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DSO with 150P Dob


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About a week ago I found M81/82, I was over the moon to say the least, been looking for them for ages. This was my first galaxy sighting and has given me a taste for finding a few more? I was going to get a 25mm BST to replace the one which came with the scope but reading through some of the posts about 8" dobs people are saying that the 18mm is better? I've only got the 6" but does the same apply? Which one should I go for?

Thanks in advance

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What eyepieces do you have at the moment? :-)

Do you have Stellarium and your telescope and eyepiece set up in it?

The most important when buying eyepieces is the aperture ratio. I suppose you have a 6" newton, 150mm aperture @1200mm focal length (fl : aperture = f/8), and it's not as critical as 8", 200mm@1200mm (f/6) or even f/5 telescopes.

With f/8 you can easily use a inexpensive wide angle (66 degree apparent field of view) for 32€/27gbp, they perform well on telescopes with f/6 and up. Below it depends on your wallet and personal preference.

The downside is that they are only available in 6, 9, 15 and 20mm, so you have to figure out what magnification, exit pupil and real field you require, depending on your current eyepieces and sky conditions.

Usually it is nice to have a eyepiece for 40-30x magnification, one around 60-80x, 100-150x, and perhaps 200x and more for planets (depending on conditions/seeing). More can be nice, but can't be used too often. For deepsky a nice overview eyepiece is nice, wide angle gives you a great space walk effect.

I have a planetary that gives me 80x, and M81 and M82 fit into the view nicely. For the ring nebula more can great, as well as some smaller galaxies. But on other DSO, you want a wide view, such as Andromeda and other larger objects.

150/1200mm with 20mm eyepiece for example will give you

60x magnification

2.5mm exit pupil (larger then 7mm or smaller then 0.6 is considered bad)

->a 60deg afov eyepiece on that telescope shows you 1degree of the sky, with a 66deg afov it's 1,1degree... It's not too much difference, but the view is nicer, and with higher magnifications a wide angle is easier to manually track planets.

When the sky is bright due to light polution, sometimes a exit pupil below 6mm can be ideal, as the sky is just too bright (and the eye's darkness adaption is a problem).

A 6mm for example is 0.7mm exit pupil, 200x, so a great planet eyepiece. HR Planetaries aren't expensive and great, too (60 degree afov but also relatively sharp on telescopes below f/6).

Calculated with http://www.sternfreunde-muenster.de/orechner.php - because I'm lazzy ;-)

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An 18mm BST would look nice in that scope I'd reckon. I used one in my 10" which has the same focal length and it showed a pleasing amount of sky. Obviously the exit pupil would be larger when used in a 10" but I think it'd be just fine in yoru scope too. as it'd give 2.2mm which is almost perfect for DSO in most peoples opinion (2mm being a sweet spot).

The William Optics SWANs would be good in your scope but if you were to upgrade in the future to a faster scope then they would stop being so good and start showing their weaknesses. The BSTs show a good amount of correction for their price and are also very comfortable to use as they have long eye relief. I'd go 18mm BST personally.

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Thanks for the advice!

I have already got the 15mm BST which I love, it is much better then the stock EPs, that's why I was going to buy the 25mm but people say that's the weakest in the series so I was looking at the 18mm. I have a Barlow so I use that instead of the stock 10mm when looking at planets 15 with 2x gives me 7.5mm, that is another reason why I was going up instead of buying a 8mm. Think I am sold on the 18mm

Does anybody know where I get a BST eyepiece case?? Not seen on anywhere

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It was referred to as the weakest in a review some time back. The problem is if you take all 6 and compare you will have an opinion that the one is not quite as good as the others, so you call it the "weakest".

The term weak for one person could be the colour balance isn't what they like, the 25mm BST is the longest so will have the widest view. Sensibly you would expect it therefore to begin to lose out at the edges - every eyepiece tends to, there is just more edge with the 25, then the 8mm.

Read reviews of other eyepieces, Hyperions come to mind, in general people will refer to one as not as good as the others. For some odd reason this has stuck with the 25mm BST and really there is no reason for it, it is a very good eyepiece. For whatever reasons I have 2 of them.

Look at it this way, stick all 6 in a scope one by one and make comment on each of them, everyone will say the one I like the best was the XXmm eyepiece, the one I liked the least was the YYmm eyepiece. That doesn't make the YYmm eyepiece the worst or even weakest, it means that for that person in the scope used they prefered one more then the others and one least. Compare using an 8" SCT then same in an 8" Newtonian and you will get 2 different answers. The 5mm will work in the Newtonian but be too much in the SCT. Same eyepiece two completely different results.

Edited by ronin
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I don't really know what to do now, apart from ignore reviews of whole series by the looks of it.

My 15mm is brilliant I love it I use it all the time, but when searching for DSOs i need an 18 or 25 and the stock ep is so hard to use, when you get the whole view it is very hard to keep I have to be some way back from the eyepiece, another reason I like the BST (extendable eyecup) I will give some thought, I will get all BSTs in the end but that's a while off yet.

Thanks all for the advice, clear skies

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