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Posted (edited)

Hey guys!i need to make this quick! I want to get a new 6mm eyepiece.I have a 8" f6 dob and i want to get a nice 200 mag and put the telescope to its limits at 400x

The most i ve seen with is 240x for Jupiter and Saturn .i ve heard they looked stunning at 400x and i am keen on trying it.Ive got my eyes on a 6mm Skywatcher UWA eyepiece.and i have a Bst Starguider 2z Barlow lens.And i seem to be having problems with my 8mm Bst Starguider (300x in total).the bst has a 16mm eye relief and a 21mm lens diameter,but when i barlowed it and look at the moon yesterday,it was hard finding the right spot to look through the eyepiece and when i moved the image disappeared.I dont know if it will have that effect on planets,when I looked at mars it was fine but i definitely want to avoid that from happening again.Will it happen?

The Skywatcher eyepiece i want to buy has a 16mm eye relief too.I dont want to make the same mistake now , i want a 6mm eyepiece at the price range of 50-70 euro either from flo or amazonuk.If the Skywatcher will have similar results,can you suggest another one?

Edited by Kronos831
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Posted (edited)
16 minutes ago, Kronos831 said:

when i barlowed it and look at the moon yesterday,it was hard finding the right spot to look through the eyepiece and when i moved the image disappeared

This is a consequence of a thing called the exit pupil. This is the diameter of the cone of light asd it leaves the eyepiece and is a combination of the focal length of the eyepiece and the focal ratio of the scope. As the focal length of the eyepiece goes down (including an adjustment for a barlow) the exit pupil will also go down and your position will produce the effect you have noticed. The focal ratio of the scope, of course, is constant.

A 6mm eyepiece (whatever one you get) with an f/6 scope will give you a 1mm exit pupil.

Edited by Demonperformer

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21 minutes ago, Kronos831 said:

it was hard finding the right spot to look through the eyepiece and when i moved the image disappeared.

Simply observe regularly and you will learn to control your head's movement unconsciously.

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Using a Barlow tends to extend the eye relief outwards which makes it harder to keep your eye in the correct place. Longer eye relief is useful if you wear glasses but if you don't then extending eyeguards, even home made with craft foam can be useful.

A second point would be not to chase very high powers. The planets are better placed for you than us in the UK currently, but even then you will often get better, more stable and contrasty views at x200 than you would at x400. The low contrast features on Jupiter can get washed out at high powers, often x180 works well. Saturn can take more, particularly on the higher contrast parts of the rings.

That's not to say x400 is never useable. On very still nights with a stable atmosphere and the planets up high you can get amazing views, that just tends to be less frequent than the nights of poorer seeing.

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If  you have an 8" 'scope, 400x is too much! :D

I have a WO 6mm SPL with my 8" 'scope and it is super.

It's about a tenner more than your budget though.

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I know 400 is too much, but it is usable at times.

41 minutes ago, Demonperformer said:

This is a consequence of a thing called the exit pupil. This is the diameter of the cone of light asd it leaves the eyepiece and is a combination of the focal length of the eyepiece and the focal ratio of the scope. As the focal length of the eyepiece goes down (including an adjustment for a barlow) the exit pupil will also go down and your position will produce the effect you have noticed. The focal ratio of the scope, of course, is constant.

A 6mm eyepiece (whatever one you get) with an f/6 scope will give you a 1mm exit pupil.

So what does this mean? Will it be unusable?

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5 minutes ago, bingevader said:

If  you have an 8" 'scope, 400x is too much! :D

I have a WO 6mm SPL with my 8" 'scope and it is super.

It's about a tenner more than your budget though.

400 is just the limit on very good nights tho,very few each year,it is usable right?

 

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9 minutes ago, Stu said:

Using a Barlow tends to extend the eye relief outwards which makes it harder to keep your eye in the correct place. Longer eye relief is useful if you wear glasses but if you don't then extending eyeguards, even home made with craft foam can be useful.

A second point would be not to chase very high powers. The planets are better placed for you than us in the UK currently, but even then you will often get better, more stable and contrasty views at x200 than you would at x400. The low contrast features on Jupiter can get washed out at high powers, often x180 works well. Saturn can take more, particularly on the higher contrast parts of the rings.

That's not to say x400 is never useable. On very still nights with a stable atmosphere and the planets up high you can get amazing views, that just tends to be less frequent than the nights of poorer seeing.

Thanks what are extending guards how can i use them and how can i make them?

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37 minutes ago, Ben the Ignorant said:

Simply observe regularly and you will learn to control your head's movement unconsciously.

So ill be able to steadily view through the eyepiece once i ve become comfortable with it?

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Posted (edited)
21 minutes ago, Kronos831 said:

400 is just the limit on very good nights tho,very few each year,it is usable right?

I have an 8" F6 scope just like yours - I suspect you have a Skywatcher 200P or very similar just like mine.  I have a really good pricey 5mm Pentax that gives me x240.  Even on the best nights I get and I live under reasonable skies in the countryside my telescope won't go to any more magnifcation than that and I rarely get the chance to use it.  8mm seems to be about the 'sweet spot' for these telescopes I highly doubt you will gain any satisfaction from going beyond my 5mm.

Edited by JOC

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NB.  From what I see and read there is a vast difference between what is theoretically possible and what is practically possible.  My take, based purely on what I have read and my own experience is that Practical magnification from a telescope seems about half that which is supposed to be theoretically possible give or take a bit.

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Good to know, will still be buying a 6mm tho,i m buying a Skywatcher heritage 130p soon so it will give me both 100 and 200x barlowed(again i dont think it will go that high very often) will use it on few nights if seeing is perfect although i expect the image to be dim and quite mushy.Thanks 

Now:Which 6mm should i buy?

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5 minutes ago, JOC said:

NB.  From what I see and read there is a vast difference between what is theoretically possible and what is practically possible.  My take, based purely on what I have read and my own experience is that Practical magnification from a telescope seems about half that which is supposed to be theoretically possible give or take a bit.

Nvm what i posted.Do you think i should get a 7mm instead?(171 ,342 on 8"dob)

And (85.5 and 171 on my future Skywatcher heritage 130p)??

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Good advice on maximum useful power above. I rarely use 400x even with my 12 inch dobsonian and that has a top quality mirror from Orion Optics in it. For Jupiter I find 180x - 240x best and for Saturn 230x - 280x. Mars sometimes benefits from 300x but it's so tiny at the moment that even then it's a little pink spot !

The William Optics SPL 6mm is a good quality and comfortable eyepiece with a slightly wider field of view than the standrad plossls and orthos. That would give you a useful 200x with your 8 inch dobsonian.

 

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7 minutes ago, JOC said:

I have an 8" F6 scope just like yours - I suspect you have a Skywatcher 200P or very similar just like mine.  I have a really good pricey 5mm Pentax that gives me x240.  Even on the best nights I get and I live under reasonable skies in the countryside my telescope won't go to any more magnifcation than that and I rarely get the chance to use it.  8mm seems to be about the 'sweet spot' for these telescopes I highly doubt you will gain any satisfaction from going beyond my 5mm.

Kronos is in Greece where the planets are significantly higher than in the UK. I viewed Saturn at x400 a long time ago when it was at nearly 60 degrees altitude and it was amazing, so it is often all about the conditions.

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Just now, Stu said:

Kronos is in Greece where the planets are significantly higher than in the UK. I viewed Saturn at x400 a long time ago when it was at nearly 60 degrees altitude and it was amazing, so it is often all about the conditions.

Stu sorry for asking, but which telescope did you observe with?

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1 minute ago, Kronos831 said:

Stu sorry for asking, but which telescope did you observe with?

I knew you were going to ask that ;)

It was my OMC200, an 8" f20 mak. Lovely scope but needed to be properly cooled and with excellent conditions to get the best out of it.

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The best views I ever had of Saturn were when it was high in the sky (probably around the time that Stu is talking about). I was using an 8 inch Celestron SCT back then at 250x. The conditions were perfect (which happens just a few times per year here) and the scope in perfect collimation and properly cooled. The image of Saturn looked like the Voyager photos !

But even then I found 200x-300x gave the sharpest and most detailed views. Going over 300x did not add to the image - it was a bit fuzzier although larger but I prefer smaller but sharper.

 

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Good lord !An f 20 8" mac i can only imagine.Well that settles it! 6mm it is! Now, do you think that the Skywatcher 6mm Planetary UWA eyepiece will suit my scope?or should i get another eyepiece?

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Posted (edited)

Guys jesus where do you find all the money to buy those scopes,also(ritorical dont mean to ve offensive)also how do you tell the wife??

Edited by Kronos831
K
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1 hour ago, Kronos831 said:

So ill be able to steadily view through the eyepiece once i ve become comfortable with it?

Yes, and you can speed the learning by moving your eye up and down a little, left and right, in all directions, decentering it on purpose and returning to center. 

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The 6mm will be great in your scope. Re. Big magnification. Expect to see a big mushy mess racing across your field of view. It is very much like buying a lottery ticket. Most of the time, nothing; very very occasionally, the air will be perfectly steady, and wow!!!!!!

Once happened once to me. X250 on Jupiter at 4am. ????. If I hadn’t had the 5mm in the box, I would have missed it!

Paul

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3 hours ago, Kronos831 said:

Guys jesus where do you find all the money to buy those scopes,also(ritorical dont mean to ve offensive)also how do you tell the wife??

Astronomy is actually relatively cheap compared to other hobbies like power boating, riding Harleys, traveling in a motorhome across the country, flying a Cessna, etc.  For under $10,000 you can get a very nice telescope setup which doesn't even begin to get your anywhere with those previously mentioned hobbies.

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7 hours ago, Kronos831 said:

400 is just the limit on very good nights tho,very few each year,it is usable right?

Well, maybe with perfect eyesight, if you pluck your eye lashes and clean the eyepiece too! :D

I've barlowed my 6mm in the 8" on the moon on a crisp, clear and steady night.

It was literally an awesome sight and at roughly an apparent 1,000km from the surface, like skimming over the craters!

However, and this is a big HOWEVER, being at the theoretical limit of the 'scope, other factors then kicked in.

I had a very difficult job keeping my eyelashes out of the way.

If you have floaters at all, this is when they really kick in and are noticeable to the point of obscuring the view.

And finally, any dust specks on the EP then appear like enormous boulders!

Other than that, because of the conditions, it was still a sharp image and well worth it for the wow factor.

I've managed this a total of 1 times!

Even on that night, it was much more comfortable and enjoyable to drop back to the 7mm and 2x barlow (x343) for most of the observing.

Reading my notes, I also commented that the barlowed 6mm gave a slightly dimmer view than the barlowed 7mm.

So, I wouldn't buy it solely for this purpose.

However I can whole-heartedly recommend the WO 6mm SPL in an 8" giving 200x magnification.

I've had my best views of the planets through this EP. :)

 

 

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