Bintel BT-252

[shaneaust]

Hi folks:

Does anyone here have experience with the above scope?
I’ve got some questions specific to it.

TIA!

[mental4astro]

So, what are your questions??? :)

[m11]

Happy to help.

As Alex has stated, need to know whats going on?
:)

M11

[shaneaust]

I can see stuff like Saturn etc but only very small. Is that becuz of the limits of the scope, or my EP, do u reckon?

[mental4astro]

Ok, we may need to review telescopes 101 a little... :)

a, Magnification = scope focal length ÷ eyepiece focal length.

b, Golden Rule : max magnification for a given scope = 50X per inch aperture

So, for your 10" scope, this is 500X

c, Max magnification on any given night is determined by the atmosphere, so even a 10" scope may be limited to only 150X because seeing conditions won't allow for more before the image begins to break down, regardless of the aperture of your scope. Most nights you we only be able to pull 150X. On good nights you can pull 250X (if your aperture allows). And you've hit the jackpot when you can max out your scope, whatever that may be.

Now, in your case, we first need to know what is your shortest focal length eyepiece is. Secondly, the plantes are always small in the eyepiece - you will never see them the size of a dinner plate.

From all of the above tech-talk, an amazing amount of detail can be seen at modest magnifications. It just takes a patient eye. Yes the object drifts across the field of view, but by now you would have figured out how to position the object in the eyepiece to maximize its drift time.

With your 10" dob, 250X will be a quite practical upper limit of magnification. The biggest challenge you will have is not the magnification but the smoothness of the action of the mount if it will allow you to easily chase the planet to restore it in the field of view. If the mount is tight or jerky it may make following the planets when using say 400X, very difficult or impossible. It's not just about magnification but also the practicalities of your scope. This is the one major short coming of your scope - the way the mount has been designed and made as a mass production instrument. Optically it is actually very good! :) But the mount is the compromise to keep its price down - it's not cheap, but compared to high end dobs it is inexpensive.

My dobs, I've built them all. I have no problem using 400X and following the planets is easy as the mounts have a silky smooth action, not jerky or stiff. You will need to adjust your mount so that it is as silky smooth as possible and maintains balance.

So lets us know what eyepieces you have, and lets take it from there :)

Alex.

[shaneaust]

Thanks so much for your advice, Alex. I’ve gotta get ready
for work now but will get back. It’s been years since I’ve used my scope becuz the mount I built has deteriorated badly, and I broke my neck a few years ago, which means picking up the scope is challenging.

[shaneaust]

Thanks guy - I’m using a Plossl 32mm and a GSO Super Plossl 15 mm. Hope this helps.

[mental4astro]

Hey Mick,

Now we are getting somewhere :)

A 15mm eyepiece will give you 83X, which is not really enough for the planets.

You will need a 5mm EP to push things along to 250X, and then things with the planets will really change. Going any higher may prove tricky if your mount is stiff and jerky, as this makes doing small and fine movements of the scope difficult.

If you would like a recommendation for a very good but inexpensive 5mm EP, consider the TMB Planetary Type ii 5mm. You will find it much easier to use than your 15mm plossl - these TMB's all have the same big eye lens (the lens you look into of the eyepiece), and they all havd the same amount of generous eye relief meaning you don't need to park your cornea on the eye lens to look into it, unlike a 5mm plossl, and they all have a 58° apparent field of view (plossls have only 52°). The 5mm also does an outstanding job in Newts (dobs are Newtonian scopes on a dobsonian mount). For their price, these are outstanding eyepieces:

https://www.ebay.com.au/sch/i.html?_from=R40&_nkw=planetary+eyepiece+5mm&_sop=15

Of the 10 individual focal lengths in this range, all are excellent in refractors, SCTs and Maks. In Newts, they all do a good job, with the 7.5mm, 7mm, 5mm, 4mm and 2.5mm being the best performers in Newts. Just avoid the 3.2mm - I have looked through three of these 3.2mm and all have been unusable.

Yes there are other 5mm eyepieces, with a wider apparent field of view, and that are also outstanding in Newts, but for price alone these are really very good. I'm sure other people can recommend other EPs.

I actually have a few of these TMBs in my kit, and the 5mm is one of them too. These $45 TMBs sit beside $450 eyepieces in my kit, and I use them all according to the conditions of the night and the scope I'm using.

Alex.

[m11]

Hey Mick,

Alex has provided a great response.

The only thing I would add is viewing regularly helps with filling in detail.

Also I would recommend to.view planets when they are high above the horizon.

I recommend also to check the collimation of the scope prior to use.

M11

[shaneaust]

Alex - thanks so much for your advice! I will be starting work on a new mount soon and as soon as I get my scope up and running again, I’ll let you know how I went with a 5 mm. 👍👍

[mcdrewsy]

I have one of these scopes. Its been in the spare room gathering dust for a long time but recently I have had it back out for a look.

I have been looking at Jupiter and Saturn with a fair quality 6.5mm eyepiece. I think the view is quite acceptable. Happy to keep nudging the scope along to follow the target but I find it really jerky and sticky when the target is high overhead. It's not so bad when the target is closer to the horizon.

Anyone got any suggestions on how to improve this aspect of the scopes performance?

..... and from one Mick to another .... enjoy your scope Mick.

I reckon my mirror needs a clean too. It's grubby. Maybe some steel wool will do the trick?

[The_bluester]

Unfortunately doing much about how they move when pointed almost straight up is an issue.

The area is one I have heard referred to as "Dobsons hole" It is just a spot where it is tricky to move them smoothly. The best approach is to pick another target for half an hour or so.

[mcdrewsy]

Thanks Bluester. I had heard of Dobsons Hole and wondered what it was.

Good advice too about spending half an hour elsewhere in the sky.

[mental4astro]

Improving the quality of action of your dob.



There are a few simple things that will help. This is what you need:

1, Metho
2, GOOD quality car wax. Don't go el-cheapo, get the good stuff, like the Turtle Wax that comes in a tub. The cheap stuff has nasty solvents in it and will not last as long.
3, rag.

You will need to take the OTA off the rocker box. Clean the trunnions (the round altitude bearings on the scope) with metho, and apply the car wax to them and polish them. The little Teflon tabs, ONLY clean them with metho, DO NOT put wax on them.

With the scope off the mount, separate the ground board from the rocker box. If the azimuth bearings use Teflon, clean the pad with metho, and DO NOT put wax on them. The laminate surface the pad push against, clean it with metho and apply the car wax to it and polish it.

If the azimuth bearing uses a Lazy Susanne devise, a LITTLE WD40 is all you can do as these are difficult to clean. I said a LITTLE.

Reassemble the mount, replace the OTA on the mount, and the quality of action of the scope will have greatly improved.

Just be careful not to over tighten the azimuth pivot bolt as this will cause the ground board to pinch at the pivot bolt which will distort the ground board and the way the Teflon pads touch against the rocker box, which will damage the pads and compromise the azimuth action over time making it stiffer and stiffer.

This clean and rewax process is good to do once a year. After the first time that you do this, you will understand the vast difference to the ease of use it makes, and this once a year bit of TLC is well worth the 20 minutes it takes. I have five dobs myself, and I do this to all of them, and it is well worth the little bit of love I give the mount :)

Alex.

PS, the Dobsons Hole once you've cleaned and waxed the mount is much less problematic. HOWEVER, I find it more of bloody brain tease than anything else as you are challenged to work out the best way to move the scope! If the action of the scope is silky smooth, not stiff and jerky, moving the scope around zenith (dead overhead) is not too hard. One tip is to not move the scope only by the focuser. One hand at the top of the OTA (focuser end), and the other hand down on the rocker box - this two-hand grip on the scope will give you much greater control over it. The mental gymnastics needed to negotiate Dobsons Hole however, man, that can be interesting! :lol:

Alex.

[mental4astro]

Ok,

We've dealt with eyepieces and improving the mount. Now we'll deal with the optics.

Collimation :scared: :scared: :scared:

OOOH...! Nasty, nasty WORD. Collimation ONLY means the process of aligning the optics. The word is what scares most people because they are 1, not familiar with the word, & 2, a Newtonian's optics is not something people encounter everyday. BUT take away the fear and it is just tweaking a pair of mirrors.

It can be a confusing thing to do at first, and in reality if you can see an image through your scope without tweaking the optics, then the scope is working. It's got a "black eye", but it works.

However, if you do take the time to learn the process, then you will be making sure that the scope is working to its very best, and the scope is giving you the very best image it possibly can, which when you think about it you didn't by that scope for an ordinary image, but a good image. If the optics are not aligned, the image will appear smeared to one side, not sharp in focus, and the stars will not be pinpoints of light, but ballooned out*. Take the moment to align the optics, and the image will be sharp and clean.

I won't go into collimation here. Not when someone else has already done a fantastic job. Look at Astrobaby's collimation of a Newt site (http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/). It goes through the process very well and gives tips on helping newcomers to the process. That site also shows that you can carry out the collimation process with very modest gear through to as much money as you care to throw at it, but if you take your time and understand the process, that modest bit of gear will leave your scope performing just as well as any other bit of gear.

Some people get put of by Newt's about the need to look at the alignment or collimation of their scope every time they set up. Thing is a Newt's optics are held in place essentially by springs (for a reason), and vibrations or the set up and take down of a collapsible type scope will see things shift. Once you get the hang of collimating, it becomes something that you do out of habit and understanding. It will take only a moment, and this moment of TLC on the scope will leave it humming at its very best for you, which is what you want in the first place.

The purpose of this post is to help you see that there are a bunch of little things that go into using an maintaining a dob, and a Newtonian for that matter. Astronomical telescopes are not the same thing as a handheld pirate thingy. They are a bit more sophisticated because their application is a very niche one, and a VERY demanding one from the optics. It then comes to you on how involved you want to get :thumbsup:

Alex.

* Eyepiece selection can also be a reason for ballooned or bloated stars. I'll talk briefly about this next.

[mental4astro]

Eyepieces.

Do your recall the catch phrase for Valvoline, "Oils ain't oils".

Well, same thing goes for eyepieces with scopes.

Everyone starting out in astro, including me when I started, thinks:

"This is a scope. This is an eyepiece. It will work."

Alas, things are not that simple :rolleyes:

There's one popular misunderstanding about telescopes, that the image they produce is all focused on a single point. The WHOLE image is to a point. Well, that is true ONLY for a single star. Remember, there are stars littered all across the field of view, so now the FOCUS of a telescope is not entirely to a single point, but across a plane.

And it is not a flat plane, but a curved one, somewhat like a bowl.

AND the design of the scope determines if that bowl shape is convex or concave (what direction the bowl shape is facing) and how deep the bowl is.

Refractors, SCT's and Maks produce a convex focal plane. Newtonains (dobs included) produce a concave plane.

Eyepieces are designed to FIRST work to a particular shape of focal plane. Get the right eyepiece in the right scope, and you have an optical match, and the image is as good as it gets. Put the wrong eyepiece in the wrong scope design, and a whole host of aberrations will become apparent, and this is an optical mismatch.

As things are, it is easier and cheaper to design and manufacture eyepieces for scope that have a convex focal plane (refractors, SCT's, Maks). Designing and manufacturing eyepieces for Newts is much more difficult and expensive. There are extremely expensive eyepieces made for refractors too, and these are also much more sophisticated eyepieces.

What you will find in MOST reviews about eyepieces is see is totally dismiss an entire line of eyepieces based entirely on the use of one single focal length piece from a model line that was used in the wrong scope design. Most people do not understand optical matching, and so will see a bunch of aberrations that are all the clues to an optical mismatch, but out of ignorance will just say that the eyepiece, and that entire line of eyepieces, are rubbish...

Contemporary eyepiece design is a very complex one. As a result, from a line that is designed for a convex plane, there may be an individual focal length or two or even three that will perform just about flawlessly in a Newt! So don't make the mistake of falling for the "trash talk" about an eyepiece line. For from a line that is dismissed as "rubbish", you may find those outstanding and inexpensive gems! :)

What are those aberrations?

Astigmatism, field curvature, pincushion, chromatic aberration, spherical aberration, coma. Scary? No, not really. Just symptoms. This is a great site that explains these, and telescope and eyepiece optics (https://www.telescope-optics.net/eyepiece_aberrations_1.htm). What's great about it is it gives a simple explanation and then leads to the much more involved mathematics behind things ONLY if you want to pursue them.

~~~~

Yeah, I know I've banged on a lot in a thread about a 10" dob and wanting to see the planets more easily. But it's as good a place as any to get you thinking a little more broadly about things.

Alex.

[mental4astro]

Ok, you've got your mount tweaked, the optics are collimated, you have a short focal length eyepiece - what can you see in the planets with a 10" scope?

A hell of a lot!!! :D

The Hubble scope it may not be, but a 10" scope, and for that matter an 8" scope too, can show an amazing amount of detail, to a patient eye:

The polar caps and soft wispy clouds on Mars.

Masses of bands of clouds, dark and light, on Jupiter, the Great Red Spot, and the transit of the shadows of the Galilean Moons across Jupiter's face.

Not just the rings around Saturn, but also the distinct main ring bands with subtle shading within each band, the Cassini Division which is a gap in the rings nearly 1000km wider than the size of Australia east-west, the dramatic difference in the shading of the bands in Saturn's clouds, and the shadow of the body of Saturn cast back across the rings which will become increasingly more pronounced over the next few months.

A wee small greenish-blue disc of Uranus, and a tiny spec of Neptune whose blue colour is much to intense to be a star.

The changing phases of Venus - nothing much more as it is cloud covered.

Mercury, barely a yellowish tiny disc.

Like a bit info and cheats on viewing the Moon and planets? Have a look at this thread of mine: Observing the Moon and Planets - the good juice and cheats (http://www.iceinspace.com.au/forum/showthread.php?t=170511)

The sketch of Jupiter below was done using an 8" SCT, and the one of Saturn an 8" f/4 dob.

Alex.

[mcdrewsy]

Thanks for taking the time to share this advice Alex. I have the wax, metho and rag ready to give things an overhaul. I expect to be back in action Saturday night.

Mick

[ausastronomer]

Alex,
.........


If what you have said is true, why is that many eyepieces, including short focal length Pentax XW's (3.5, 5, 7 & 10), Nagler T5's and T6's, Televue Radians, Vixen LVW's, Nikon NAV HW's and Docter Optics 12.5mm all work equally well with excellent flat field views, in all telescope types, including Newtonian reflectors, refractors and Catadioptic Telescopes ?

Further, if what you say is true, why is it that my 14mm and 20mm pentax XW's show Field Curvature when used in both a Newtonian Reflector and in a SCT ?

John B

[mental4astro]

Simple.

The complex nature of contemporary eyepiece design allows for many modern eyepieces to be designed to work virtually equally well between different scope designs (variations in performance with individual focal lengths will happen across different scope designs). This is also testament to the clever designs that have been developed for these lines. The LVW's that were mentioned were a particular favourite of mine because of this reason. I have always said this too. If any doubt, I suggest one do a search for all my discussion about the LVW line. No misrepresentation on my part.

What all these eyepieces have in common is they are pricey.

Similarly, like I said in post No. 16, between EPs designed for a particular scope design, say a refractor, the majority of focal lengths in that line may not perform well in a Newt. However, there will be individual focal lengths that will perform very, very well in Newts. This is not to say that they will perform the same as a Nagler, but for a fraction of the cost the end user may be very happy with that more modestly priced eyepiece, even if the very edge may show a small amount of astigmatism.

What these eyepieces can offer is very good performance in a Newt for not so large a price tag - those inexpensive gems.

Some bedtime reading: https://www.telescope-optics.net/index.htm#TABLE_OF_CONTENTS

Discusses different scope designs with their pros, cons and inherent aberrations, plus eyepieces, their designs, aberrations and relationship with scopes.

[Oddity]

I’d like to think you’re joking but there was no wink sooo... in case you’re not:

PLEASE do not!!!! You’ll be on a fasttrack to a new mirror. It’s like taking steel wool to your eyeglasses. But worse. Much, much worse.

Here’s a useful link

https://www.skyatnightmagazine.com/advice/how-to-clean-a-reflector/

[mcdrewsy]

Yes, its a joke.

But now that I have taken it out of the tube I think I am up for a new mirror anyway. It's not grubby its stuffed.

.... but I was still able to find galaxies NGC 5128 and NGC 4945 in centaur the other day with a quarter moon up, so maybe I should just live with it.

[Bobbyoutback]

I think the OP would like answers to the following question :)

I'll refer to the objective size of the OP scope ' 10" F/5

Question ' If that mirror was of the same quality but figured to F/7
would it show better contrast on planets ?

Cheers
Bobby

[Ukastronomer]

Errr I think we all know he was joking

If not then I highly recommend THIS same method, works for video cameras as well as telescopes and binoculars

https://www.youtube.com/watch?v=lrdkFXsr5Us


.

[ngcles]

Hi Bobby & All.



The short answer is: Only if the secondary mirror was also smaller and therefore the central obstruction became smaller -- though there probably wouldn't be much in it and the difference would only be detectable on nights with good seeing. Because of the longer f.l, you can get away with a smaller secondary.We don't know the dimensions of the secondary mirror in this 'scope, but GSO 10" 'scopes are typically about 60-65mm for the f/5 and they are therefore about 25% obstructed.

Conventional wisdom says if the obstruction is kept below 20% its effect on contrast is negligible. A 10" f/7 could happily use a 40mm minor axis diagonal and the central obstruction would drop to just under 16%. Such a telescope, assuming a good figure on the glass and good collimation would be a superb planetary telescope -- almost refractor-like images. The extra focal length would also mean moderately high magnification (say x200) could be obtained with relatively simple eyepieces (like a 9mm abbe Orthoscopic for example) that would also offer reasonably comfortable eye relief. The f/5 to achieve the same magnification would need an eyepiece 5/7ths that of the f/7 -- about 6mm.

The downside to f/7 is that truly expansive fields (at very low magnifications) aren't going to be nearly as wide as the f/5. The other downside is the long and heavy tube about 1800mm would need a more substantial mount and the whole package would be less transportable.

Hope that helps.

Best,

L.

[Bobbyoutback]

Thanks it did help ! the longer focal will give better contrast transfer on the image ( planets )

Cheers
Bobby :)

[ngcles]

Hi Bobby,



It isn't really about the increased focal length, but more about the smaller secondary mirror. As I said, once the size of the secondary is reduced to below the 20% mark (by diameter, not area) its effect on visual contrast is shrinkingly small. Higher than 20%, particularly once it approaches 30% there will be a noticeable difference on nights of very good seeing. Probably 9/10 nights, it isn't really detectable but on that 10th night, you'll go "Oh"!

I know this is a bit off-topic but all commercial Schmidt-Cassegrainians have comparatively (to an Newtonian optimised for visual observing) quite large secondary mirrors -- in the order of 35% by diameter. Commercial Maksutov-Cassegrainians are similar-ish but come out at about 28-29%. Though there is a lot to like about both those designs, particularly when it comes to dimensions and portability, contrast on planetary images isn't their long suit.

Unfortunately (also), many of the less-expensive commercially-made Newtonians are sold with slightly over-sized secondary mirrors. I think it is because it renders the telescope a tad more forgiving of slight mis-collimation of the secondary mirror and also on the basis that a larger secondary is much more desirable for imaging and photography (even though 99.9% of those telescopes will never be used that way).

If you are prepared to put in a little effort, buy a smaller secondary and swap it in for the original it will make a slight improvement on nights with very good to excellent seeing. On deep-sky objects, it would be a virtually undetectable difference.

The trouble is, sourcing a size that is exactly what you want. Usually if you want "X" you have to choose between something like 0.9X and 1.2X.

Antares Optics (in the U.S) sell various sizes, and for the 10" f/5 GSO 'scope, the 54mm would be a good choice and provides a 21% central obstruction with a good-sized 100% illuminated field for about $100 USD. Unfortunately the next size down, -- the 46mm is probably a tad small and will result in a very tiny 100% illuminated field -- not optimal for observing deep sky stuff at low magnifications. You could overcome that to some extent with a lower profile focuser than the original equipment So you'd probably be better off with the 54mm. It would be a matter for the owner to weigh-up whether such an investment would be worthwhile.

In the alternative, BINTEL sell a GSO 50mm (in a perfect world I'd probably want 52mm) for just $49- AUD.

Best,

L.

[Bobbyoutback]

That was an excellent post !

Just a question regarding say a F/5 system verse a F/6 with both having the same aperture , the faster scope will need to be more accurately collimated ' may suffer coma & be harder on eyepieces plus with the larger central obstruction = softer contrast , would you agree :question:

Seems the owner of the scope below must have liked looking at planets :)

Thanks for the info about Bintels 50mm secondary .

Cheers
Bobby

[ngcles]

Hi Bobby & All,



Apologies to Yoda but there is no "more accurately" here, either the 'scope is collimated or not. There is no "try". An f/5 is no more difficult to collimate than an f/6, or an f/8 for that matter because the same techniques and/or tools are used. Yes, simple eyepiece designs are tolerated far better in slower 'scopes than fast ones -- they show fewer and less obtrusive aberrations.

Having said that, the slower system is a bit more tolerant of slight mis-collimation than the faster one. You would be hard pressed to pick the difference between slightly imperfect and perfect at say f/6, but at f/4, the difference is fairly obvious. Though I used a traditional collimation tool (Cheshire) until recently, I have now moved to a barlowed laser that is simply quicker and more convenient in use (particularly when you are on your own and the focuser is 3m away from the collimation nobs bolts).

I know there is a movement among observers toward faster (large) Newtonians and is is becoming increasingly common to see (particularly in the U.S) 'scopes like say a 24" f/3.5 or even faster. Mel Bartels recently finished a 25" f/2.8.

Yes, there are some attributes that make such a system attractive like having both feet on the ground while you observe (nearly all the time), not having to invest in tall, cumbersome difficult to transport ladders etc. If collimated well, and the 'scope holds collimation well at all angles, a coma corrector (like a Paracorr) can deliver very good images, well corrected over a wide field. There some drawbacks however (isn't there always)?

These very fast Newtonians do require a quite large secondary mirror and this will play a role in contrast degradation particularly on planetary images when the seeing is either very good or excellent.

Last year I purchased a 25" f/5 classic Obsession second-hand in the U.S and had it shipped out here. The guy who sold it to me used the money to put a deposit (yes, just a deposit) on a 24" f3.3 from New Moon telescopes that is now nearing delivery. That 'scope has a nearly 7" secondary mirror (yes, you read that right a 7"). The 'scope will only ever require one step to observe when at zenith (a very nice thing) but, (this is the down-side) is also 29% obstructed. The mirrors in these ultra fast 'scopes cost a lotta-lotta money once the f/stop becomes less than f/4 because they are so difficult (time consuming and labour-intensive) to figure and parabolise. But, horses for courses, he lives in an urban environment and all his observing involves a 45 minute drive + set-up. So I can understand (even if I don't agree) with his choice.

Okay, the 'scope I bought is probably a bit longer in focal length than I'd want. If I could have everything and money were no object I would manufacture something bespoke for myself, I'd pick a 25" f/4.3 because you can still use a 4" secondary mirror and the central obstruction is kept below 20% **and** you don't absolutely need a coma corrector at that f/stop (It would probably be desirable though) ... and you're one or two steps down the ladder than f/5.

The other thing is, that being a truss-tube dob (getting back to that "tolerant" thing), f/4.3 while still pretty twitchy collimation-wise is quite a bit more forgiving than f/3.3. Truss-tube dobs unless they are built like battleships do suffer a little bit of sag at different angles of elevation. For f/3.3, it would need to be as tight as a drum to be collimated at all angles.

But, given my own circumstances, the 'scope I purchased (and it is the first telescope I "bought" in my life) doesn't need to travel (happily, I live under excellent skies), it stays permanently assembled and just rolls out to observe. I spent a few hundred dollars on a sturdy, well-engineered ladder so I feel quite secure even four and five steps up in total darkness. One very nice thing about f/5 is that the secondary mirror is just 3.5" resulting in a teensy-weensy 14% central obstruction. Save for diffraction spikes on bright stars, the images are genuinely refractor-like. At f/5 coma is only an issue at the very edges of low-power fields without a coma corrector.

The other nice thing is this: I landed this 'scope that was almost brand-new (had only been assembled and used a half-dozen times) with every Obsession-extra, bell and whistle etc (Nexus DSC + Servocat etc) (Galaxy mirror 0.94 Strehl ratio tested and certified), shipped to Australia, tax paid for ... $17,200

The f/3.3 New Moon 'scope (brand-new) he bought cost all-but $30,000 AUD + Servocat + DSC + import taxes and fees + shipping etc -- so about $41K AUD if delivered here.

I'm happy to walk up and down a few ladder steps for the rest of my observing career for that magnitude of saving.

Before I end, it would be remiss of me not to commend to all readers the excellent advice provided by Alex Massey above that undoubtedly took some time to prepare and type it. It is all good stuff and well worth knowing. These relatively inexpensive Chinese or Taiwanese Dob/Newtonians are really quite good value for money, can be improved upon and made to work even better very cheaply and generally provide a whole lot of bang for buck. As you can tell, I am a fan of Newtonian telescopes -- I make no apology for that. They really are a great choice for both beginning or advanced/expert observers alike.

Best,

L.

[croweater]

Excellent post Les. Is there an equation to work out the smallest secondary mirror size possible for a certain aperture and focal ratio without losing light. Cheers, Richard.

[Bobbyoutback]

Thanks L ,
Another top post !

Regards
Bobby

[ngcles]

Hi Richard & All,



The best advice I can give you is to obtain the long booklet "Newtonian Notes" by Peter Francis that speaks about and gives examples of calculations pertain to this sort of thing. Once upon a time (back 30-40 years ago) nearly every astronomical society had people who knew and understood this stuff thoroughly, but like the sub-editors at the newspapers, they have largely disappeared.

Woo hooo !! I found a .pdf of this handy booklet on the web:

http://www.alpo-astronomy.org/jbeish/NEWTNOTE.pdf

There used to be a freeware called Newt that was like a CAD for Newtonian telescopes and calculated all this stuff automatically and you can see that by altering this, that or the other parameter, what effect it had on the size of the 100% field, 75% field and whether the focuser or the secondary was vignetting the telescope, secondary mirror offsets (if desired -- I don't bother with that -- I centre everything).

Hold on, I found it! It's now web based and is here:

https://stellafane.org/tm/newt-web/newt-web.html

All this stuff is one of the nice things about actually sourcing components for yourself and assembling a Newtonian telescope that best suits your own observing (and other needs regarding portability) needs -- you can, by varying a few components, f/ratios, focusers, secondary sizes etc etc, optimise the telescope for visual observing (all-round) planetary observing or richest-field type telescopes -- or indeed photographic Newtonians. There are a few reasons why some other designs (R/C or other exotic astrographs) (in a perfect world where funds were unlimited) work better or at least more easily than a Newtonain photographically, but they can still be quite good for photography too.

Best,

L.

[croweater]

Thanks for the links Les :thumbsup: Realy handy. Cheers, Richard.