#21  
Old 09-08-2019, 10:10 PM
Oddity (Andrew)
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Originally Posted by mcdrewsy View Post
I reckon my mirror needs a clean too. It's grubby. Maybe some steel wool will do the trick?
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/a...n-a-reflector/

Last edited by Oddity; 09-08-2019 at 10:22 PM.
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  #22  
Old 10-08-2019, 06:24 AM
mcdrewsy (Mick)
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Originally Posted by Oddity View Post
Iíd like to think youíre joking but there was no wink sooo... in case youíre not:

PLEASE do not!!!! Youíll be on a fasttrack to a new mirror. Itís like taking steel wool to your eyeglasses. But worse. Much, much worse.

Hereís a useful link

https://www.skyatnightmagazine.com/a...n-a-reflector/
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.
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  #23  
Old 10-08-2019, 06:35 PM
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Lets sort ideas please .

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
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  #24  
Old 10-08-2019, 09:02 PM
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Ukastronomer (Jeremy)
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Quote:
Originally Posted by Oddity View Post
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/a...n-a-reflector/
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


.
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  #25  
Old 10-08-2019, 09:44 PM
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ngcles
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f/5 -v- f/7

Hi Bobby & All.

Quote:
Originally Posted by Bobbyoutback View Post
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
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.
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Old 10-08-2019, 11:25 PM
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Originally Posted by ngcles View Post
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.
Thanks it did help ! the longer focal will give better contrast transfer on the image ( planets )

Cheers
Bobby
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  #27  
Old 12-08-2019, 11:07 AM
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ngcles
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Hi Bobby,

Quote:
Originally Posted by Bobbyoutback View Post
Thanks it did help ! the longer focal will give better contrast transfer on the image ( planets )

Cheers
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.

Last edited by ngcles; 13-08-2019 at 11:34 PM.
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  #28  
Old 13-08-2019, 03:40 PM
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Originally Posted by ngcles View Post
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 passes 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.
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

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

Thanks for the info about Bintels 50mm secondary .

Cheers
Bobby
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  #29  
Old 13-08-2019, 09:40 PM
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ngcles
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Collimated -v- Collimated

Hi Bobby & All,

Quote:
Originally Posted by Bobbyoutback View Post
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

Cheers
Bobby
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.

Last edited by ngcles; 13-08-2019 at 10:54 PM.
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  #30  
Old 13-08-2019, 09:54 PM
croweater (Richard)
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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.
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Old 13-08-2019, 10:21 PM
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Thanks L ,
Another top post !

Regards
Bobby
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  #32  
Old 13-08-2019, 11:12 PM
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Hi Richard & All,

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Originally Posted by croweater View Post
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.
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.
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Old 14-08-2019, 12:22 AM
croweater (Richard)
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Thanks for the links Les Realy handy. Cheers, Richard.
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