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150P or 150PL?

Hedgerow

By Hedgerow
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Hedgerow

Hedgerow

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Hi,

I have been looking at scopes on various websites and think the SkyWatcher 150 looks about right for my budget. However, I am not clear about the difference between the P and PL models. I appreciate PL has a longer focal length and the P is 'faster'. The blurb on retailer websites does not make clear if one is better for say planetry work and the other for deep space. In fact both scopes are claimed to be suitable for both. So what is the story?

Any advice greatfully received. Thanks :)

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brianb

brianb

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For visual work you want the PL - f/8 is plenty fast enough, collimation is very much easier & a lot less critical. f/5 scopes are pretty temperamental from the collimation point of view and require very expensive eyepieces to get full benefit, f/8 scopes work just great with budget eyepieces.

The short focus scope is preferred for DSO imaging (with a coma corrector) but you will require a mount much more solid than the EQ3-2 usually supplied with it to attempt DSO imaging at all.

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JKB

JKB

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Hi Hedgerow

I have the 150p and the views are amazing! I have not tried the PL but as posts above say it depends what you want to do with it.

For Deep space go for the P general obs go for the PL. Collimation was no probs as it was set up 'out the box'. the mount is fine and you get what you pay for, but I am more than happy.

Cheers

JKB

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Moonshane

Moonshane

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one thing not mentioned so far is that the PL will give more magnification and less field of view with a given eyepiece than the P and the widest field possible will be narrower in the PL than the P.

the PL has a focal length of 1200mm and the P 750mm

this means that for a 10mm eyepiece, the magnification in the PL will be (1200/10) 120x but in the P will be 75x. This means you need a 6mm (approx) eyepiece to get the same mag in the P as in the PL. Some cheaper eyepieces at this 6mm focal length are very tight on eye relief (how close you place your eye to the lens) and not suitable for spectacle wearers (you often don't have to wear your glasses to actually observe though).

Furthermore, the field will be narrower. eg the field of view with the 25mm plossl you will possibly get with the scope would be 1.04 degrees in the PL and 1.67 degrees in the P. this calculator might help you choose eyepieces in the future Telescope Field Of View And Power Calculator

I don't necessarily agree that you need expensive eyepieces with the P (f5). many people use cheaper eyepieces (<£50) and are quite happy with them in fast scopes. also, collimation for visual use is the same whether the scope is fast or slow, you just need to be slightly more accurate with the P than the PL. BUT ideally you should be just as accurate with both. collimation is not something that should put you off newtonian designs like these. it's easy after a short time.

good luck with your choices.

Shane

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brianb

brianb

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many people use cheaper eyepieces (<£50) and are quite happy with them in fast scopes.
Not when they've seen what the scope can do with premium EPs.
collimation for visual use is the same whether the scope is fast or slow, you just need to be slightly more accurate with the P than the PL. BUT ideally you should be just as accurate with both
Yes, the mechanism is the same ... but the diameter of the coma-free field is proportional to the third power of the focal ratio, so a f/5 scope is four times as critical as an f/8 ((8*8*8)/(5*5*5))
collimation is not something that should put you off newtonian designs like these.
No, it shouldn't put you off, but why pick a scope that's awkward?

The narrower field of view is easily got round by using longer eyepieces, and (as you correctly point out) short eyepieces needed to get "optimum" high powers with a short focus scope have their own issues.

Something like f/8 is ideal for a beginner. IMHO.

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Moonshane

Moonshane

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Not when they've seen what the scope can do with premium EPs.

Yes, the mechanism is the same ... but the diameter of the coma-free field is proportional to the third power of the focal ratio, so a f/5 scope is four times as critical as an f/8 ((8*8*8)/(5*5*5))

No, it shouldn't put you off, but why pick a scope that's awkward?

The narrower field of view is easily got round by using longer eyepieces, and (as you correctly point out) short eyepieces needed to get "optimum" high powers with a short focus scope have their own issues.

Something like f/8 is ideal for a beginner. IMHO.

don't get me wrong Brian, I agree with you but thought it important to explain the differences to someone new in a little more detail than the posts so far had done.

I am also a fan of the expensive eyepiece but for a beginner I think this is sometimes overplayed (and I have done so myself in the past) as many cheaper eyepieces (like the BSTs and TMBs) are available for less than £40 and do a decent job. Personally though I agree that a used TV plossl or Baader Gen Ortho for about £50 would be a better buy in the long run. I'm arguing against myself now! :)

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michael.h.f.wilkinson

michael.h.f.wilkinson

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I would go with the PL if you are using it visually, or for planetary imaging. For observing DSOs speed has nothing to do with it. In fact, the PL has the smaller central obstruction (secondary mirror size), so it transmits a few percent more light (not noticeable in practice), and for a given magnification, the image in the PL would be ever so slightly brighter. Given the advantages of using higher eye relief and lower budget EPs, and more tolerance to minor mis-collimation, I think it is the easier scope to use.

There are drawbacks: it is longer, and for DSO imaging the P is better (though a smaller refractor would be better still).

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great_bear

great_bear

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I am also a fan of the expensive eyepiece but for a beginner I think this is sometimes overplayed (and I have done so myself in the past) as many cheaper eyepieces (like the BSTs and TMBs) are available for less than £40 and do a decent job.

Actually the expense thing is a bit of a red herring at shorter focal lengths (and indeed, I'd say the TMBs do more than a decent job; their Smyth lens making them work just fine in an F5 scope without issues.)

The point is, that the 150PL will give better results with a much wider range of eyepieces than the 150P will.

However, the choice isn't as quite clear cut as it used to be since - on the flipside - the 150P version now comes with a two-inch Crayford focusser, opening up the possibility of super-wide views.

Tough choice really...

(although I'd personally still go with the PL)

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Parus major

Parus major

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Something like f/8 is ideal for a beginner. IMHO.

I took Brian's advice for buying mine and I've never had a moment that I thought I should have bought something else instead. My choosing between the P and the PL was budgetary as it was nearing my top end and these two provide the most bang for your buck in terms of size and mount quality at this price so I was happy to go with the PL after reading what Brian said.

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great_bear

great_bear

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I was happy to go with the PL after reading what Brian said.

Yes - Brian has a habit of being right about such matters...

- I've never regretted following a brianb recommendation, but have certainly had cause to regret going against them...

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Hedgerow

Hedgerow

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Gentlemen,

Thank you for your input, discussions and disagreements. The net result is that the PL is probably my best option. However, I suspect that as I get into regular observation, I would like to spend more time on photographing ds images, but that would probably push me down an entirely different path re kit. I am not going to rush into anything yet and I'll spend some time with members of the Wycombe AS to get the hang of things first. Nevertheless, thank you to all who have contributed to this thread.

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