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Eyepiece Help


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Hi All

Im still asking lots of questions, i am new to all of this and need help on a new eyepiece.

I know the constellations, when viewing through the finder scope, all is well, once i look through my lowest mag eyepiece, im lost.

My lowest mag eyepiece at the moment is 25mm, would it be better for me to use a 32mm or 40mm eyepiece first to coordinate myself and then use stronger mag to, as it were, zoom in? (Reading this back to myself im not sure if i make sense or not) hopefully someone get the gist of what im trying to say.

I understand the easiest way would be to follow coordinates out of books, but im not sure if my mount is up to it, im using a Skywatcher 150p on a EQ3-2 mount.

Once again thanks for the time

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57 minutes ago, Plumb71 said:

Im still asking lots of questions..

Paraphrasing Tyson: It's probably only people who don't ask questions who remain clueless and dumb. You ask on and you'll be the wisest amongst all :thumbright:

 I'm not sure if it helps but a useful aid is to make circles either on a transparent piece of plastic or from wire that correspond to the field of view of your main low and medium power eyepieces.  Use these circles on your star atlas to help plot your field of view and to map out your star hops from the brighter stars.

Other tools I've found really useful - but acknowledge they aren't for everyone - is a Telrad or Rigel Quickfinder etc to get in the neighourbhood, then use a RACI finder scope. Just orientate the star chart so it aligns with what is being seen through the finder and star hop from there.

If I've grasped your question correctly, I also think Moonshane's Locating Objects in the Night Sky  an invaluable aid.

 

Edited by Rob Sellent
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51 minutes ago, Rob Sellent said:

..Other tools I've found really useful - but acknowledge they aren't for everyone - is a Telrad or Rigel Quickfinder etc to get in the neighourbhood, then use a RACI finder scope. Just orientate the star chart so it aligns with what is being seen through the finder and star hop from there.....

 

 

Those are the finders that I use with my 12 inch dobsonian and very effective they are too. My favourite star chart most often is the Sky & Telescope Pocket Sky Atlas.

With regards to your eyepiece question, yes, using a low power to start with and then applying a bit more to get the view framed as you would like it, is a good approach. The 25mm eyepiece is already quite low power but a 32mm plossl would show you a little more sky. In the 1.25 inch fitting a 40mm eyepiece won't show any more sky than a 32mm can and also, with a scope which has a focal ratio of F/5 (ie: the Skywatcher 150P), the exit pupil generated by using a 40mm eyepiece is too large to be effective.

 

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I didn't know that about exit pupil John, so thank you for the information. As a rough estimate do you reckon about 4mm exit pupil to be a useful maximum? Also, it looks like we have a similar taste in finders 🙂. I just can't along with any other type and remain in awe of folk who can star hop with upside down, back to front finders :happy72:.

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A 40mm eyepiece gives an 8mm exit pupil at f5, which as John suggests is rather too large. I think much depends on  how much light pollution there is and whether you are filtering or not. 4mm unfiltered under average skies. 5 to 7mm under dark skies or when filtering seem somewhere about right.

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Nice one, Stu and what you say makes a lot of sense. In my f5, I don't like observing with anything lower than about 20mm and even then the skies often seem a little too washed out. Curiously, regarding this 'washed out' appearance do you think optics or aperture also play a part? I only ask because sometimes with a lower power eyepiece at around 4mm exit pupil, a smaller aperture scope with a longer focal length seems to offer me a darker background in the eyepiece. Is that because with a smaller aperture there's less 'noise' getting through or am I making an observational error here and talking rubbish and really ought to get back to the scopes and double check my dubious findings?

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I'm not sure about that one Rob. My understanding is that same sized exit pupil in any scope should give the same image brightness, the thing that changes is the image scale depending on the aperture.

What I'm wondering is how you get the same exit pupil in a smaller aperture, longer focal length scope (which sounds like it should be a longer focal ratio). Can you give examples of the scope/ eyepiece combinations you are thinking of?

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20 hours ago, Plumb71 said:

Hi All

<snip>

I know the constellations, when viewing through the finder scope, all is well, once i look through my lowest mag eyepiece, im lost.

My lowest mag eyepiece at the moment is 25mm, would it be better for me to use a 32mm or 40mm eyepiece first to coordinate myself and then use stronger mag to, as it were, zoom in? snip>

What I'm perceiving is a disconnect between when you are looking through the finder scope and when you are looking through the telescope, so I have some questions for you:

1. are you aligning your finder scope with the telescope beforehand? The center of your finder scope should be pointed at the same spot as the center of your telescope.

2. are you using any kind of tracking motors? If you are moving from the finder scope to the telescope too slowly (without tracking) the target is very likely to have transited out of the field of view of the telescope.

As mentioned earlier, a 1.25" 40mm Plossl isn't going to gain you any more FOV than a 32mm Plossl, simply because of the field stop. But a 1.25" 32mm Plossl will give you a wider FOV than a 23mm. However, the FOV of the 23mm is such that you should be able to move from a finder scope to the telescope without losing the target.

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13 hours ago, Stu said:

I'm not sure about that one Rob. My understanding is that same sized exit pupil in any scope should give the same image brightness, the thing that changes is the image scale depending on the aperture.

What I'm wondering is how you get the same exit pupil in a smaller aperture, longer focal length scope (which sounds like it should be a longer focal ratio). Can you give examples of the scope/ eyepiece combinations you are thinking of?

Aye, I guess it's me being dumb. What I had in mind is something like this:

  1. 24mm in a 10" f5 (1250mm): 4.8mm exit pupil, 52x.
  2. 24mm in a 3" f6.3 (480mm):  3.8mm exit pupil, 20x.

It's a mm difference but I find the second option sometimes gives a darker background than the first. I figured - erroneously - that the 1mm exit pupil difference would be compensated by the higher magnification but that isn't always the case. Sometimes the background in the second option appears a little less washed out than the first. 

 

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50 minutes ago, Rob Sellent said:

Aye, I guess it's me being dumb. What I had in mind is something like this:

  1. 24mm in a 10" f5 (1250mm): 4.8mm exit pupil, 52x.
  2. 24mm in a 3" f6.3 (480mm):  3.8mm exit pupil, 20x.

It's a mm difference but I find the second option sometimes gives a darker background than the first. I figured - erroneously - that the 1mm exit pupil difference would be compensated by the higher magnification but that isn't always the case. Sometimes the background in the second option appears a little less washed out than the first. 

 

Exit pupil is exit pupil Rob, and is a good way of comparing the brightness of view in different scopes putting aperture and focal length aside. It doesn't matter that the first example has a higher mag, the larger aperture in this case is enough to give a larger exit pupil and the result will be a brighter sky background.

Put a 19mm eyepiece in the 10" and a 30mm eyepiece in the 3" and the positions are reversed and the smaller scope will give the brighter view, albeit at x16 mag vs x65 in the newt.

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Plumb71, I went through something similar early this year when I developed a severe case of the astronomy bug and started in earnest. It can be overwhelming at first finding your way around the sky and I'm still learning. I have GOTO on one of my scopes but have never used it yet, wanted to learn the basics first. Make sure your finder scope is adjusted properly first if you haven't checked. I pick a star like Vega that's exceptionally bright and use that to align the finderscope, but as mentioned, you have to move quickly between the eyepiece so it doesn't move out of view too quickly. This might take a few tries before you get it dialled in accurately.

Also everything is upside down through the eyepiece, so that's another bit to get used to. Right is left, etc etc

Another recommendation here for Sky & Telescope's Pocket Sky Atlas. I have the jumbo version, spiral bound. It's excellent.

A relatively cheap but decent pair of binos are a quick and handy way to learn the sky as well. 

I also use two phone apps which have helped me greatly - absolutely brilliant ways to start learning your way around.  

The first phone app is SkyView and the second on which I use the most now is SkySafari. With SkySafari, I can advance the clock to see where my preferred target will be at say, 11pm tonight, and I can set up my scope to try and pick it up from there as the night sky gets darker. Skyview had a handy quick identify feature as well. I wish they'd combine the best features from both, but that's a different discussion.

Can you use 2" eyepieces to gain a wider field of view and show more sky? Regarding eyepieces, I started out with the 2" 28mm skywatcher eyepiece which is really good for the money - cheap as chips as they say. I have an older 200p that requires the funky bevelled adaptor for 2" eyepieces, don't know if yours is the same. 

I've also used a 21mm Televue Ethos with a 100 degree field of view in a 130PDS which is kind of like cracking a walnut with a (very expensive) sledgehammer, but the low mag views around 37x (I think) and 100 degree field of view were simply astounding for this little scope.

Maybe an eyepiece in the 20-30mm range with a wider field of view (66-82 degree) over a plossl (52 degree normally I believe?) will help show more sky and let you get your bearings easier? Plus the wider field is just nice to have, full stop. There's the Baader Morpheus line at 76 degree FOV, Explore Scientific 82 degree, etc etc. I find looking through my 25mm SW 52deg eyepiece is fairly limited, like looking down a pipe, and I don't use the 10mm plossl, never got on with it. The 28mm 2" plossl is brilliant by the way.

Just to make the exit pupil police cringe, I have a 55mm TV plossl I've used in a large f4 dob, giving an exit pupil the size of a mini roundabout. It probably wouldn't even work in a 150, too little mag. However, I originally bought it to adapt to a night vision tube which I'm slowly in the process of sorting. This eyepiece gets a lot of flak, but in a big scope I think it offers great views on it's own and is just razor sharp. You are reducing the effective aperture considerably in a fast scope however.

Anyway, align finderscope really well, use phone apps, Sky Atlas, etc to learn your way around - these have all helped me.

It's like moving to a new town when you really start stargazing, you have to learn the streets. 

 

 

 

 

 

Edited by Ships and Stars
typo
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20 hours ago, Ships and Stars said:

Plumb71, I went through something similar early this year when I developed a severe case of the astronomy bug and started in earnest. It can be overwhelming at first finding your way around the sky and I'm still learning. I have GOTO on one of my scopes but have never used it yet, wanted to learn the basics first. Make sure your finder scope is adjusted properly first if you haven't checked. I pick a star like Vega that's exceptionally bright and use that to align the finderscope, but as mentioned, you have to move quickly between the eyepiece so it doesn't move out of view too quickly. This might take a few tries before you get it dialled in accurately.

Also everything is upside down through the eyepiece, so that's another bit to get used to. Right is left, etc etc

Another recommendation here for Sky & Telescope's Pocket Sky Atlas. I have the jumbo version, spiral bound. It's excellent.

A relatively cheap but decent pair of binos are a quick and handy way to learn the sky as well. 

I also use two phone apps which have helped me greatly - absolutely brilliant ways to start learning your way around.  

The first phone app is SkyView and the second on which I use the most now is SkySafari. With SkySafari, I can advance the clock to see where my preferred target will be at say, 11pm tonight, and I can set up my scope to try and pick it up from there as the night sky gets darker. Skyview had a handy quick identify feature as well. I wish they'd combine the best features from both, but that's a different discussion.

Can you use 2" eyepieces to gain a wider field of view and show more sky? Regarding eyepieces, I started out with the 2" 28mm skywatcher eyepiece which is really good for the money - cheap as chips as they say. I have an older 200p that requires the funky bevelled adaptor for 2" eyepieces, don't know if yours is the same. 

I've also used a 21mm Televue Ethos with a 100 degree field of view in a 130PDS which is kind of like cracking a walnut with a (very expensive) sledgehammer, but the low mag views around 37x (I think) and 100 degree field of view were simply astounding for this little scope.

Maybe an eyepiece in the 20-30mm range with a wider field of view (66-82 degree) over a plossl (52 degree normally I believe?) will help show more sky and let you get your bearings easier? Plus the wider field is just nice to have, full stop. There's the Baader Morpheus line at 76 degree FOV, Explore Scientific 82 degree, etc etc. I find looking through my 25mm SW 52deg eyepiece is fairly limited, like looking down a pipe, and I don't use the 10mm plossl, never got on with it. The 28mm 2" plossl is brilliant by the way.

Just to make the exit pupil police cringe, I have a 55mm TV plossl I've used in a large f4 dob, giving an exit pupil the size of a mini roundabout. It probably wouldn't even work in a 150, too little mag. However, I originally bought it to adapt to a night vision tube which I'm slowly in the process of sorting. This eyepiece gets a lot of flak, but in a big scope I think it offers great views on it's own and is just razor sharp. You are reducing the effective aperture considerably in a fast scope however.

Anyway, align finderscope really well, use phone apps, Sky Atlas, etc to learn your way around - these have all helped me.

It's like moving to a new town when you really start stargazing, you have to learn the streets. 

 

 

 

 

 

Thanks for the reply, i have aligned the finder scope, which is pretty accurate (to me) i have ordered sky atlas, i do have the 2019 sky guide which is fine when using the finder scope, but once i look through the eyepiece i go beyond the initial stars which is where im getting lost, hopefully the sky atlas will help me with that, im going to hold off getting another eyepiece at the moment. Im slowly getting used to everything being upsidedown and round the wrong way, which at first blew my mind.

Once again thanks for the advice, i will let you know how i get on

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