Hello, New to Board and Hobby

[-George-]

Hello, I am looking at buying myself a Telescope. I was part of the Cloudynights forum, but they told me I may find more relevant information here since I am In NSW - Australia.

So here I am.

Basically I have 1 question for myself and one for a friend of mine who already owns a telescope.

1: I want to buy a 12" telescope (maybe a 16"). I live on a farm, past Lithgow in NSW, where it is pretty dark and to naked eye I can see a 1000s of stars.

Having said this, I have seen the Mead Light Bridge 12", is this good?

Are there better things worth looking into/investing in?

I want something that I can get nice planet views with and everything else. I understand more aperture allows for more magnification without losing detail and thus contrast etc is greater (greater detail available to view). Same time I also understand the quality of the mirror is highly important as well otherwise, there is not much to magnify on that will look 'wow'.

Anyways I did not want to buy something that later I will feel ripped off on because I should have got 'the other thing' which was a little more $.

So how is the Meade Light Bridge 12"?

Are there any other ones (LIke Orion) that perhaps offer better stuff?

The Lightbridge is Bk-7 the Orions offer the faster cooling mirrors (pyrrex, or low dispersion) something rather... *scratches head*.

And, would a 16" be worth looking at, or start with the 12 and perhaps later as I gain experience, look into a 16? The 16 Mead Lightbrige as example is $1000 more than the 12". 12" good started, but same time, If I do love what I am doing, the 16" could have avoided a 'needed upgrade' later... so I am considering that as an option. However, If I get a 12" later I can buy a very high quality mirror 16" rather than have wasted it on a cheaper sort... so perhaps the 12" is good to start on from this angle rather than having a 16" to upgrade to a better quality 16".


I hope you get where I am coming from.


Next: Friends question.

My friend has a 10" Dobsonion mounted SAXON reflector telescope.

Optical Design Newtonian (Parabolic)
Diameter 254mm
Focal Length 1200mm
Secondary Mirror Diameter 58mm
F/Ratio f 4.7
Highest Practical Power 508x
Faintest Steller Magnitude 14.7
Resolving Power 0.56
Finderscope 9x50 Finderscope
Focuser Diameter 2" with 1.25" adapter
Focuser Crayford
Eyepieces Plossl 25mm and 10mm (1.25")
Solid Tube style
-

He wanted to know what eye piece can he buy that will give him the maximum magnification without losing quality... wants something for good planetary viewing, such as mars etc without it being a 'dot' in the sky (like binoculars) but also not being a haze because he over-shot the magnification where all detail is lost. So a balance between surface detail and size. Kind of like what you see in pictures, to get that 'wow' sensation rather than buy the eye piece, look at the sky and go 'hmmmm yeah its oooooo k" and feel bummed out.

Help appreciated.

He lives in Melbourne, he usually drives out of Melbourne to view in the country, I live myself as said out past lithgow, in a nice dark area.

[barx1963]

Hi George!:welcome:

You have asked quite o lot of questions so I will attempt to answer them one at a time.

The Meade 12" Light Bridge is a very good scope. It is a truss tube version of the standard GSO 12" dob that GSO make for Meade. The optics in these are mass produced so can vary a little in quality so there is the occasional poorer quality mirror but all the ones I have seen have had pretty good mirrors. Mechanics are OK and overall they represent very good value for money.
As to whether there is something better to invest in, well yes, but like all things telescopic it a matter of finding the best fit for you and your circumstances/budget. Moderate sized dobs like this give a large aperture at a very reasonable price. There are lots of scope that are better optically but cost many time more or have much smaller aperture. Just also be aware that they tend to be bulky, despite the truss tube construction.
They are not really a planetary scope, they give pretty nice images of planets but I found with my 12" that it struggled if I pushed the magnification beyond about 115x so planets were still rather small.
Your query about the BK7 versus Pyrex is one that crops up occasionally. I believe that Pyrex cools down slightly faster than BK7 but the main issue with BK7 versus Pyrex is that BK7 has a higher Coefficient of Thermal Expansion than Pyrex. That means that it expands and shrinks more than Pyrex as temperature changes. The effect on the figure of the mirror is potentially detectable but at f5 it would be very minor. A faster mirror would show it more. Personally, I wouldn't worry about the mirror material.
As regards the 12" versus 16" query. A 16 will show you more, more deep space objects are within reach, you can use more power on the planets people start queuing up at your scope at Star Parties!! BUT, besides the cost it is a bigger scope. I would always advise someone to at least have a look at one of these scopes before committing to buy (and that also applies to the 12") and make sure they are happy with the physical commitment and storage space required.

As to your friends query. A 10" newtonian ( remember that a dob is a type of newt) will usually be ok up to about 100 to 150x power so the 10mm EP he already has will give a pretty good idea of its capability as that gives 125x. At that power planets are still going to be small. Pushing the power also magnifies any atmospheric problems so one you push into higher power you are really waiting for those fleeting patches of still air to enable clear views. He could go out and invest in a better EP, but no EP will compensate for the atmosphere.

Malcolm

[billdan]

Gooday George,

Welcome to ISS, sounds like you have the perfect spot for enbarking on this wonderful hobby.

First up the golden rule for amatuer astronomy - The best telescope is the one that gets you outside regularly to use it.

There is no point buying a heavy 16in scope if you can't move in and out of Garage etc. You can't rely on the wife to help you carry it out each time she'll end up despising your new hobby.

You could put it on wheels and run the risk of it being unstable. Plus you will have to learn to love balancing your body on a step ladder in the dark to reach the eyepiece.

However if you are serious about the hobby, build yourself an observatory, an 3x2 Metre metal shed on a concrete slab, that can be jacked up and placed on wheels to roll away is the simplest.

This is what I did and only took about 3 weekends worth of work, I just laid some pavers behind the shed for the wheels to roll onto, cut a new door frame at the front of the shed, for the telescope to pass through, then hired some axial props to jack the shed up and bolt the wheels (castors) underneath attached to a 3x2 inch wooden frame.

If you are prepared to do this then go for the 16 inch, otherwise if you are content an carrying it out each night, get the 12 inch.

As far as brands go (Meade, Lightbridge, Orion, Skywatcher etc) they are all good and only differ in some features, like mirror type, focuser provided, whether or not they can track the stars or electronically GOTO to a chosen destination.

Don't worry about Pyrex or BK7, yes Pyrex does cool down quicker but also attracts dew quicker IMO. This cool down time is only relevant for maintaining a stable focus, if you have to turn the focus knob a few more times with BK7 than with Pyrex, it doesn't really matter.

By the way Dobsonians are a general purpose instrument, great for nebula's and galaxies, and average on planets, the best telescopes for planets are long focal length refractors.

For your friends question, a 6mm eyepiece will give more magnification but is really only suitable for steady seeing, otherwise stars will be bouncing around in the eyepiece. I hardly ever use my 6mm and only go down to the 12mm eyepiece because of seeing issues, I mostly use the 25mm.

Regards
Bill

[Allan_L]

Hi George,
and welcome to IIS :welcome:

The meade Lightbridge is a good quality scope.
The truss design makes it easier to transport.

Yes, Orion is a good brand also, I believe optically better (IMHO).
and they do a truss as well.
I also like Skywatcher.
and they do a collapsible (as opposed to separable truss).
I have a 12" Skywatcher and really think it is a good (midway) option.

Orion and skywatcher are available in GoTo as well (add about $1000 to the basic price).

Problem with bigger aperture is moving it about.
The 16 is a big heavy beast, and may be difficult to handle alone.

The Best scope is the one you will use more, so size and weight do affect this.

But all are Good quality mirrors and have many fans.

For your friend, the theoretical highest magnification useable is twice the aperture mm. So for a 250mm (10") would be 500x.
As magnification is focal length (1200) divided by eyepiece mm., that could be attained with 2.4mm.
BUT you would need perfect seeing and atmospheric conditions to get a clear image.
Commonly held is that 300x is about as good as you will ever be able to use and that therefore would be a 4mm eyepiece.

Now a standard plossl with 50* field of view would be a bit like looking through a straw at 4mm.

So a more expensive eyepiece with a larger field of view would be much easier to use. But more expensive.

But know this, photos are obtained by long exposures (to capture more light photons) and this is not possible with real time observing of the human eye. So even then there may be some degree of OK hum!

Best solution for both is to get along to an observing session near you.
Look though other peoples scopes and eyepieces and ask questions.
I think there is a planned session for Katoomba airport next Saturday.
Not sure for Melbourne, but check out the Star-parties forum.

Hope this helps a bit
Clear Skies

Looks like Malcolm and Bill pushed Post before me, so sorry for duplication, but it all sounds similar.

[-George-]

Just an interesting read I read a while ago:

http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/360923/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1

The idea of the entire thread was to show that the general consensus of Refractors = Planetary viewing at its best, not the case compare to Reflectors (if set up right).

Can someone explain how does 'magnification' vs Aperture work then?

I am under the understanding that

More aperture means, more magnification possible.

At the same time however, if the sky is the Limiting factor, to about 300x or so, then whether one has a 24" dob or 6" dob, doesn't make much difference in terms of Power.

That means a planet (Like Saturn) would be the same size in both the 20"+ and the 6". However, Saturn should have more detail to it in the larger one simply because the larger one gathered more light?

Does that also mean that galaxies etc would be more visible with larger aperture even if the 'size' of what you are seeing would be the same (based on magnification which has been limited by the sky)?

That also then says if you 'do' get a very nice night, where you can go 500x magnification (like these people did in the link provided), then obviously yes, the larger aperture because it provides more detail allows more for magnification to be used without sacrificing detail?

Having said this, I can not understand how a small telescope is as good as a larger telescope for planets. More convenient perhaps, but detail comes from Aperture.

I have a shed where I can store my telescope. I can get the scope mounted on some sort of wooden frame with wheels on it (like a trolley) if needed. We have seen the 12" in person, not bad when its trus-rod style in terms of size.

O and off-topic, how do I set the forum to show latest post at the bottom as I read top down now down up? This is all reverse order!

[raymo]

Hi George, You are right in that Saturn would look the same size in a 6 or a 24 at the same mag, but you would get a brighter image in the 24 due
to it's greater light gathering power, and you would see more detail, but not because of it's light gathering ability, but because the smallest
feature that can be resolved is a function of primary objective
mirror/lens diameter. Moon craters illustrate this well. The smallest
crater resolvable with a given size scope ia as follows.
4"[100mm] 3.5km diam. 8"[200mm] 1.8km diam. 12"[300mm] 1.2km.
Moon rilles minimum width 4" 250 metres 8" 110 metres 12" 70 metres.
Due to the aforementioned light gathering power of the 24, a huge
number of faint objects would be totally invisible in the 6, but readily
visible in the 24. A good illustration of light gathering is the number of stars visible in different size scopes. If you could see the whole sky at
one time, a 4" would show you approx. 3 million stars, 8" 12 million,
and a 12" 30 million.
raymo

[PRejto]

I will only add that under most conditions the sky is the limiting factor. So, regardless of theoretical resolution, a lower resolution is mandated by the sky. Also, keep in mind that the larger the aperture the more seeing effects the image. Put a 6" refractor next to a 12" and in poor/medium seeing the refractor will often give a more satisfying image of a planet. On the other hand, even in poor seeing, a view of a deep sky object that is faint will usually look brighter and more satisfying in the larger scope. So, in my view there is always a trade off going on and it depends on what you want to observe. There is no "perfect" scope for all objects. Like many of us - down the road - you may find yourself with more than one scope!

Peter

[-George-]

I understand that. However, when sky is good, obviously the large one would be beautiful to look through?

I think I will start with a 12" (so I dont have to start on something that requires ladders etc) - having a solid 12" tube or Truss Rod style 12" does it make a difference in terms of quality etc of viewing or identical?

Now just one question regarding my friends scope.

He told me he has an eyepiece that allows him with a Barlow to get 250x magnification and Mars is a 'little red dot in the sky'.

What does he need to get this more visible like a planet (size of our former 1 cent coins) rather than a Dot...

[barx1963]

George
The last 2 answers give pretty good explanations. The thing to remember is that planets are bright objects. Even a small scope will gather enough light to create a reasonable view so a long refractor with small aperture will usually work fine. But as the others have said, aperture enables smaller features to become visible so negating some of the effect of the atmosphere.
I have seen this in a very practical way. In my old 12" as I stated earlier it was a struggle to push it much beyond 150x on Saturn, but in my 20" I have already had Saturn at 317x with plenty of clarity. In the 12" the Cassini Division ( that's the main gap in the rings) was just visible most of the time but in the 20" it looks like it has been drawn with a texta it is that dark and sharp.

Malcolm

[astro744]

Buy the best optics you can afford whether it be a reflector or refractor. Yes, more aperture will always show a brighter image at the same magnification and more aperture is capable of greater resolution. How sharp an image will be will depend on many factors, see below.

Exit pupil is eyepiece focal length/focal ratio of telescope. A 0.5mm exit pupil is generally considered the minimum below which you start seeing floaters & other eye defects over you image and the image really starts getting a bit too dim. However some people can observe at smaller exit pupils so try it and see for yourself.

e.g. If your telescope is a 254mm mirror, f.l. = 1200mm, f.r = 4.7 then using a 2.35mm eyepiece will give 0.5mm exit pupil (4.7 x 0.5). Using such an eyepiece will give 1200/2.35 = 510 magnification.

Eyepieces that fall in this range are a 4-2mm Nagler Zoom or 2.5mm Nagler.

The zoom would have an exit pupil range of 0.85 to 0.43mm and magnification range of 300x to 600x. The 2.5mm Nagler would have an exit pupil of 0.53mm and magnification of 480x.

Now I am not suggesting you go out and buy this eyepiece since first some conditions have to be met.

1. You have an exceptional mirror.
2. Your mirror has cooled and is stable.
3. Your telescope is well collimated.
4. Your local seeing conditions are excellent, i.e. no heat radiating from nearby surrounds.
5. Your atmospheric seeing conditions are excellent. i.e. steady air above you.
6. Your eyepiece is is such as to give excellent on axis performace and good contrast, vital for planets. (Note this does not have to be an expensive eyepiece as a good Plossl will give fine results). The expensive eyepieces generally give wider fields and excellent performance over the entire field and that is what you are paying for. Note though expensive Plossls and Orthoscopics (Clave, Brandon, Tele Vue) also exist (Clave only on the used market). Tele Vue Plossls only go down to 8mm and they really are exceptional. A well made Plossl will give better contrast and sharpness on axis. Note Tele Vue Radians and Naglers also give exceptional performance on planets and are more comfortable to use.
7. You do not suffer from significant astigmatism. If you do you must wear glasses which means your eyepiece must have a long eye relief (20mm) for you to use it comfortably.
8. You have an equatorial mount or GOTO tracking so that you can concentrate on the image rather than constantly chasing the image across the field. (This will not affect the quality of the view but may affect its perception).
9. How often do you want to observe at 500x? You can use a 2x or 2.5x Powermate or Barlow to achieve the higher powers with longer focal length eyepieces. Quality is very important here. Powermates are essential 'invisible' as are quality Barlows. There are however many poor Barlows.
10. For deep sky objects (e.g. galaxies) quality aperture rules. However the mirror must still be of very good quality. There are so many people that call their telescopes light buckets and think that light gathering is all it is about. I would take a quality custom 25cm over a mass produced 30cm any day and likewise a custom 30cm over a mass produced 35cm or even 40cm. That's not to say though that mass produced telescopes are no good only that you wont know until you get one and even then you may not know for many years or until you've looked through a quality custom telescope.

I'm not sure what you budget is but have a look at http://sdmtelescopes.com.au/

Mars is quite small this year and is currently 15.2 arc seconds reaching 15.6 maximum in a few days. It can attain 25 arc seconds and will get close to this in a few more oppositions time (about 2 years and 2 months between oppositions). Note Jupiter can reach just under 50 arc seconds but is currently only 36.8 arc seconds, still over twice the size of Mars. Note you will never see spacecraft quality images in a telescope! (Most telescope advertisements feature spacecraft quality images of planets).

I have seen Mars in 23cm f15 refractor and it was simply awesome as was Saturn & Jupiter. The refractor has the advantage of a closed tube so tube currents are not normally a problem as in a tube reflector that has warm air rising up the tube walls from the mirror end to the top.

I too have invested in a quality hand crafted 26cm, f6.4 Newtonian mirror and it too gave me an exceptional view of Mars a few oppositions ago using Clave Plossl eyepieces.

However the best view I have ever seen of any planet and on this particular occasion it was Jupiter was with a friends 40cm" f5 Newtonian mirror. It was better than anything I ever remember on the large refractor with swirls, festoons, ovals all over Jupiter. The eyepieces we used on the night were Naglers.

You will soon find that no one telescope does it all and you will one day own reflectors, refractors perhaps and SCT of various sizes. The best telescope is the one you will use the most and for me that is currently a Tele Vue 60. Why did I buy such a smaller telescope? Because they didn't make a 40! I love my TV-60 and it can show me Mars, Saturn & Jupiter in all their glory. Yes the images are small even at 120-180x but oh what a view! And best of all I can be observing in minutes and pack up in the same time.

Back to my first statement. Buy the best optics you can afford and you wont look back. Enjoy!

[-George-]

Thanks for the feedback.

Well my friends telescope as said is a standard 10" SAXON as posted above.

He asked me 'How do you even know what quality mirror you have?'

So based on his telescope, can someone give me the 'most powerful' (useful) magnification? he said at 250x mars is a red dot. So what must mars be at to actually 'see' it, 500? And would 500 even be any good on the scope he has (given whatever mirror it has which im sure you would know more than me)?

[barx1963]

Mars is really small. At the moment is about 15" in diameter. As a comparison the full moon is 30' or 1800", so it is 1/120th of the moons size. Personally I never been a big fan of observing Mars. I have probably only once had a really nice night that coincided with a Martian opposition and all I managed to get was Syrtis M and the north polar cap. Ther are lots of good DSO's much more interesting than Mars IMHO!

Anyway if he is really determined he needs to understand that patience and a lot of time at the eyepiece is required, especially if he is going to go for higher power. To push up to 500x he will need a 2.5mm EP. I wouldn't worry about getting a super expensive one as it not get a lot of use and even cheap ones are still pretty sharp in the centre of the field.

Malcolm

[Renato1]

Hi George,

Just an aside, keep in mind that with a big dob, you can make something which approximates the performance of an APO refractor for a few dollars. Using cardboard and tape in the first instance, or plastic (from a large folder say) and velcro for something more solid, you can make an aperture mask with a hole cut out of it, with a diameter which starts from the edge of the secondary to the outside edge of the primary. Because I have a half circle spider vane holding the secondary, I can get nearly 6" of clear aperture (though if you have four spider vanes, the circle has to fit within them and the outside edge, and will thus be smaller).

It is quite remarkable to use that aperture mask on nights when the stars are big blobs in the dob. Suddenly, they become pinpoints and you have a high f ratio telescope (but galaxies really dim out though).

Regards,
Renato

[astro744]

See http://www.backyardastronomy.com/Backyard_Astronomy/Downloads_files/Appendix%20A-Testing.pdf for start testing a telescope. You can also test on an bench using a Foucault tester with knife edge or Rhonchi grating. Search the web for testing telescope mirrors.

Note at 250x Mars is equivalent to over two Moon diameters, i.e. over 1 degree.

15" x 250 = 3750"
3750/3600 = 1.04 deg.

The Moon is 1/2 deg in the sky and you can see plenty of detail on it unaided so you should easily see detail on Mars at 250x and even less. Note favourable conditions must prevail as per my previous post.

The art of observing is to be able to detect small detail amongst low contrasting features. However Mars has a lot of high contrasting features such as Syrtis Major (dark) and the poles (light). You need to know which part of Mars you are observing as it does change night to night.

If the image is simply a blob then either your telescope is not cooled, atmosphere is turbulent (most likely); again refer to my previous post.

Inexperience of the observer also comes into it and what one observer sees as a dot another will see a wealth of detail.

Learn to do a star test to see what the mirror is like (do it on a few different nights as you really need good seeing for the test).

Patience and perseverance for good seeing is what is required for observing Mars or any other planet. Note in two oppositions time 27/7/2018, Mars will be 24 arc seconds in diameter and this will be the best opposition for another 15-17 years from then. (There is an opposition every 2 years and 2 months but only every 15-17 years is there a very favourable one). Note also sometimes there are two very favourable oppositions either side of a conjunction two years and two months apart.

See http://www.uapress.arizona.edu/onlinebks/MARS/APPENDS.HTM

[Monstar]

Just a general question, there are a load of formulas spread throughout this thread, is there anywhere here or on another site that has all these and perhaps others collected together for easy reference?

[astro744]

See http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=79#.UzoeH6iSy5I for some very handy info.

Note true field of view is eyepiece field stop diameter x 57.3/telescope focal length.

It can also be calculated using apparent field of view of eyepiece/magnification given by eyepiece. However the numbers are not always the same but close and this is mainly due to distortion effects.

I always use the field stop diameter formula if the field stop diameter is known and you can also use the field stop diameter formula for imaging. e.g. say you have a full size DSLR having a 36mm x 24mm chip. Your true field of view when using a 600mm focal length telescope (no eyepiece, i.e. prime focus) would be 36 x 57.3/600 by 24 x 57.3/600 = 3.4 deg by 2.3 deg.

1 degree = 60 arc minutes.
1 minute = 60 arc seconds.
1 degree = 3600 arc seconds (60 x 60)

The Moon is approx 30 arc minutes in diameter.

Just search the web for astronomy formulas.