Eyepieces for a 12inch dobsonian

[DivyaD]

Hi,

After a lot of research I purchased my first advanced telescope a 12inch Orion SkyQuest Open Truss Dobsonian. Being into astronomy for years now this is the first gaint telescope I am using. Today was first light and I was rather confused as to what  I was seeing. Comparing my tiny 4inch view of Jupiter with the view from this 12inch; it was nearly similar! So the question is which eyepieces should I use to get better detail? I have a 28mm, 32mm, 15mm, 6mm etc. and a Baader zooming hyperion. I viewed Jupiter from both the telescopes using a 32mm wide angle eyepiece.

This is probably a really rookie question so go soft on me

Which eyepieces and of which make should I attach to fully utilize the capability of this telescope? I tried the Baader and the 15mm and the 32mm for seeing Jupiter tonight. 

Any help would be highly HIGHLY appreciated.

 

 

Clear Skies,

Div

 

[John]

Hi and welcome to the forum !

Your eyepiece focal lengths seem good for the 12" dob.

The zoom at the 8mm setting would probably give the best views of Jupiter although the 6mm might be OK if the seeing conditions are really good. The 6mm will be better for Saturn, the Moon, Mars and double stars though.

I don't know which 4" refractor you also have. The 12" should show more planetary detail but if the seeing conditions are just mediocre then there might not be much difference over the 4" refractor.

The size of the planet in the field of view will depend on the eyepiece focal lengths used and the focal lengths of the telescopes. It won't automatically appear larger with a 12" scope but the larger aperture should support higher magnifications than the 4" scope, when the seeing conditions are good.

Make sure you allow the 12" scope time to cool down and check that the collimation is reasonably good. Both will make a difference to the quality of planetary images.

[Piero]

Also to me your issue seems due to cooling time and/or collimation. Possibly cooling time could be discarded if there is a minor difference between the place you store the telescope and the place you observe. The views through a poorly collimated telescope can be severely affected as soon as you start pushing the magnification. Stars can appeared blurred (when you add a bit of magnification). Planetary details will be compromised even more. 

[Buzzard75]

Welcome, Div! I'm not sure if you have the one with the IntelliScope system (XX12i) or the GoTo system (XX12g), but they are essentially the same scope. I personally have the XX12g. I have a 28mm Orion DeepView eyepiece that came with the scope, a 14mm Explore Scientific 82deg eyepiece, a 10mm Tele Vue Delos, and a 2x Orion 4-element Barlow.

Bottom Line Up Front:

To get down to the crux of the discussion. We have the exact same telescope. However, what works for me and what works for you may actually be two entirely different things because of our personal preferences and our intentions of use. Likewise, what works well in one telescope, may not work very well in another, such as the difference between a 12" and a 4". Not knowing what that 4" telescope is, it's hard to tell you what you should be seeing compared to a 12" dob other than everything will be smaller and dimmer in the 4" when using the same eyepiece. Typically for planetary viewing though, you're going to want the shortest focal length you can get to bring out the most detail while not enhancing distortions from the atmosphere (more on that below). Some evenings I can push my magnification all the way up to 300x using the 10mm and the 2x Barlow. Other nights I'm only able to get it to 214x using the 14mm and the 2x Barlow. At 214x I can get Jupiter and all four of the Gallilean moons in without any issues and I can make out most of the cloud bands. If the seeing conditions are optimal and I can push it to 300x, I may start to lose Callisto depending on where it is in its orbit, but I can make out all of the cloud bands and even see the Great Red Spot if it's facing us. When looking at Saturn, I can see the Cassini Division. Using the 10mm and a 2x Barlow I essentially have a 5mm eyepiece. If you're not starting to see this when using your 6mm eyepiece on your 12" dob and it looks exactly the same in your 4", then there's either something wrong with your eyepiece, the telescope, or you're not looking at what you think you're looking at.

 

Getting to the meat and potatoes of it. Be prepared, this is somewhat of a lengthy post and some of it will probably be information you already know. Since I have the same scope though, I want you to understand how I made my decisions as to which eyepieces to purchase and all my logic behind them.

Finding the right combination of eyepiece and telescope is somewhat of a trial and error. That being said, there are some general guidelines that can be applied upfront to help steer you towards the right focal lengths. For instance, the rule of thumb for theoretical maximum useful magnification of a telescope is typically defined as 50X the aperture in inches. In our case, this results in a magnification of 600x (50x12). This is under ABSOLUTELY PERFECT seeing conditions. Given all of the atmospheric interferences we have to deal with (humidity, air turbulence, clouds, dust in the air, etc.), it's a very rare thing that we can even push magnification beyond 200-300x depending on the quality of the optics in the scope and the eyepiece. Not only are you magnifying the object you're trying to view, but you're also magnifying all those disturbances between you and it. It can make for a very hazy image and it may appear to shimmer. This shimmer is actually the atmosphere you're seeing. So let's assume 300x is our maximum. I'm sure you're already aware, but if you take the focal length of your telescope and divide it by the focal length of your eyepiece, you'll find your working magnification. Consequently, if you take your focal length of your telescope (1500mm) and divide by your maximum magnification (300x), you'll find the shortest focal length eyepiece you'll probably want to use (5mm). The other thing you need to worry about when getting to the short focal lengths is exit pupil. To find your exit pupil, you take your the focal length of your eyepiece and divide by the focal ratio of the telescope. The focal ratio of the telescope is defined as the focal length divided by the aperture (1500mm/305mm =  4.9 or commonly noted as f/4.9). The exit pupil of a 5mm eyepiece would be 1.02mm (5/4.9). This is a perfectly acceptable exit pupil. You can go smaller, but anything smaller than 0.5mm will be like looking through a pinhole. Some people even begin to see floaters in their eyes and begin to experience discomfort as the exit pupil gets smaller. It should also be noted that a smaller exit pupil will also appear dimmer when viewing nebulas and galaxies than a larger exit pupil. Small exit pupil sizes are something that is a personal preference based on what you find comfortable and acceptable.

Moving on to the other end of the spectrum, the longer focal length eyepieces, one of the biggest things to concern yourself with is actually the exit pupil again. Only this time we're talking about large exit pupils. The human eye is only capable of dilating so large. Some people's pupils can get wider like owl eyes, but as we age, they typically can't dilate as much. With refractors there really is no limit to how large an exit pupil can get before developing any negative impacts. With reflectors like ours however, we have a secondary mirror in the path of our vision. If the exit pupil gets too large, the center of the image will actually begin to dim due to the obstruction of the secondary mirror. I personally use 7mm as my guideline, but yours may be different and you may be comfortable with a larger exit pupil. Using this as a guide though, we take the exit pupil and multiply it by the focal ratio, giving us 34.3mm (7x4.9) for a long focal length eyepiece. Now, 34.3 seems like an odd number. I would feel comfortable bumping that up to 35mm. This would also give us a magnification of 42.8x (1500/35). Coincidentally, if you look at the specifications for the telescope from Orion, you'll see that they actually have already defined the lowest useful magnification as 42x. You could have just taken those two numbers and figured out the appropriate focal lengths, but I hope this explanation has provided some more insight into how they achieved those numbers. Conversely, they defined the theoretical maximum magnification as 610x and the highest useful magnification as 300x.

So you have a large and a small eyepiece, what do we put in between? Really anything that tickles your fancy, however there's one caveat to that and it's Barlow lenses. You seem to have a wide range of eyepieces and quite a bit of overlap. I personally decided on getting a 2x Barlow so that I could effectively double the number of eyepieces I have rather than having a plethora of individual eyepieces. In the case of a Barlow you want to try and avoid factors and multiples of eyepieces you already own as you can get Barlow lenses in 2, 3, 4, or even 5x. I have three eyepieces, 28, 14, and a 10mm. With the addition of a 2x Barlow, I have essentially doubled the number of eyepieces. I now have the equivalent of a 28, 14, 10, 7, and 5mm. But wait, that's only five not six, you say. You're right. With a 2x Barlow my 28mm is essentially a 14mm eyepiece. I've broken my caveat of factors and multiples, but I don't intend to use the 28mm for much longer and I'll get into why later. I knew the shortest I would ever go would be 5mm. Starting with that and deciding to get a 2x Barlow meant that I could actually get a 10mm eyepiece rather than a 5mm and double the number of eyepieces I had. So every eyepiece I purchase from here on out is like a 2-for-1 sale. On the opposite end of the spectrum, the largest I'll ever go is 35mm. However, I also needed to consider the apparent field of view (AFOV). Some people prefer a narrow field of view while others prefer a wide field of view. Both have pros and cons and a lot of it is personal taste. I personally wanted a wide field for object location and that space walk feeling when looking through the eyepiece. There are some nice 35mm eyepieces out there. However, some of them have a somewhat narrow AFOV (68deg or less) when compared to others. I can actually get a slightly shorter focal length eyepiece (increasing the magnification) with a larger AFOV (cramming more stuff in), and actually have a slightly larger true field of view (AFOV/mag) for that space walk feeling I'm looking for. Right now I have my eye on a an 30mm Explore Scientific 82deg. The magnification is only slightly reduced from the 28mm I already have, but the apparent field of view is greatly increased from 56deg to 82deg. This means that my true field of view goes from 1.04deg to 1.64deg of the night sky.

And here's where we get into eyepiece construction. The quality and construction of the eyepiece can play just as big of a part in the quality of the imaging as the combination of the scope and eyepiece focal lengths do. The 28mm eyepiece that came with the scope has a fairly narrow field of view at 56deg, compared to the 82deg of the Explore Scientific and the 72deg of the Tele Vue. It also only has four elements. The Tele Vue has somewhere between 6-8 as does the Explore Scientific. Neither has published how many lenses or what type they are, but these are what they are believed to be. Typically the more lenses, the better the imaging, but that's not always the case. You also have to take into account coatings of lenses, lens groupings, and construction of the actual optical tube they are held in. All of this plays a part in the image quality obtained from the eyepiece and this is where the trial and error I mentioned at the very beginning come into play. Each manufacturer has their own proprietary coatings, lens orientations, and eyepiece manufacturing techniques. They can all have a variety of types of eyepieces that they offer as well. Two eyepieces from two different manufacturers, or even the same manufacturer, with the exact same focal lengths, and the exact same AFOV's may actually provide very different imaging qualities. For example, the 28mm Orion I have is a fine starter eyepiece. But it is absolute junk compared to something from Tele Vue, Baader, Explore Scientific, or even some of Orion's own higher end eyepieces. It is Orion's absolute lowest-end entry level eyepiece. It's not something I use for regular viewing. I only really use it for object location and then switch to my 14mm. This is why I said I was looking for something else. I want something that will still provide me with the object locating capability, yet with better imaging capabilities that I would actually like to use for those larger deep space objects (DSO's). In order to find what works best for you, you may have to try a number of eyepieces. That's not to say that you can't be happy with what you already have. What you have may be perfectly fine, they are just suited for specific situations. Short focal lengths are great for planetary observing while longer focal lengths are great for DSO's. Some of the eyepieces may have better coatings on them that lend themselves to providing better anti-reflective or light scattering properties. I recommend you continue to use what you have, find out which ones you like the most, and determine why. If you have the ability, I'd also recommend you join a local astronomy group and attend a few viewing sessions. These can be great places to meet other knowledgeable people and possibly even try out their eyepieces to see if there's something out there that you like even better than what you have.

[Louis D]

The key to getting the most out of a large aperture scope is to have steady seeing and spot on collimation.  If you have these two working in your favor, you can observe an object at the same exit pupil as with a smaller scope (let's say 2mm), but the image scale will be larger making it easier to pick out fine details.  All of this assumes your mirror is well figured.  If it isn't, you'll never get as sharp an image as in a smaller, well figured telescope.  That's why people pay big bucks for Zambuto, Swayze, and other super premium mirrors.  They want to be sure to get the best image possible for a given aperture.  My best view of Jupiter was through a 12" Portaball with a Zambuto mirror under steady Texas skies.  I could see blue swirls and dark barges quite clearly.

[Stu]

Some great answers already so I will try not to repeat the already valid points.

Planetary observing has a number of key criteria to get right, as I see them they are:

Cooling - a large scope has a large mirror which takes time to reach ambient temperature. Before this, it generates tube currents which can really affect the view, particularly at high magnification. Unless it is kept at outside temperature it is best to put the scope out anything up to an hour before using it.

Collimation - this is key to sharp planetary views so make sure you know how to get it right. It is not difficult, just takes a little practise.

Seeing conditions - this relates to the atmospheric turbulence which spoils the views at high power. I see this as two types, local and high altitude.

Locally, you can get turbulence off the roofs of houses, from areas of tarmac or concrete and from central heating flumes. You can avoid this by trying as far as possible to observe on grass and avoid observing over houses. This is not always possible but worth trying as far as possible. My best planetary views  for instance are when the planets are above the gaps between my house and the neighbours. Often you will find that the atmosphere locally calms down a lot in the early hours of the morning once the heat of the land and houses has dispersed.

High level seeing conditions depend upon the weather and the position of the jetstream. You can't affect these, but can understand from what you are seeing whether conditions are good, bad or middling. When seeing is poor it is best to concentrate on low mag targets.

Planetary altitude - the lower the target is, the more atmosphere you are looking through in order to see the target. When looking up at the zenith you are only looking through a few tens of miles of thick atmosphere. Looking towards the horizon you are viewing through hundreds of miles of potentially turbulent atmosphere. Unfortunately at the moment the planets are at quite low altitudes from the UK so the views of Jupiter and Saturn for example are nothing like they were ten years ago or so. You can help this by observing when they are at their highest for the night which is when they transit the meridian in the south. The other option is to hop on a plane and head somewhere south where they are at a higher altitude but that's not practical with a 12" scope! The altitudes gradually improve over the next years but won't be at their best for quite some time.

Observer skill and patience - taking a quick look into the scope will never give you a detailed view. You need to spend time at the eyepiece in order to pull out the detail. Even in good seeing conditions you will get short periods of excellent seeing when the detail becomes much clearer. Over time you will find your brain builds up a picture of this detail and you see far more. Many of us spend an hour or more just looking at, say Jupiter to draw out all the detail. In addition, there are events such as moon and shadow transits which take a number of hours to complete so you can easily spend quite a few hours at the eyepiece watching these take place. I often find that x180 ish works well for Jupiter, x200 ish for Saturn and up to x250 ish for Mars. This varies of course and can be pushed more if the seeing is very stable.

From a scope perspective, all things being equal a large scope has greater resolution than a small one so will show more detail. Ultimately this is the case, but you can often find that if seeing conditions are poor then the smaller scope cuts through this more easily and delivers more stable images. A large reflector is more susceptible to seeing conditions, cooling and collimation etc than a small refractor so that may explain your experience. Keep viewing with the reflector to catch the good conditions when you are setup correctly and you will be surprised what you can see. When I put my 4" frac next to my C925, I find the frac almost always delivers nice sharp images, but the SCT gives far more colour and resolution if I am willing to wait in between the wobbly periods!

Sorry for waffling on, got carried away  

[John]

Great post Stu

 

[alan potts]

Think I will just sit this one out, great advice guys!!

Alan