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Would a 7mm be better..


NoRain
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Hi

In USA for vacation,pick up a ETX90 EC. So far its pretty good it came with a Meade 26mm I also have a Vixen LVW-13mm,ES 8.8mm. I have been using the ES 8.8 on Saturn looks OK but do you think I can get a little higher so I can see more detail maybe a 7mm will there be much of a difference between the 8.8. and a 7mm.

Thanks for any help,I am still a little confused about how high I can go I am thinking that 6mm would be the highest ?

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A good rule of thumb is that the Maximum magnification is two times the apperture in mm, or better, the exit pupil Not smaller then 0,5mm (eyepiece nn devided by focal Ratio of the telescope).

So on a 90mm telescope, 180x would be Max. On moon and planets more can work but usualy you loose contrast.

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Firstly welcome to SGL and a big hello to you and Norway, I have never been there but would love to someday.

I can't disagree with that information. On your larger planets I tend to stick to the X160 to x180 area. You should get a reasomable image with this type of magnification, however with knowing the focal length of your scope I cannot comment on specific eyepieces.

Alan.

Edited by alan potts
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Looks like the focal length is 1250mm so the 8.8mm gives x142 and the 7mm gives x178. You would definitely notice the difference and it would be worthwhile as a highest power ep I reckon, though it assumes the optics are reasonably good. I've not used an ETX, maybe others can advise on that point.

Stu

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I recently picked up a ETX125 and from my very limited experience and based on what others have said about the ETX's the general rule about max magnification doesnt apply to these scopes! :)

On the moon a few days ago i pushed it up to 475x and was able to focus no problem. View was a little dim but i was just testing it to see if i could focus :)

Can i suggest you check out Weasners mighty ETX site. Its and amazing site for etx owners. So much helpful info. And if you cant find what your looking for email him and he gets straight back to you! This guy knows everything there is to know about these scopes!

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It's true you can use very high mags on the moon and double stars and get good results still. It's a different story on planets though where you need just the right magnification depending on the planet and the sky conditions.

x150 to x180 is often good on Jupiter, up to x200 on Saturn when it is high up, and Mars often needs, or at least benefits from more, I've found x230 to x250 works well if the sky is stable.

No hard and fast rules though, I've had Saturn at x400 looking pin sharp on one very steady night, a long time ago!!

Stu

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Of course the same magnification rules apply to those telescopes, but as they are just a rule of thumb no one prevents you from exceeding the recommended magnification. But not only does the image get darker, you also loose contrast. While this is no big deal with the bright moon, it is vital to choose a decent maximum magnification for deepsky objects and even planets so that you can see them large and with the best contrast possible.

I have viewed the moon and jupiter with 600x on a 102mm Mak, but that's rarely possible due to seeing conditions and you can't see more then with perhaps 200x.

On star clusters such as m13 it is important to magnify high enough to see the single stars outside the center, but if you overdo it you won't see them at all. Also on the ring nebula sometimes 260x works like a charme, sometimes light pollution is bad and the overall contrast is best at much lower magnification and the nebula becomes invisible with 260x...

So it's best to choose an eyepiece that's usable regulary even if conditions are not good, as those maximum magnifications rarely are practicable... BUT I would never leave the house with something to magnify more, as there are those rare moments when planets and dso will look amazing around 300x :-)

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600x on a 102mm? That's brave ;).

Often - at least in my experience, you don't actually gain anything from magnifying above an exit pupil of about 1mm unless the seeing conditions are good, and 0.5mm if the conditions are excellent.

Yes, you can use an exit pupil of, say, 0.1mm on any scope (5x barlow and an eyepiece with a focal length of half your focal ratio), but there is no point because the view is much sharper and generally better with less magnification ;). Perhaps 0.1mm exit pupil is taking it too far - but the point still stands, below about 0.5mm exit pupil you don't really gain much apart from size - you begin to lose more than you gain - unless conditions are perfect.

This reasoning, however, doesn't apply with double stars - some have such a small angular separation that you need a huge amount of magnification to split them - of course, clean and sharp stars aren't what you're after when splitting at 200 - 400x +, you're after the 'gap' between the pair.

The Moon does appear to take magnification well, but even it doesn't benefit from a smaller exit pupil than 0.5mm

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Hi

Ok thanks, I was just a little worried that I would go to high and waste some money. So it looks like if I stay under 200x I should have no problems like 6.4mm - 6.7mm should be the highest I should go for now.

Then just get a higher one from a friend to test out 1st.

Thanks

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