition

[tyler_jrb]

hey again brantuk. would be nice as you say to get some s/h eyepieces but i dont know of any reliable websites where I can buy them from. I havent been on for a month so i cant access the for sale section on flo. as for the barlow yes I will try to get hold of the ultima ive also heard some good reviews for the omni series but just thought id ask if you have used them and what you thought of them for the price in the post before. I have also read the primer link which has helped me. thanks again.

[tyler_jrb]

hey tbird thanks for the offer and will very much try to attend if i can be taken that is as i am only 16 years of age. would be nice to test out some of the ep's.

[tyler_jrb]

hey when you look at a star through the 10mm does it have the same effect as mine?

[tbird]

TBH I don't use the Skywatcher EP's anymore but when I first got my scope the 10mm seemed OK if not great. Mine is the 750mm focal length so the magnification is 75x whereas it'll be 120x in your longer scope.

[tyler_jrb]

do you have any clue why it is like that when using the 10mm on a star, is it the eyepiece or the scope? its like a multicoloured star with what looks like other stray scattered light around the edge of the star(close to the star). but then when using the 25mm seems fine and pinpoint. do you know why?

[RikM]

It's a combination of seeing, star brightness and mis-collimation. These show up more in the 10mm because of higher magnification.

[tyler_jrb]

ahh ok.