Dob Trumps Refractor

[glend]

I spent a couple of days up at Bretti Reserve this week and left the dob at home this time to give my Bresser AR102L, f9.8, 1000mm fl refractor a good go at a dark site. This refractor is identical to the Explore Scientific model with the same AR102 designation, but sold mainly in europe through Bresser (Meade EU). The scope sits on a Vixen Porta II Alt/Az mount with JMI Train-n-Track motors.

Useful magnification is around 200x given the usual refractor edict of 2 times the aperture in mm.

The two Allan's were up at Bretti as well so I had some large dob perspective on the views for comparison.

I was mainly aiming to give the planets a real go: Jupiter early on and then Mars and Saturn later in the night. The views are great for contrast and lack of coma, and of course no need for collimation at all. So in many ways more pleasing a device to use. But as with all but the longest Acrhos there is some CA to deal with, but this was ably handled by a Baader Semi-APO filter, and being a long focal length scope it was not very noticable anyway.

While I could get good views of the three planet targets I was really pushing that refractor. The relative size of the object in the EP just did not compare to that of the dobs, and Maritan detail in particular required at least 182x with the trusty 1mm Nagler. Barlowing just made the image quality worse.

Last night back at home, I got out my 16" GSO Dob and did the same comparsion again. The big dob killed the refractor on every level. It is perfectly collimated, and edge coma is never an issue with planets anyway. The sheer size of the panet in the EP and the brightness could never happen with the refractor because of the small aperture. Object detail was outstanding with the dob, picking up things the refractor could not see. Contrast was on par with the refractor on planets but for pin point stars it is hard to beat a refractor - but there is the rub: not enough aperture to make it really useful for DSO work. The reactor can resolve bright double stars (or triples like Beta Moncer..) well but it has no real reach. Even for widefield star work on clusters and Eta Carinae, the refractor can't provide the expanse of view that the dob has.

Of course it's not a fair comparison I suppose, 102mm verses 406mm gives you heaps more light to work with, and the refractor did well to stay in the ballpark with the dob albeit with a much smaller image to work with at its useful magnification limits.

Not sure where this leaves me with the refractor, it's a great grab and go setup but it leaves me wanting more aperture. I am building a 127mm refractor at the moment but I suspect it will still fall well short of the dob in pure breathtaking observation.

So why do people buy refractors (maybe they live in small apartments), or do mostly guided astrophotography on expensive mounts. But for stunning visual vistas, it's the dob hands down.:eyepop:

[jenchris]

I don't mind a newt, but I like goto - (I know you can get Dob gotos).
It's always horse for courses - I'm off to UK in a fortnight for 6 months - I'm taking the 102 skywatcher with me - since the 8" SCT on an EQ6 is not going to pack too easily (!!!) especially considering the tripod and counter weights...

[glend]

I almost did buy the Vixen NA 140SS but decided to go with a DIY build using the iStar 127mm f12 R30 objective.

I was not rubbishing refractors, rather praising good dobs. 'I come not to bury Ceasar but to praise him'.

Refractors continue to be the most expensive scope per inch of aperture, and exotic glass formulations designed to improve performance do not help change that equation.

I grant you that if I was comparing a 102mm refractor and a 102mm dob the refractor would win hands down but if you compare equivalent visual value propositions the dob will always win. A 16" dob cost less than a good 4" APO refractor, and the dob comes with a usable mount.

[Profiler]

Have to also keep in mind there are refractors and then there are "refractors"

I think the quality differences in terms of glass, design and craft in construction really come to fore when you start pumping up the magnification on targets such as planets etc

[sn1987a]

True,true.:P

[Profiler]

:rofl:I agree with all of this.

I am not trying to defend refractors but if I have one gripe about this age old question which keeps on coming up is that whenever arguements about how reflector X beats refractor Y the advocates of such arguments always seem to be comparing modest refractors and don't actually have any or much experience with "refractors" at the top end.

For example, I haven't yet come across someone who says "I used an AP Starfire for 5 years but then I sold it in a heart beat because I got fantastic/better views etc etc from a 10' Dob/Newt":rofl::rofl::rofl:

[Steffen]

Their respective cost should provide a valid frame of reference.

Lately on IIS threads about beginner scopes tend to descend into boutique gear boasting, along the lines of "my $15k ortho APO on its $5k+ mount shows much nicer stars and almost as much deep space as your 8" Dob. That is about as valid as saying "my giant $20k SDM smokes your SkyWatcher 80mm achro"...

Cheers
Steffen.

[David Niven]

Refractors owners may not like or admit it but IMHO, the aperture per Dollar is the starting point to compare value. Then you can talk about contrast and sharpness and CA or lack of it.
A piece of glass, no matter how well made or what exotic material it is sculptured from, cannot create photon and without sufficient photon, you are absolutely in the dark.

[PlanetMan]

Problem is cost is not being cited as determinative variable - simply that folks with AP refractors don't bother with reflectors - everyone knows mirrors are cheaper than refractor glass

[Steffen]

That is my impression, too. For what reasons, I don't know, but I suspect they're not all entirely rational. If you have $20k to drop on a visual rig, why would you choose a 6" instead of a 24" scope? You don't hear people say they chose a TOA150/EM200 over an SDM because of space constraints, but because the Tak is presumably "the best" :screwy:

Cheers
Steffen.

[cometcatcher]

Mmm 16 inch dob beats 4 inch refractor. Who would have thought? :P My 16 inch dob also slays all my refractors and smaller reflectors except.... when the object is really big or really bright.

I remember the disappointment at seeing the Helix nebula through the 16". I had a hard time finding it. The light was just too spread out to see properly. It looked better in binoculars! Of course, the Sun and Moon are more convenient in a small refractor as are comets.

[N1]

Doesn't that depend on the size of the DSO and the exact nature of the "work"? I've looked at M31 through 10 and 12 inch dobs versus a small fast 'frac and was somewhat underwhelmed by the "improvement". More detail? Yes. Different? Yes. Better? No. Each has its merits. Every DSO has a context. And some DSOs are just too big for big dobs. To me the key components are exit pupil and FOV. They are the end result the optical train provides and all that matters. Aperture is just a means to that end and only needed if you want high power. Power is a matter of taste. I do want to be able to view at high and low powers, do so efficiently and enjoy the advantages each has to offer. The FOV a dob provides at 15x is not good enough - that's if you can find an eyepiece for this in the first place. A big dob is as useless for low power as a small frac is for high power, IMHO. Please correct me if I'm wrong here.

The statement you make re the "expanse of view" that a refractor can't provide as opposed to the dob - Is that really the case when a fast refractor is used?




I have bought a small refractor:

- because I do not do AP and want stunning visual vistas. For that, a wide FOV is key. Even with my 1.25" back, I can take in over 4 degrees of sky.

- because I do not want an expensive mount. It sits on a photo tripod.

- because I want viewing comfort. I observe best standing up, feet on the ground, line of sight horizontal. The refractor setup provides exactly that, no matter where it's pointed. Viewing comfort means more detail seen at lower powers.

- because I want ultimate speed & portability. The fastest I have set up my visual rig ready for pointing was 33 seconds, out of the boot of my small car (Loading the gear into the boot at home: 10 seconds). Cool down time: Nil. Try to beat that with a 16" dob.

- because I want flexibility. Spotting Syrtis Major from the lounge room? Possible. Observing Jupiter while cooking dinner? Piece of cake. Counting feathers on that Rosella in the garden? Easy.

- because I chase eclipses (some). The Sun doesn't care about aperture unless you want the finest of details. Plenty of light for everyone. The Sun in Total Eclipse through a low power scope - I won't even try to describe it. Outside of the moon's shadow: White light and H-alpha, both excellent.

- because I want to observe planets during the day. What's the current phase of Venus look like? 33 seconds - you get the idea. Try moving a light bucket along with the moving shadow of a building to stay safe when the target is close to the Sun.

- because of the Moon. It's there on most nights. No other object shows that much detail. Again, plenty of light.

- because the best scope is the one used most often.

And finally, I have bought a small refractor because I can always use a dob for the higher magnifications. The 10" has recently arrived and is waiting to be put to work :party:

[julianh72]

+1

[Larryp]

I'm afraid I'm a refractor man through and through. Only high quality ones, though.
The views that knocked my socks off in over 30 years in this hobby have always been through a high end APO, like an Astro-Physics, Tak, or Televue.
I have never looked through any Newtonian that I subsequently wanted to own-apologies to the dob owners, but that's how I feel!

[jenchris]

Nicest scope I ever looked through was a 20" homemade dob.
I nearly fell into space..through the eyepiece.
Blown away is a phrase that spings easily to mind.

[Renato1]

Different telescopes for different uses. I suspect you would have a hard time splitting the tiny companion star from Antares with the big dob.

Who is to say that that particular view is a lesser view than seeing a tiny smudge of a galaxy in the big dob, which would have been invisible in the refractor?

And it is extremely nice seeing pin point star images in big clusters with the refractor.

That said, I only ever took my 4" f/9 APO to a dark sky site once, felt like you did, and it bacame a backyard telescope.
Regards,
Renato

[clive milne]

Well... now you can say that you have come across someone who used to own a 7" AP Starfire (one of the originals with the NASA glass) and without prejudice I sold it because my home made 12" dob was significantly better in every respect bar 1. The view through the refractor was slightly less contaminated with light scatter. This advantage was only relevant 0.5% of the time (trying to resolve Antares)

In my experience however, absolutely nothing comes within a bulls roar of a well built binocular Newtonian for impact and image fidelity... the 7" AP wasn't even on the same page, it was not even close to being included as an honourable mention in the foot notes.

2c

[Satchmo]

Hmm ..as the owner of three refractors and two reflectors I would have to say in my experience this is not true unless you frequent Newts with poorly cooled , poorly collimated optics of dubious quality . I'm afraid my Newts trounce the refractors on every task I can offer and I would expect them too .

If I want to see perfect mathematic stars on any night I can put a small aperture stop on the Newts to cripple their ability to resolve the atmospheric turbulence ( and any other target for that matter ) and I can drop the magnification down to mimic that in the smaller refractor. Win - pinpoint stars . It is a `myth ' that a smaller aperture scope will show _more_ than a lerger one at any time ...all the detail will be there at least as much as the smaller aperture and maybe not as clean but it will be there.

You can't beat the portability , freedom from collimation responsubility and dust seal of a smaller refractor but that comes at a big cost considering you can buy a good 8" dob for $399 which will probably be superior for most tasks .

I don't feel threatened in these discussions because I own them all but I don't like to see Newts maligned as they are so effective at such a lower cost .
I dont like to see refractor owners over aggrandizing the capability of their instruments. Refractors work so well for some people but please don't peddle nonsense about Newtonians .

[Satchmo]

Its funny you should say that - but I have related on this forum before of the night at ASNSW 's Wiruna property when Clives 7" Starfire stood unused with a crowd of people viewing Jupiter through an older stock 10" F6 Dob circa 1980's that stood next to it . There were a load more features visible as there was now colour differentiation between festoons in the bands. The planet disc in the 7" was sharp but all the bands were gray in fact all the detail was fairly black and white compared to the Newt. It was just plainly a more satifying view in the 10"

That 7" Starfire was a masterpiece but so long as the optics are good quality aperture simply rules - no free lunches there...

[Varangian]

One of the greatest joys of owning a reflector is collimating it. I just feel a great sense of grounding when my mirrors come together in perfect harmony = beautiful experience.

[barx1963]

Actually, that is true! I get a lot of satisfaction from getting my dobs just right. Knowing I have contributed even in a small way to the superb image in the eyepiece is a pleasure in itself. And doubly so with my little 8" where I designed and built the collimation system myself.

Cheers

Malcolm

[Varangian]

Yes indeed! I use a film cannister with a little hole in it and then do a star test. Loads of fun.

[ausastronomer]

Then you've never looked through a good newtonian that's been properly cooled and collimated. I have used some of the best refractors on the planet, including a Tak TOA 130, a Tak TOA 150, an AP 160, a Televue NP 127 and a 15"/f12 D & G, just to name a few. Just this past week I have had a few views through a Tak TSA 120 at Coonabarabran. All of these scopes have provided excellent views. None of these views have convinced me I should swap one of these scopes for one of my 3 newtonians, all of which are premium scopes.

Not even a close contest really. When both scopes are properly set up and cooled a larger aperture high grade newtonian is going to beat a smaller aperture high grade refractor every time. When one considers cost, a high grade 10" newtonian will cost a lot less money than a high grade 6" refractor and easily outperform it.

Cheers
John B

[Renato1]

The spider vanes - if one has the standard four ones - limit the size of the clear aperture of the stopped down reflector. And it is still affected by tube currents some of the time. And one optical expert did tell me that there was some deviation from using the stopped down mirror, which made the result slightly different in the reflector from using an APO refractor (can't remember exactly what it was).

And on the few occasions I tried using an aperture mask on my 14.5" dob, I was unable to split Antares - but that is nothing conclusive, as I could only achieve splitting Antares with a 4" APO refractor on every 2nd or third night's attempt (the sky had to be really steady). Have you managed to cleanly split it with your dob and aperture mask?

That said, the best views I've ever had of Jupiter and Saturn were through a club member's 10" Dob which he had built himself with the best possible parts, and a Panoptic and Powermate. He had a Parks mirror in it, and the tube was made of octaganol wood. I was in astonishment as he pulled it out of the back of his station wagon, set it up on the ashphalt and aimed over various school buildings at Jupiter and Saturn - no cooling down time required - to deliver the sharpest most detailed images imagineable. But then again, his mirror alone cost more than my entire 14.5" dob.

You are very right though about people putting down reflectors unreasonably. My very first telescope was a 4.5" Focal reflector from Kmart, and it still delivers better images than my 4" and 6" Maks do - which I am still struggling to collimate each correctly to give me a view of planets akin to those of that reflector.
Regards,
Renato

[clive milne]

The attainment of perfect collimation in a Newtonian is an ideal... but it is not necessarily a prerequisite for an an excellent visual experience. All it means is that the diffraction limited portion of the view is centred in the eyepiece field stop. You can be slightly off and still see a perfectly sharp image in the eyepiece (albeit with the sweet-spot de-centred). Refractors are different in that the magnitude of off axis aberrations are generally better controlled (mostly a function of the focal ratio) and the mechanical requirements of lens assembly necessitate an ota that stays orthogonal. If you were to throw $10k at a 6" newtonian you could also make it impervious to collimation errors.
To reiterate... the primary advantage that refractors have over any other optical format is that they scatter slightly less light across the image rendering what can be perceived as a slightly more 'pure' view.
However, I disagree with the maxim that implies that the merit function of refractors is intrinsically superior to reflecting telescopes as a priori. It simply doesn't stand up in the real world .... at all.

[ausastronomer]

That is simply not true. I have cleanly split Antares on hundreds of occasions in newtonians from 6" to 25", which is the sizes I have tried on it. The mirror needs to be cool, the telescope needs to be collimated, the optics need to be decent, the seeing needs to be decent and Antares needs to have reasonable elevation, to achieve a clean split. I have also split it in high quality APO refractors down to 4". The smallest being a Takahashi FS 102. The same type of conditions apply when splitting it in a refractor.

Two nights ago at Coonabarabran under reasonably good seeing we split Sirius at 300x in my 18" Obsession and Antares at 200x in my 14" SDM. These were at different times. Had I swapped the scopes around the 14" would have split Sirius easily and the 18" would have split Antares. My 10" SDM has split them both under favourable conditions on many occasions.

I first split Sirius many years ago, when it was infinitely harder due to the closer separation, in my 18" Obsession, under excellent conditions at 1050x (5mm Pentax XW + 2.5x TV powermate).

You should try 1050x in a refractor, (I am sure a 6" APO can pull 175x per inch of aperture) :D, it does an excellent job of showing extended detail in targets like the homonculus and planetary nebula like the Ghost of Jupiter (NGC 3242).

Cheers
John B

[astro744]

10.1" F6.4 Suchting Mirror, Clave Plossl eyepieces, evening twilight; piece of cake!

This was the first time I ever split Antares and was surprised how easy it was. A good mirror and eyepieces help but I think it was the twilight that made the task easier.

I have Newtonians, Refractors & and SCT. I have looked through both 7"/f9 & 6"/f12 Starfire, 9"/f15. At one point I was convinced the 7"/f9 was simply the best ever telescope. It was mounted on top of a 16"/f5 Newt/Cass that simply never produced images worth looking at mainly due to local atmospheric disturbances and city light.

I love the pin point stars and and flat field of my 2.4" & 4" refractors and the fact that I can get well over 4 degrees of stunning star fields. Planets are very sharp and contrasty but I often want a brighter image as I simply run out of exit pupil diameter; that's how much I can push the refractor.

I love my 9.25" SCT because it is compact and yet has ample light gathering and good focal length. Higher powers with bigger eyepieces is nice.

I love my short 6" Newt when combined with Paracorr delivers a near 3 degree field of pinpoint stars that rivals that of the 4" refractor. One day I'll do a side by side comparison.

I love my 10.1" Newtonian because it is the largest aperture I own and gives me the brightest images of deep sky objects and also gives me plenty of light when pushing up the power on planets. The mirror can certainly take it. This telescope is also a great galaxy buster not because it is a big Dob but because it has a great mirror that gives me excellent contrast and lets me pick up very faint galaxies.

I have seen Jupiter in a 16"/f5 Suchting mirror that was better than any view I had ever seen of any planet in any of the above mentioned telescopes including that of the 9"/f15 refractor. However the big refractor had tracking and a good long focal length and looking at a 'big' image of a planet for extended periods without having to nudge the telescope has considerable advantage.

There is no one perfect telescope and after 30 Years I have acquired a few different types. I'm yet to own a large reflector but I am having so much fun with my TV-60 that I don't think I really want and definitely do not need one.

[Larryp]

To me, visual astronomy is all about aesthetics, and as I said, the views that have knocked my socks off over the years have all come from APO refractors.
You like reflectors and I like refractors-its as simple as that! I get a bit tired of this "mine's bigger than yours" argument :)

[N1]

Yes, the discussion has reached a point where even this argument would seem valid:

KECK TRUMPS DOB

where does that leave us?

[cometcatcher]

Hubble trumps Keck. :P

[julianh72]

James Webb trumps Hubble

[barx1963]

Not yet it doesn't!!

[N1]

:rofl::nerd::screwy::doh::poke::P:l ol::hi:

[sn1987a]

My Suchting, Zambuto and Kennedy stand ready to meet and take on any puny Refractor :P

[Renato1]

Thanks John, I did say "suspect" and am happy to be corrected. The best views of Jupiter I've ever had with any of my telescopes is through my 14.5" dob, but that's only been on a couple of occasions at the end of a four or five hour observing session, and in the middle of rural fields. Unfortunately, I can't do those sessions any more, and have never been able to get anything sharp from my old suburban backyard or current semi-suburban backyard. Even my C8 struggles where I now live - after six weeks of lousy views of Jupiter, Mars and Saturn, I had a friend pop in who I showed them to, and magically, the C8 worked perfectly for a change.

I'll try 175X per inch in my 4" APO when I remount it in a few weeks, though given seeing conditions here, I'm not optimistic.
Regards,
Renato

[Renato1]

Certainly you are a proponent of different telescopes for different purposes. Thanks for sharing your star splitting experience. I'm still fascinated with the concept of the 7" telescope mounted on the 16" one, and the 7" one being the superior performer.

Yes, one can see thousands of galaxies with a 9.25" and 10" telescopes, one of the delights of life. Oddly though, on occasions I was in really dark places in central Victoria for work, I've been quite amazed at the views with the performance of my 25X100 binoculars, where each sweep across Virgo was showing dozens of tiny galaxies that stood out like sore thumbs. I never got that effect with my small travel telescopes.

Fascinating to hear of your enthusiasm for the TV-60. I have a little WO 66mm refractor I bought for camping (when all my wife's stuff left me with no room for my 80mm in the boot), and it was quite a lively telescope out in the country, but I must confess to not having used it at home. You have piqued my interest.
Regards,
Renato

[astro744]

See at the bottom of http://www.questacon.edu.au/science-communication/international-engagement/questacon-and-japan/gifts-from-japan

The observer is looking through the AP 7"/f9. The orange one on top is a Celestron Comet Catcher. Not the best picture for showing off the refractor but you can just make out the tube assembly.

I worked there for many years many moons ago well before the fire in 2010 which destroyed the observatory & planetarium. I haven't kept in touch and am not really sure if any of the telescopes were saved.

[cometcatcher]

I just dug out my 40 year old Tasco 80mm F15 refractor for some solar and lunar observing. Not sure why. Think I might be going senile in my middle age. :screwy: Oh yeah that's what it was, I want prime focus to fit in the field of the APSC sensor without a barlow and for the Sun and Moon I don't need aperture. But man, I forgot how loooong this thing is.

[N1]

Sure?

Here are some candidates:

Suchting:
http://www.cfa.harvard.edu/hco/grref.html

Zambuto:
http://en.wikipedia.org/wiki/File:Wilder.jpg

Kennedy:
http://en.wikipedia.org/wiki/File:Potsdam_Great_Refractor.jpg

:poke:

:scared3:

[sn1987a]

Ha!, it would be a joy to be smashed by those. :D

[doppler]

It should be noted that these old clasic big refractors are all constructed at the optimal f15 focal length. I think this is where the difference between modern refractors and newtonians becomes muddled, most are made to be less cumbersome by reducing tube length and therefore f ratio. 30 years ago long tube f15 refractors were the norm, and 8", 10" and 12" newtonians were standard at f6 and f7. You have to get scopes with these longer focal lengths custom made these days.

[Renato1]

Thanks for the photo. It sure looks substantial.
Regards,
Renato

[N1]

Are you suggesting that their FOV may be a bit narrow? ;)

[doppler]

I think that with achromats and newtonians image quality improves with focal length. Visually this is not as obvious, but photographically the difference is very noticable. The increase in CA is easily seen between my f7 and f5 achromatic refractors and although the image is much brighter in my 10" f4.5 dob than my 8"f6 newtonian the coma in the 10" is really noticable.

[strongmanmike]

As an owner of one of the worlds finest commercial 6" APO's - an AP Starfire 152EDF which I have done some good observing time with under dark skies before relegating it to it's true calling (astroimaging) and having spent many hours at Wiruna at several SPSP's with high quality 12", 16", 20" and 24" dobs I can happily say the overall viewing experience through all the dobs left the views through the Starfire for dead - end of story :D

Of course as an astrograph the Starfire rocks and as mentioned gives text book perfect crisp stars and great contrast :thumbsup:

Mike

[astro744]

I couldn't agree more and I'm glad you used the term 'High Quality' as this is important both for planets and deep sky as a good mirror will perform better on all objects.

[sn1987a]

Took the 28" f4.2 out last night. Peter Read had sprinkled his magic fairy dust on it a few days earlier. Magnificent viewing of Mars and Saturn about 1 hour east of Perth, best views of those planets ever. Air was cold and still with some dewing. Mars was like looking at Gorbachovs head, the dark areas looked like continents on a pink ocean, ice cap had a black eye. Saturn was awesome, the crepe ring was ash grey I swear I could see the planet through it, best encke ever. I'm ruined, all nights from now on will compare this one. Also split Antares for kicks Antares B a lovely green. I wish I had witnesses.

[strongmanmike]

Yeah...sounds exactly like a night with the 6" Starfire :lol: (not)

Sounds like an awesome scope Barry

[Allan]

Just waiting for the invite Barry. :D

[sn1987a]

Anytime Allan, the bigger the Dob the bigger the showoff!

[OICURMT]

I wonder why modern professional scopes are all mirrors? :question:

[MrB]

Next time! :D

[barx1963]

Not all are reflectors but certainly the vast majority are. Simply put it is much easier with large optics to make a mirror that is supported underneath than a lense that is only supported at the edge. The largest refractor is the Yerkes 40" in the US. Any larger than that and the lense sags under it's own weight and is not useable.

Malcolm

[brian nordstrom]

;) My Gawd! you large reflector owners are too much :lol: .
Go ahead and keep patting your selves on your backs ( like a winning politician :help:).

Its like my sweet 5inch f8 istar refractor TRUMPS ! my 50mm finder ,, you know I think its the 5x the aperture that gives it the advantage ;) , like Barry's 28 incher will do to my refractor that's what 5x plus aperture will do and yes mate I will be a witness to your report , I look forward to that Barry .

Look at these photo's , I was set up looking at the moon and Venus in under 10 miniutes , the 20 and 22 inchers were ready about 30 miniutes after this photo was taken ,,, if lucky .
Note the 2 man lift in the background in the 1st photo ? not much fun at 3am after a 9 hour session :D , but I stayed and helped these mates strip down .
Yes these 2 monsters perform , but at a price and in Darwin its very hot and humid at 3am , horrible .


Aperture rules ,,, sometimes .....:thumbsup: .

I like Laurie just like the way a refractor shows stars , the planets and moon , at the 200-250x , and that's about the atmosphere will allow 90% of the time .

I did have my (sold) Tak M210 at 806x on Saturn one night and will never forget it ,, but it was driven , I can only imagine what 1025x on a DOB was like ,, 4 seconds between nudges ? :rofl: .

Brian.

[Kunama]

Because they are in an observatory all set up all the time, they just flick couple of switches and away they go.

Most amateur "dobs" would be lucky to be used once a week due to the hassle of setting them up.

[ausastronomer]

If that was the case those telescopes are a very poor design, or your friends are painfully slow at setting their scopes up.

Here is a link to Alan Dyer's image of the observing field after the scopes were set up at last weeks Ozsky event at Coonabarabran.

https://www.flickr.com/photos/amazingsky/13574066573/

Lachlan McDonald and I were the first to set up scopes.

Lachlan and I set up

30"/F4.6 SDM
25"/F5 Obsession
3 x 18"/F4.5 Obsession Classics
18"/F4.2 Obsession Ultra Compact
14"/F4.5 SDM

It took us less than 70 minutes to set up those 7 telescopes, which included pegging down tarps under my 2 scopes.

Andrew Murrell arrived a bit later and it took Andrew just over 30 minutes to set up his 25"/F5 Obsession and his 18"/F4.5 Obsession.

Your friends obviously mixed a lot of talking in with the setting up.



My dobs all track very nicely at high powers. They all have GOTO and Servocat Tracking. 1050X in the 18" Obsession is a walk in the park.

Cheers,
John B

[ausastronomer]

Hi Matt,

That's not necessarily always the case. My 10" and 14" SDM's are left fully assembled in the garage all the time. It takes me less than 3 minutes to wheel a scope outside, set it down, drop the wheelbarrow handles out and connect the power supply. Another minute to fine tune the collimation and I am observing in less than 5 minutes from the time I open the garage roller door. Similarly, it takes me less than 5 minutes to pack the scope away at the end of the night. The biggest chore of the setup and pull down is carrying the eyepiece case in and out of the house.

Cheers
John B

[PlanetMan]

Bravo - Thanks for proving my point - although the crowd was looking at the dob I suspect "Clive" wasn't going to be parting with his AP despite what may have occurred on this one anecdotal experience

[sn1987a]

Yep me too, I leave them set up ready to go at the drop of a hat. I've got the 16 and 18 loitering behind me and the 28 is out in the yard right now tracking the moon waiting for Mars and Saturn later on. I have modified the back door and can wheel in/out the 28 fully assembled. Its not that hard to set up, wheel out the mirror box on wheelbarrow handles, attach truss poles, attach truss ring, attach uta, hook up cables, collimate, go. Piece of piss. :P

[MrB]

See post #17 of this very thread ;)

[sn1987a]

Easy! :D

[barx1963]

My 20" is a sort of Obsession style scope but uses many older design features that do slow down the setup. Nevertheless I can be up and running inside 20 minutes if I don't muck about too much. My record is about 13 minutes including collimation. I have nowhere to store it setup so have to wheel it out and put it together each time.
I have seen Erick setup his 12" SDM in about 3 minutes, plus about another 5 to finish off plugging in cables etc for AN, Servocat and dew heaters.

Luckily the views make it absolutely worthwhile!!

Malcolm

[N1]

Dude, that's one small house you've got there :P
Great instrument though. Quality skies would be critical to use that thing to its full potential. Must be pretty good where you are.

[astroron]

I am sorry to say this, But, every time I see the heading of this thread
" Dob Trumps Refractor"
I cannot help thinking it is about Donald Trump:rolleyes:
Sad isn't it:(
Cheers

[sn1987a]

I couldn't afford a big house I spent all my money on scopes :sadeyes:

[N1]

Agree. It should be called "Dob Complements Refractor" :astron:
Not sure if that Trump guy would know anything about scopes. Or astronomy. Or physics. Or science.

Incidentally there is no smiley icon showing a dob on here. :shrug:

[MortonH]

While I've looked at a few deep sky objects through large Dobs at star parties I've never seen high-power planetary views through one. I'm curious how often it's possible to use the 1000x powers being talked about here.

Not trying to be controversial - if anything I'm wondering how much time to spend pestering big scope owners at IISAC :D

[Steffen]

There is usually a night sky tour at IISAC, open to all who are interested. Last year we enjoyed some great views through an 18" (I think), with Argo Navis and ServoCat to make things easier with the crowds.

With Mars close by I'm sure it will get a lot of scope time this year.

Cheers
Steffen.

[MortonH]

I'd like to find a big scope that isn't part of a sky tour so I can have a really good look at Mars :D

[sn1987a]

I'm looking forward to a 1000x night myself. Favouring the 10mm Ethos and Paracorr II I usually go to 300-350x when the scope has cooled sufficiently. On rare really good nights I can push it to around 600x with the 6mm but that's about it so far. :)

[julianh72]

Or anything.

[Satchmo]

If the seeing is good there is really not much advantage in taking the power much beyond 1 mm pupil - as at this power the first ring of the airy disc becomes visible , and like putting your face close to a newsprint photo at which point you see the dots the magnification becomes empty . ( this all assumes you have well functioning eyes ). I demonstrated this on a good night with my 12" 8" and 3" refractors al;l showing the first ring of the Airy pattern at magnifications of 300, 200 and 80X respectively .

Above this power floaters in the eye and the crystalline structure of the eye also begin to obscure and compete with subtle detail on planetary surface.

At 1mm there is still a good brightness gain of the disc compared to features coming from the eye. While you can hear tales of retractors being pushed to 100X per inch on bright high contrast objects like the Moon and Saturn there will be no gain over lower magnifications.

My current 350mm reflector is revealing to me astonishing detail - all that the aperture is capable of in moments of steady seeing on Mars at 350X with a 5mm Radian ( an eyepiece of focal length equal to the f number will give a 1mm pupil ! ) . So while I often here anecdotally of people using up to X1000 `without image breakdown ' - I know it is nonsense in the sense that low contrast detail on the planets will become compromised by the small exit pupil which will cause a lot of `grain' in the image in all but say a 40" telescope at these magnifications. You may use X1000 but you certainly wont see more and much more likely to see a lot less.

[sn1987a]

That would explain why I keep coming back to the 10mm as my favoured eyepiece because 350x is the sweet spot on nights of good seeing?. The other night Mars and Saturn were spectacular at 350x best I've ever seen them. When I went higher the view wasn't as good.

[MortonH]

I'd heard of keeping the exit pupil at 0.5mm or greater to avoid seeing your floaters, but not the 1mm 'limit' Mark is talking about.

However, now that I think about it, my 6.7mm eyepiece gives around 1mm or more in all my scopes, which might explain why it gives particularly satisfying and comfortable views of the planets.

[ausastronomer]

Hi Morton,

I have no problems at all using exit pupils down to about .5mm, particularly in my 14" and 18" scopes and gaining additional detail to what is visible with a 1mm exit pupil. The smaller exit pupil provides a larger image scale. If the seeing supports it, the telescope aperture supports it and the observer's eye supports it, then additional detail can be obtained. Larger telescopes are better supported to use small exit pupils, as the larger telescope collects exponentially more light for a given exit pupil. Using a .5mm exit pupil in an 18" telescope is a whole lot different than using a .5mm exit pupil in a 4" telescope, simply because the larger telescope collects over 18X more light and throws up a much brighter image. The ability to use small exit pupils can also depend notably on the individuals eye physiology. To make a blanket statement that exit pupils under 1mm provide no additional details is somewhat misleading. It depends on a whole lot of variables.



Not very often at all. Maybe 4 or 5 times in the 8 years I have had the 18" telescope. I used to get excellent seeing from my backyard at Killarney Vale looking north across Tuggerah Lakes. The seeing over large bodies of water can be exceptional as the body of water does not emit thermals like land masses. This occurs because the temperature of the body of water is very stable from day to night. About 4 or 5 times a year I could push the 18" scope to 750x on the Moon, Mars, Saturn and double star splits, but the more common magnification cap under normal very good seeing nights was 525X, which I could attain about 1 in 4 observing sessions. The more common magnification cap was 300X which I could reach on 2 of the 4 nights and one night in 4 I was limited to about 200X, or just under. From my present location in Kiama, due to the close proximity of the escarpment and its prevailing thermals I haven't had any of my 3 telescopes over 400X in the 3 years I have lived here. It's worth noting that an 18" telescope at 300X shows a whole lot more detail at 300X on any given target than a smaller telescope due to its greater light gathering power and greater resolution. It's probably worth noting that in my 18"/F4.5 scope 420X represents a 1.1mm exit pupil.

Cheers,
John B

[ausastronomer]

Andrew Murrell and Gary Kopff will have two of the 3RF scopes at IISAC this year. A 25"/F5 Obsession and an 18"/F4.5 Obsession. I cannot attend this year as I have to work on ANZAC day, being our single busiest trading day of the year.

We had an exceptional view of Mars last week in Andrews 25"/F5 Obsession at Coonabarabran. We had a couple of nights of excellent seeing at Ozsky and took advantage of the conditions for some excellent views of Mars and Saturn. We had a photograph image of Mars in the 25" scope at 450X using a 7mm Nagler eyepiece.

Cheers,
John B

[Steffen]

I'm looking forward to that! Last year I caught jaw-dropping, detailed views of the homunculus nebula, in less than perfect conditions...

Cheers
Steffen.

[MortonH]

Thanks John. Useful info.

Shame you won't be at IISAC.

[Satchmo]

Hi John
Its a general rule of thumb I use assuming good seeing and good optics. Because it is enough to reveal the diffraction ring in these circumstances there is not so much value in increasing the magnification - the image will be dimmer. If the seeing is not so good it may be that you can see more with 0.5mm pupil simply because you are making it easier to see as the scope is not working at the diffraction limit, and the point of empty magnification will be much less apparent. . 1mm pupil is just the point that if the seeing and optics are good , the image can already reveal everything that there is to be seen resolution wise at least for high contrast features. If the seeing is good enough to resolve some diffraction pattern structure at 1mm then there should be no gain in resolution at a higher power / smaller pupil.

I would imagine you would use 450X on the planets much more frequently than 900X ( 0.5mm pupil ) with the 18" - " seeing" effects the planetary detail just the same at both powers but the 1mm pupil will be just a lot more satisfying aesthetically as the `signature' of your eyes structures in transmission and floating internal debris will appear to be a lot less objectionable.

[FlashDrive]

Oooooh my....never a ' truer ' word spoken ....:D .... com'on not having a ' go ' at anyone ... but there's a lot of truth in that..... cause I was like that decades ago.....just plain lazy I was :rolleyes:

Flash.....:D

[Stardrifter_WA]

I know that feelin' Flash, but I would never admit to being lazy, so I just say that I don't like to work any harder that I absolutely have to. :rofl:

I had the same issue with my Ligthbridge 16", however, I solved that problem of putting lockable wheels on it, and they work well. :D

Recently selling my ED80, has started a trend though, as I now may part with this one too. :sadeyes: A move may be imminent. :)

Cheers Pete

[cometcatcher]

I did the same thing with my 16" dob with added wheels. It was a success in that I could easily wheel it out by myself but also found that the damping time was longer for vibrations to settle.

I really like Barry's truss. I should redo mine into something like that.

[sn1987a]

Splitter!

[sn1987a]

My modified Lightbridge 16.:D

http://m.youtube.com/watch?v=YwFcYzohscw