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Total newb - Just pulled the trigger on a Skywatcher 200P 8" dob. need advice on eye pieces.


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On 17/10/2021 at 13:49, Ricochet said:

Probably not. It looks like a 26mm plossl unnecessarily put into a 2" fitting for marketing purposes. 

As someone with an 8" dob I would suggest that at some stage you will need one 2" eyepiece for your telescope, but it needs to be one that shows a wider field of view than a 1.25" eyepiece can show, and it needs to be one of decent quality so that it is reasonably well corrected to the edge. If the eyepiece isn't well corrected you might find that the usable field is actually less than in your widest 1.25" eyepiece. 

For your initial upgrades, I suggest that something shorter than your 25mm is probably the way to go in order to give you a range of options to try on different objects. I would start by picking something in the 12-14mm range and then dividing whatever focal length you choose by 1.4 to find your next focal length. 

thanks. As a total noob I am finding myself thinking that I would like to push the magnification envelope even though I have been told that's not a good idea XD.  What would you say the TRUE max magnification for an 8" dob is?  on paper they say 400x, but I have read even 250x can be a challenge? 

my thought was that when I am going to try to push the magnification I would do so with an extremely high quality eyepiece to get the desired result - perhaps something like a 4.7mm televue ethos , or 8mm ethos w/ 2x Barlow. 

 

BTW, I agree that those cheap eye pieces look like a 26mm plossl unnecessarily put into a 2" fitting for marketing purposes, but I asked the seller to send a picture of the bottom (which they dont show in the ad) , and it appears to have a large chunk of glass in there so I don't think they are a 1.25 put into a 2" barrel - what do you think? 

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6 hours ago, clean said:

What would you say the TRUE max magnification for an 8" dob is?

It depends what you are observing. For double stars you can probably push to 400x, but for planetary the optimum for the scope is more like 200-250x, for star clusters up to 200x (if it fits in the fov), galaxies 100x, filtered nebulae less than that. All of that is assuming the atmosphere allows you to push that high. In the UK once you hit 150x you can start to hit atmospheric limits and they will override anything your scope may or may not be capable of under perfect conditions. 

6 hours ago, clean said:

it appears to have a large chunk of glass in there so I don't think they are a 1.25 put into a 2" barrel

I didn't mean it was literally a 1.25" set of lenses inside a large barrel. The glass can still be 2" but the eyepiece can be stopped down internally. 

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7 hours ago, clean said:

As a total noob I am finding myself thinking that I would like to push the magnification envelope even though I have been told that's not a good idea XD.  What would you say the TRUE max magnification for an 8" dob is?  on paper they say 400x, but I have read even 250x can be a challenge? 

Bear in mind that the maximum useful magnification is limited by a number of factors, not least of which is the stability of the atmosphere you are looking though.  To put this into context, when observing say, Jupiter, on a reasonable night with my 10" dob, I find my 8mm BST eyepiece gives me a very satisfying view at a shade over 150x magnification.  The Gallilean moons are visible within the field of view and I can see some decent detail on Jupiter itself.  If I step up to 250x with my 5mm eyepiece, unless the seeing is pretty good, I'm merely seeing a larger fuzzy disk, rather than the smaller fuzzy disk that, being smaller, gives the impression of crisper detail! So although it might be theoretically possible to go up to about 500x with my telescope, in reality I know I'm never going to do that.  The eyepieces I regularly use are 25mm (50x), 12mm (about 104x) and the 8mm.

Secondly, and not insignificantly, if you're using a dobsonian, you'll find that the object you are observing can move pretty rapidly through the field of view at high power!  And in order to keep it in sight, you're going to have to be constantly nudging the telescope.  Personally, I find it more relaxing to have a wider field of view and more leisurely experience.  For a laugh, I once tracked the ISS through my telescope, just to see if I could - it wasn't easy!

Pete

Edited by Orange Smartie
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With my 8" dob, I find I use x150 most regularly for planetary work. Occasionally x200 for Saturn, but often the seeing is too poor for anything higher, especially as it's low.

Last month I had Jupiter at x300 on a rare night of exceptional seeing. Normally I save that magnification for trying to split really tight stars. Lots of nudging involved at that magnification, too.

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I'll echo some of these other posts on magnification. Jupiter as the reference, I chill at around 180x mag in a place with consistently poor seeing. I like using this view calculator.

Some things to learn about while you're playing with the stock eyepieces is exit pupil, field of view and how fast your scope is vs. the quality of glass.

Good choice btw, I stuck with a 4" dob with stock pieces for months and months before I leapt for a better scope/EP setup. I'd have made all kinds of errors otherwise, especially surrounding EPs and especially surrounding magnification so you're on the right trail.

Grats on the scope!

Edited by HiveIndustries
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On 20/10/2021 at 07:22, clean said:

thanks. As a total noob I am finding myself thinking that I would like to push the magnification envelope even though I have been told that's not a good idea XD.  What would you say the TRUE max magnification for an 8" dob is?  on paper they say 400x, but I have read even 250x can be a challenge? 

The late Sir Patrick Moore is often quoted to have said: "...50x per inch of aperture".

 

On 20/10/2021 at 07:22, clean said:

my thought was that when I am going to try to push the magnification I would do so with an extremely high quality eyepiece to get the desired result - perhaps something like a 4.7mm televue ethos , or 8mm ethos w/ 2x Barlow. 

Under a perfect clear sky getting an unrealistically high magnification maybe possible... even with a high quality eyepiece. That said, I would not go above the 200x for visual. Simple reason is that what you are viewing target will rapidly drift/move across the field of view in a very short space of time due to the Earth's rotation and you are forever manually tracking the 'scope in alt-az to compensate it and after a while it is not going to be fun. For imaging you maybe able to get away with it.    

Edited by Philip R
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On 20/10/2021 at 07:22, clean said:

thanks. As a total noob I am finding myself thinking that I would like to push the magnification envelope even though I have been told that's not a good idea XD.  What would you say the TRUE max magnification for an 8" dob is?  on paper they say 400x, but I have read even 250x can be a challenge? 

my thought was that when I am going to try to push the magnification I would do so with an extremely high quality eyepiece to get the desired result - perhaps something like a 4.7mm televue ethos , or 8mm ethos w/ 2x Barlow. 

 

 

Given that the maximum magnification you'll find useful will be different depending on the seeing conditions, I went for the Televue Nagler 3-6mm zoom. It's a tighter field of view (50deg) than the Ethos or Delos (or even the Delites) but you get a consistent 10mm eye relief which is better than you'll get on shorter FL orthos or plossls.

However, the real key advantage of the 3-6mm zoom is you can set the magnification to match the seeing really easily just by twisting the barrel. There's 4 click stops for 3,4,5 and 6mm but you can also use intermediate settings. It's an option to consider as a useful and slightly more unique tool..

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High magnification sounds wonderful in theory, but as several have said above, it comes with penalties. Nothing, nothing to do with telescopes comes without a 'but ..... ' .

The more you magnify that view, the more you magnify the effects of the wavering, rolling air masses of the troposphere , our 10km ish thick atmospheric band where our weather mostly happens, and thinner stuff right out to around 500km above where we stand.

The more you magnify that view, the smaller the piece of sky you are looking at, so the faster any target will zip across and out of your particular piece of sky : the 'scope needs shifting to keep the target in sight. Bellieve me, I've tried it ... little 150 heritage dob, 750mm focal length , with 8mm eyepiece and 2x barlow, trying to see detail on Mars last year, that's 187.5x magnification. Out of the many nights I tried, Mars at 94x ( no barlow) was maybe half of the time too much for the conditions, and when I tried the barlow, because it seemed the air was steady enough, I found the constant adjustments of nudging the tube to keep up were just not practical. 

I've no idea how limiting your particular atmospheric conditions will be, but the dob nudging is a world wide constant of optics and geometry, :evil4: , and spending plenty on a wonderful eyepiece will not do a thing for the atmosphere or the need to nudge , well, apart from if you buy a really wide field of view eyepiece , which gives you a little more time until the target drifts off the edge.

You cannot push the magnification beyond what the atmosphere will let you, we are at the mercy of the weather.That's one of the reasons big observatory telescopes are best sited up mountains with less atmosphere for the photons to battle through.

Don't be thinking you made a mistake buying the dob though, it's a great telescope, use it , use the supplied eyepieces, explore as far as they take you, then consider eyepieces. If you must try pushing the magnification, rather than potentially wasting a lot of cash on something at the extreme edge of what you can use, buy a decent eyepiece which gives you 125x or thereabouts , and a 2x barlow .

Heather

Edited by Tiny Clanger
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Thanks for all the help and suggestions everyone! greatly appreciated. 

the scope showed up this morning. I wanted everyone's opinion on something I noticed. one of the secondary mirror supports seems to be twisted somewhat - all the others are dead strait. Could this throw off the collimation at all? or interfere with entering light? or does it even matter?

 

thanks again. 

 

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Edited by clean
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It could increase the strength of the diffraction spike associated with that vane, but if you consider that the light being reflected is only in the central 8" of the tube, you may find that the twist is far enough over that it doesn't matter. However, as it is twisted it could have potentially been weakened so I would contact your supplier and see what solution they propose. 

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thanks guys, unfortunately the supplier told me to "  twist that vein into a better position with your fingers" .

At this point ill try to get in touch with the manufacturer and see what they say.  I get that its a hard item to ship back / forth, and the supplier obviously does not want to deal with that , but yes your right - its new and should be right.

 

 

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14 hours ago, clean said:

thanks guys, unfortunately the supplier told me to "  twist that vein into a better position with your fingers" .

At this point ill try to get in touch with the manufacturer and see what they say.  I get that its a hard item to ship back / forth, and the supplier obviously does not want to deal with that , but yes your right - its new and should be right.

 

 

Here in GB/UK 🇬🇧 'we' are protected by various consumer and distance selling laws/rules protecting the customer. It is the responsibility of the reseller/supplier to put right... not the end user/consumer/customer. 

Edited by Philip R
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It looks like the nut either needs screwing into the tube a bit more, or rotating a bit to straighten it up. If it were me I'd have a play and see how it looked after, but I can understand why you'd want it right from new. 

How is the stud attached on the outside of the tube? Is it a screw thread?

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It looks as though the vane would straighten pretty easily. However, it shouldn't be like that from new and you risk making it worse. Definitely try to get the supplier to resolve it. If you can't, I'd be tempted to try it out as-is and if the effects are noticeable I'd have a go at correcting it. If you do try, the golden rule is to have the tube horizontal so nothing gets dropped down the tube onto the primary mirror.

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Get a quote from a local machinist to fix the bend and send it to the seller requesting reimbursement for having to make it right locally.  Remind them of those UK consumer protection laws.  The seller may come back with a counter-offer to make it right after all.

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