Beginners Lens Question

[Col 130EQ]

I have a Celestron Astromaster 130EQ that came with a 10mm and 20mm eyepieces. Its a 650mm focal length scope so the 2 eyepieces I have take it to 65X and 32.5X magnification. But I have some questions if anyone cud help me?

1. It says that in the spec of my scope that the highest useful magnification is 150X. So does that mean with or without a barlow lens? ie... if I got a 4or5mm eyepiece then that would be the highest, and using a barlow lens on top of that would be pointless as it would take it above 150X?? Therefore I could only use a barlow lens with my 10mm eyepice to keep it under 150X. Bit confused about this??? Could I use a 4or5mm and then use a Barlow on top of it to get it up to 300X?

2. Which eyepieces are best for Planets? Moon? Stars? Nebulae? I always assumed the higher magnification the better but other posts have hinted otherwise. Anyone explain this to me? or point me to a good site I could look at on eyepieces and barlow lenses explanations?

3. Someone said any 1.25 eyepiece would fit my scope, but are some makes better than others?

Thanks in advance for any replies. The Stellarium software someone suggested is great, finding everything really easy with it!!!

Cheers

Col

[daz]

Col

The highest useful magnification depends on lots of things, but the theoretical limit is 40-50x per inch of aperture. Your 130 is about 5" so 200-250x is its highest potential. The biggest factor will be the "seeing" of the atmosphere, the better this is, the better your viewing ability. It could be that the spec is quoting the highest magnification with the supplied optics....

With a 2x barlow, as you now know, it doubles the magnification, but the light grasp is reduced by 4 times. So what you see is bigger, but dimmer (because the same light is spread over a larger surface). Bear this in mind.

So, the answer is yes, you could use the barlow and a 5mm EP, but remember that the object will be very dim at this power.

2. Some say that the orthoscopic eyepieces are very good for planets as they have a narrower field of view, therefore the planet fills more of the view. It is down to personal preference though, as some people like a slightly wider view. A good plossl will be the best place to start and try it on different objects. Take a look through the Equipment section, there's no end of threads on eyepieces.

3. Yes, but for what you will need for now, Meade 4000's are very good, Antares Elites are another good one. There is a Revelation set which gets very good reviews, as does the Celestron set as well.

Hope this goes someway to answer your questions...

[Col 130EQ]

Thats real helpful my friend. Thanks for taking the time. I didnt know about the light grasp. I think my best bet is probably to purchase a 5mm, a barlow 2X and maybe a wider angle eyepiece just to see what the difference is.

Suppose the only other question to follow this is price range. There are plenty of eyepieces on ebay for under 10pounds but I assume its like anything in life, you get what you pay for? As all the other EP's from respectable online stores are more in the region of 30-50pounds. Going to check out the ones you have suggested.

Thanks again

Col

[coulsaa]

Col,

I'm finding the 'Backyard Astronomer's Guide' by Dickinson and Dyer very good at explaining this sort of thing.

Andy

[John]

.......but I assume its like anything in life, you get what you pay for ?.....

Very much so with astronomy equipment - you really need to budget £30-£50 per eyepiece unless you can pick up some 2nd hand. Used astro equipment is well worth considering - if looked after it never really wears out - all my equipment is 2nd hand in common with a number of members of SGL

This site is worth keeping an eye on:

http://astrobuysell.com/uk/

John

[ENWOD]

Thought i'd add a slight bit to this topic. In the handbook the quoted Useful magnification is x306. As i have the same scope as you, shall be purchasing some barlows and lenses and shall see where i get with those. Most likely a x3 barlow and a 6mm EP.

[John]

Thought i'd add a slight bit to this topic. In the handbook the quoted Useful magnification is x306. As i have the same scope as you, shall be purchasing some barlows and lenses and shall see where i get with those. Most likely a x3 barlow and a 6mm EP.

The theoretical maximum useable magnification for a 130mm scope is 260x. In practice on most nights it will be somewhat lower as seeing conditions, particularly in the UK are rarely optimum.

TBH 306x would be too much on all but the finest of nights and even then you would not be seeing any more detail.

Ultra-high power is pretty overrated IMHO - the most satisfying views often come at low and medium powers I've found.

John

[Col 130EQ]

Thanks for the replies everyone. Its an interesting one about defining the useful magnification. I wonder if the manufacturers have a set mathematical way of working it out. But in practice, defining what is actually 'useful' I suppose depends on many factors like you guys point out.

I think the best way for me is to purchase a 2X barlow, a 6mm eyepiece like ENWOD suggested and a higher spec wide angle. Should have a pretty decent range. Build up slow, keep checking for offers on ebay, and on sets etc. Keep seeing these sets with different colour filters as well. Not too sure bout what use they are tho.

Stay in touch fellas

Col

[radioactive]

Tthere you go Col you have now hatched a plan

how wonderful this astronomy bug is !!

here's wishing you countless hours of great viewing over the 2008 season

[Col 130EQ]

Had a look on ebay and found a load of lenses, company called MA, all bout the 4.99 region. Is this as the price suggests just cheap and nasty???????

http://stores.ebay.co.uk/LOTNABOX_Telescopes_W0QQcolZ4QQdirZ1QQfsubZ8QQftidZ2QQtZkm

Please could you check out and give feedback???

Cheers

Col

[cygnusx1]

I read somewhere that the approx max mag under average seeing conditions ( define that! ) is 50x per inch of aperture.

So on your 130mm = +- 5" that would be 250x, sounds about right.

[gary1968]

Col

With a 2x barlow, as you now know, it doubles the magnification, but the light grasp is reduced by 4 times. So what you see is bigger, but dimmer (because the same light is spread over a larger surface). Bear this in mind.

Daz, just a quick question, slightly off thread, sorry but from the above quote you say the use of a 2x barlow decreases the light grasp by 4 times. So, my 8" f5 with my 20mm EP and a 2x barlow has 4x less light grasp than with my 10mm EP and no barlow.

So, my question is, would it be better to use a 8" f10 with no barlow rather than my current f5 and a 2x barlow?

Gary A

[barkis]

I think, but I may be quite wrong, that Daz may mean that doubling the focal length of your system by introducing A Barlow lens, will slow the system as a photographic one by a factor of 4, which means, for example, that if you were to image a nebula without doubling your focal system and got a reasonable result obtained in 1 minute, then doubling the focal length would require 4 minutes to get the same result.

Using an amplifying device like a Barlow, is normally used to increase image size of such as the planets. In those cases, increased exposure times are not too disadvantageous. Of course you wouldn't dream of using a Barlow to image a faint galaxy or nebula. At least I don't think you would. I hope Daz. doesn't mind me butting in on a question asked of him. If I have got it wrong, he can slap me round ear at Astrofest in February.

Ron.

[daz]

Perhaps 'light grasp' was the wrong phrase!

Ron is correct in his explanation of the photographic example - and no, you generally wouldn't barlow a neb or galaxy, you'd use a narrower field of view (a larger telescope).

What I mean is that, as the magnification increases, the light coming from the object has to be spread over a larger and larger area - therefore, its overall brightness will diminish.

In answer to your question Gary, then I think yes, it would be preferable to use an f/10 system rather than barlowing an f/5. Mainly from the point of view that there is less glass in the optical train. If you increase aperture as well, then of course its even better as you gather more light as well.

This is why the longer focal lengths and bigger apertures are ideally suited to planetary imaging.

I hope I didn't confuse anybody before - apologies if I did.

[daz]

Had a look on ebay and found a load of lenses, company called MA, all bout the 4.99 region. Is this as the price suggests just cheap and nasty???????

http://stores.ebay.co.uk/LOTNABOX_Telescopes_W0QQcolZ4QQdirZ1QQfsubZ8QQftidZ2QQtZkm

Please could you check out and give feedback???

Cheers

Col

I would stay away from them. The key phrase for me is "an excellent image at the center of the field of view" which means that off-centre is going to be pants, kidney beaning is likely to be bad.

IMO of course!

[Ant]

If I can just chip in my 2p worth here as well.

Remember that an Eyepeice collection will stay with you for life - even if you sell the scope, you'll keep the EP's.

So IMO it is better to buy better quality EP's then making do with cheaper ones...

Ant

[daz]

I think that's 4p Ant - but sound advice!!

[Whippy]

Got change of a quid?

[philsail1]

I'm sure I've heard somewhere that whatever telescope/eyepiece you use in the UK, generally, you can't go above about 300x (due to the atmosphere)?

Regards,

philsail1

[Andrew*]

in the early morning when the air is steadiest, you can push the magnification quite a bit sometimes. I recall Russ once got 450x out of his scope in the early morning :shock:

However, this is extremely rare and not worth having an expensive eyepiece for such occasional use. I've pushed my newt to 333x once on the moon in good seeing but the image started to degrade...

Andrew

[Whippy]

I'm sure I've heard somewhere that whatever telescope/eyepiece you use in the UK, generally, you can't go above about 300x (due to the atmosphere)?

Regards,

philsail1

Hm, I'm not sure about that. I had my 6" Mak up to x360 on the Moon last time I was out (about 2 weeks ago) and it looked pretty good t'me. I'm also fairly sure I had about x300 out my newt before and it was 'acceptable' on M27. But obviously they were the execption rather than the rule.

Tony..

[thing]

I'm sure I've heard somewhere that whatever telescope/eyepiece you use in the UK, generally, you can't go above about 300x (due to the atmosphere)?

Regards,

philsail1

I've had my 8" Newt up to x400 just for a laugh, the images (of Mars) were pretty clear but TBH it looked a lot better at x200 and there's not a lot of apparent difference in size either. It's not like the difference between x200 and x400 is massive which might sound weird but that's my experience. The Moon invariably begins to 'boil' badly at above x300 for me, so again about x200 is fine for the Moon as far as I can see. So a 130 mil scope should be about right for the seeing in UK, although of course it won't see as brightly as a bigger scope.

[gary1968]

Perhaps 'light grasp' was the wrong phrase!

Ron is correct in his explanation of the photographic example - and no, you generally wouldn't barlow a neb or galaxy, you'd use a narrower field of view (a larger telescope).

What I mean is that, as the magnification increases, the light coming from the object has to be spread over a larger and larger area - therefore, its overall brightness will diminish.

In answer to your question Gary, then I think yes, it would be preferable to use an f/10 system rather than barlowing an f/5. Mainly from the point of view that there is less glass in the optical train. If you increase aperture as well, then of course its even better as you gather more light as well.

This is why the longer focal lengths and bigger apertures are ideally suited to planetary imaging.

I hope I didn't confuse anybody before - apologies if I did.

Thanks Daz, that'll be that cleared up........................

Gary A

[daz]

Most welcome