SkyWatcher 200p with Celestron 93230 and Barlow xN

[MathewRimmington]

Hi,

Following on from some great advice here a few months back I bought a Dobsonion Skywatcher 200p and I've been having a great time viewing the night sky using the supplied eyepieces. I feel like I've developed a feel for what the standard eyepieces can and can't do now, and am ready to upgrade.

To start me off, I have ordered a Celestron 93230 8mm to 24mm which should hopefully arrive next week. I've read some mixed reviews about the eyepiece, specifically when using it at the max/min of the zoom range. But my thinking was that to start with, an eyepiece which is pretty much a 4 in 1 for a beginner like me is probably fine, and it'll save me continuously swapping pieces in the dark. If I find after some experimenting that I'm struggling with viewing at 10mm zoom or less I'll happily buy another dedicated eyepiece.

In addition to the eyepiece, I'd like to get a Barlow lens but I can't decide which one.

My gut feeling from the research I've done is that I should just buy a x2 as that'll pretty much see me to 8mm/2, so 250x magnification on my scope. I live in the Cheshire countryside and it's fairly dark, but I'm guessing I won't be able to do much with more than 250x magnification. To give you context on darkness, I've seen the milky way very faintly here once this winter but it's generally not visible.

The avant-garde idea I've had is to buy a x3 Barlow lens instead. That would see me get down to 8mm/3, which is pretty much 375x magnification (give or take a few decimal places). I know that realistically 375x is too much, but the zoom eyepiece with a Barlow x3 would give me flexibility to get further than 250x magnification should I be able to use it, e.g. if I zoomed to 11mm/3 I'd be getting 272x magnification. When I think about it mathematically, I feel like the x3 should do everything the x2 should do and more, but something is telling me that it's not as straight forward as I want to believe.

Any thoughts, experiences or advice will be gratefully received.

Thanks,

Mat

[wulfrun]

The thing to remember with a zoom is that their FOV gets smaller as you zoom out (longer FL). Given that a 3x Barlow takes you into the realms of unusable magnification (maybe with rare exceptional nights), you'd get a slight advantage in FOV by using a 2x and just accepting the "wasted" overlap range.

The other thing with a zoom is that you'll probably want to supplement it with a fixed low-power eyepiece for better FOV at that end.

Just food for thought...

[Spile]

I would wait until you have thrashed the living daylights out of your zoom. I occasionally use the Barlow with my zoom but more often than not, and if I can, I will use it sans Barlow.

What I do like is the flexibility of being able to see rather than guess what is the optimal power for the object in the eyepiece.

 

[Ricochet]

13 hours ago, MathewRimmington said:

8mm/2, so 250x magnification on my scope.

The standard Dobsonian Skyliner 200p has a focal length of 1200mm so a 4mm eyepiece will yield a magnification of 1200/4 = 300X. If you have bought an Explorer 200p that has been converted to a Dobsonian then that would have a focal length of 1000mm so a 4mm eyepiece would then give you 250X.

For a telescope with an aperture of 200mm the optimum planetary magnification lies in the region of 1 to 1.2X aperture, which is 200 - 240X, however the atmosphere in the UK often limits useable magnification to the 150-200X range. This would indicate that the 2X barlow will suffice and allow you to use the zoom in the range in which the apparent field of view is largest. For Uranus and Neptune (sometimes Mars when it is far away) you may find it useful to push the magnification to an equivalent of 4mm to increase the disk size but diffraction will be scrubbing detail and beyond this is unlikely to be of use.

If you get into double star splitting then the standard 2-2.4X aperture limit applies (if/when allowed by the atmosphere) and you could use the full range of the zoom and a 3X barlow, but if this were to happen I suspect you would be looking for a sharper eyepiece than the Celestron 8-24mm.

[Soligor Rob]

I have just purchased a Sky Watcher Dobsonian 200P, I find a 2x Barlow is excellent for getting in close, I also bought a Celestron X-Cel LX 18mm EP which gives good clear views, as a beginner I found M42 in Orion easy enough to find and adding a UHC filter really enhanced the view.

A recommendation from SGL I also bought a BST Star Guider 8mm EP. which has excellent eye relief and at £49 is great value for money, we are lucky living less than 5 mins from Chester city centre but have very little light pollution (Bortle 5) so when & if the sky ever clears again we hope to continue exploring the night sky.

I also bought a right angled finder which makes locating your target so much easier.

https://www.firstlightoptics.com/finders/astro-essentials-9x50-right-angled-erecting-finderscope.html

I would definitely recommend buying a copy of "Turn Left at Orion" about £18 from Amazon a great book with loads of information.

Good luck.

[MathewRimmington]

1 hour ago, Ricochet said:

The standard Dobsonian Skyliner 200p has a focal length of 1200mm so a 4mm eyepiece will yield a magnification of 1200/4 = 300X

Yeah, you're absolutely right. Mine is a standard and the label on the side does say FL is 1200mm. Not really sure where I got 1000mm from, but it does mean my numbers were wrong and magnification is higher even with the Barlow x2.  Thank-you

[MathewRimmington]

Thanks for the info everyone, it is much appreciated.

I'd not heard of the Turn Left at Orion book until now, have added it to the wish list  

[Second Time Around]

Some 2x Barlows can also be used at 1.5x, although it's not always mentioned in the blurb, and it's one of these I'd recommend.  These dual 1.5x/2x Barlows allow the black lens cell to be unscrewed from the body of the Barlow and then screwed into the filter thread at the bottom of an eyepiece to give approx 1.5x.   First Light Optics do one in their Astro Essentials range that even has a standard T thread at the top for attaching a camera.  It's just £25.  Go to https://www.firstlightoptics.com/barlows/astro-essentials-125-2x-barlow-with-t-thread.html

The exact amplification varies from eyepiece to eyepiece depending on where the field stop is located.  At 2x amplification with the 8-24mm zoom this will give you magnifications of approx 100-300x.  Most nights the seeing (atmospheric turbulence) won't be good enough to go as high as 300x and you'd get more use from the approx 75-225x that 1.5x amplification will give you.  Additionally, at a given magnification the field of view will be bigger with 1.5x amplification.  This is because the vast majority of zooms have a wider field of view at the high power end.