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Dob mob eyepieces ??


spaceboy

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This may sound like a stupid question but what eyepieces do the dob mod use? With long focal lengths and large image scale offered by 16" + light buckets there doesn't leave much in wider field eyepieces at the focal lengths needed to keep magnifications within seeing conditions of the UK. Or do bigger dobs allow for higher magnifications as the image is more resolved and brighter ??

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It seems that the ES82's are the main ones at the mid to high magnifications. One factor to remember is that even with an 82 degree eyepiece your eye accepts a field of around 68 degrees so in effect even with 82's when looking down it what you are getting is not everything. To get the remainder you need to move your head/eye a bit. It does raise the question of: Are wider the 68's worth the outlay in cost.

For longer then 14mm - assuming no swapping to 2" - then have to "longer" half as Starguiders or Meade 5000's or similar. Not sure of the ES68's and there seem few 70's or 72's around. Didn't Meade make a 68 also at some time?

Owing to the way that eyepiece outlay tends to follow scope cost a good number go for the Delos eyepieces - 72 degree and £330. The other factor of TV EP's are they are good with a fast scope, but you still need the mirror to be good also.

A 16" is often to simply collect more light to view an object at the same magnifications as you can get with an 8", just being around 4X brighter you see more of/in the object. They are not necessarily 16" for the purpose of increasing the magnification. As you say it is a "light bucket" it collects more light.

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I use a 27mm Panoptic instead of my 14mm Pentax XL in my 15" dob with a 1900mm focal length for most of my mid-power viewing.  You're right in that you can't get down to low power views as would be possible with a shorter focal length scope.  The 30mm ES-82 also works well in it.  For higher powers, I just use roughly double the focal length I'd use in my 8" dob.  The 17mm ES-92 works well for planets and gives a fair amount of dwell time.  Balance can be an issue with it, though.

I can definitely see much more detail in Jupiter's belts with the bigger dob.  I can also see much more detail in the Veil nebula when using an OIII filter than an 8" will show.  I suppose if the atmosphere were unstable, it would be worthwhile to use an off-axis aperture stop to make the system unobstructed.  That alone would increase contrast quite a bit, at the expense of absolute theoretical resolution.

Going to a larger aperture at the same magnification gives a noticeable increase in image resolution and brightness.

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I have a big fast Dob (20 inch F4.1) Because it doesn't track I like a wide FOV to give a longer 'dwell' on the target between pushes. I also like premium EPs because they do far better at the edge of field than cheap ones. I don't use a coma corrector and don't feel the need - when using the right EPs. For me the right EPs are a 13 Ethos (ex Moonshane, thanks you Sir!) and a 26 Nagler. Very occasionally I might pop in a 10mm Radian for a Wow moment on M13 but basically I just use the 13 Ethos and 26 Nagler. I don't often do the planets in the Dob, be it said. I prefer a tracking scope.

Olly

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6 hours ago, ronin said:

16" is often to simply collect more light to view an object at the same magnifications as you can get with an 8", just being around 4X brighter you see more of/in the object. They are not necessarily 16" for the purpose of increasing the magnification. As you say it is a "light bucket" it collects more light.

On the contrary @ronin, a lot of the dob mob are looking for maintained surface brightness at increased image scale. They hunt for smaller fainter galaxies which need power to show them properly.

My understanding is that often a 21mm Ethos is there as a lower power/Finder, with 13mm or even 8mm used for observing.

An 8mm eyepiece in a fast f3.7 scope gives just over 2mm exit pupil at high power which is just about ideal.

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I use 100° Ethos EPs in a 15" manual dob: my base set is 17/10/6mm.  I picked up  an 8mm, and recently added a 31mm Nagler for 1.6° tfov.

5 EPs is a bit overkill, but they do all get used - depending on the target, sky darkness and seeing.  I leave a Paracorr 2 in the focuser for these EPs.

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On 11/09/2017 at 13:17, ronin said:

A 16" is often to simply collect more light to view an object at the same magnifications as you can get with an 8", just being around 4X brighter you see more of/in the object. They are not necessarily 16" for the purpose of increasing the magnification. As you say it is a "light bucket" it collects more light.

 I understand that a 16" scope and an 8" scope can  provide the same magnification,  by  simply changing the focal length of the eyepiece, but I'm interested in the size of the final image?  Would it be larger in the 16" if using say the 10mm Delos, or as you describe, just brighter! If that is the case, looks like I'm keeping the 8" ( though it was my intention to buy the 12" Skyliner,  on the grounds of producing a  larger image ).

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Many like an exit pupil of .8mm-1.5mm on the moon and planets for nice rewards with respect to the eye, of course many use smaller ones. The extra aperture gives more fine detail assuming the seeing etc co-operates thus more can potentially be seen on certain objects like the moon.

Low power binoviewing the moon using a big dob will give unforgettable views and is relatively unaffected by seeing (within reason).

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31 minutes ago, Charic said:

 I understand that a 16" scope and an 8" scope can  provide the same magnification,  by  simply changing the focal length of the eyepiece, but I'm interested in the size of the final image?  Would it be larger in the 16" if using say the 10mm Delos, or as you describe, just brighter! If that is the case, looks like I'm keeping the 8" ( though it was my intention to buy the 12" Skyliner,  on the grounds of producing a  larger image ).

By definition magnification is how large the image is compared to viewing with the naked eye. If you choose to have the same magnification in two different scopes then you choose for the image size to be the same. However, the larger telescope is capable of showing an extended object at a larger size if you choose to view at a higher magnification than in the smaller scope. 

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26 minutes ago, Ricochet said:

By definition magnification is how large the image is compared to viewing with the naked eye

Cheers Ricochet.........sometimes its just the way something is written, and how it's written makes sense, makes it click ?

I was also reading earlier today to the effect that the image at the focal plane, observed from the objective end ( I know that sounds back to front!) will be the same as you would see with the naked eye, however by using an eyepiece, at the focal plane, were able to magnify that image to something more suitable for studying with the naked eye.
Its your mention of ............" If you choose to have the same magnification in two different scopes then you choose for the image size to be the same. "  Doh! Why was that not apparent before I asked?.......pretty obvious if two different focal length/focal ratio telescopes  are producing the same magnification, say x100, then yes,  the image will/should be the same.
It was my ongoing thought that a larger telescope in some way, also produced a  slightly larger image, forgetting for a moment (could be an age thing?)  that the telescope itself, its  purpose,  to capture photons,  not magnify anything, again, that falls to the eyepiece.

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So the dob mob don't really need a lot of ep's but wide for are an advantage.

 

Also on another note when we say magnification is it so that we don't actually magnify the image. ie: get any closer to the subject as the subject remains in the same place relative to us. So aperture is king because this increases the brightness and in doing so makes the object easier to see. Which is kinda why we want to up the "mag" despite not actually gaining anything from doing that, as while we make the object appear larger it is also duller so in truth our eyes see less. So is this where your ep choice comes from Shane? Exit pupil has more gain than magnification.

I admit I always try to keep to the 1mm rule for solar unless the seeing is exceptional where I take it to stupid floater filled 0.3 territory.

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The brightness of an object is determined by the telescopes aperture, it's  light grasp, and technically can't be brighter than what the eye already see's.

Then comes the eyepiece? as you magnify the image at the focal plane, the  light  you see under magnification spreads out, therefore it loses brightness, the more you magnify the greater the loss in brightness!

Hard to fathom, I know!

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51 minutes ago, spaceboy said:

But are you not collecting more light by using the bins? Increasing aperture? Same as a larger exit pupil. Your not making the object brighter but collecting more light from that object so "making it brighter to the eye"

Nick, I think I'm right in saying that it's the surface brightness which cannot get brighter than you see with the naked eye.

A larger scope allows you to magnify smaller objects whilst maintaining the surface brightness. My assumption is that the total brightness of the object does increase as the area increases.

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8 hours ago, Stu said:

Nick, I think I'm right in saying that it's the surface brightness which cannot get brighter than you see with the naked eye.

A larger scope allows you to magnify smaller objects whilst maintaining the surface brightness. My assumption is that the total brightness of the object does increase as the area increases.

Thus you can see it better ? I still prefer fracs and thier ability to cut through the cloud ?

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14 hours ago, Stu said:

Nick, I think I'm right in saying that it's the surface brightness which cannot get brighter than you see with the naked eye.

Correct.  That's why a lot of folks in the states who already have APOs and big dobs are moving on to image intensified astronomy with the latest night vision tubes.  They can make faint nebula look like monochromatic photographs in real time.

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