The perfect scope … advice required!

[okiscopey]

I’m considering in the near-future going a bit mad and purchasing something like a 24-inch SDM with my retirement money. It’ll be in a permanent set-up and will be my attempt to avoid getting aperture fever this late in life. (If my employer was more generous, what the hell, I’d go for the 32-inch!)

I’m not sure where my true interests lie yet (fairly new to practical stuff), but I’m keen on visual, plus having a set-up that can tackle most types of photographic work to a high standard. I think I’ll be keen on DSO imaging with narrow-band filters, but probably not serious planetary imaging.

I’m aware my own knowledge and experience would be the limiting factor for a long time, but I’d rather be limited by my grey matter than the equipment.

What I’m thinking about at the moment is whether this type of scope can be made to do just about everything well, given the right combination of peripherals … Argo Navis, Servo Cat, field de-rotators, equatorial platforms, etc.

It seems to me that the big aperture, fairly fast focal ratio, and the respectable three-metre focal length is good for all sorts of tasks … but what about the tracking ability?

The critical question is: when a big dob like this is permanently set-up, is it possible to achieve the accuracy of, say, a G11 mount by any combination of add-ons? (Is that the sound of guffaws and ROFLs I can hear in the backgound?) If this is possible, then surely this scope becomes ‘nearly perfect’.

I assume that add-on equatorial platforms do not have anywhere near the accuracy I’m thinking of. If so, is there another solution, possibly using a field de-rotator? Comments on IIS in the past have given me the impression that these are ‘more trouble than they’re worth’.

Am I on the right wavelength, have I missed something, or do I still have to end up with two or more different scopes to do different types of work?

[netwolf]

Quiet a lot of people image DSO's using a large Dob with servocat/Argo combo. Or other goto option.
http://homepages.uni-tuebingen.de/johannes.brachtendorf/Kinder/Galaxies.htm
http://www.obsessiontelescopes.com/imaging/index.html

Planning is the key to avoid field rotation, you need to study the subject and when it would be the best time to image it to avoid FR. You can use Field derotators as you have mentioned.

If the drive system is designed correctly you can also avoid PEC and backlash.

Regards
Fahim

[okiscopey]

Many thanks Fahim, you’re first on the guest list for my new observatory!

I’ll have to look more into what needs to be planned to avoid FR - I don’t know anything about this.

I looked at your links, and realise it’s all possible. It seems like people get great images with fast scopes and modern CCD’s with short imaging times. I’ve looked at most of this before, but had sort of assumed they used very long exposures. (Haven’t I seen this recently though … many minutes or hours through filters?)

From the Obsession site:

“Imaging with a large aperture reflector like the Obsession is exciting. The large light gathering area means CCD exposure times are very short - usually under a minute. Minimal guiding or none at all. Multiple images are stacked in your PC to build up a vivid image. With proper software, field rotation at the focuser is eliminated.”

Surely an Obsession f/5 is no different in this respect from any other f/5 system? I thought stacking was more the domain of planetary imaging, not DSO’s. Also, what is this field rotation software? Presumably it’s part of the astro stacking programs. Rhetorical questions really, I can look into this myself at some stage. Sigh! So much to learn!

[netwolf]

Mike, there is one other issue inherent with Alt/az Though you can use a Field derotator to avoid FR, inacuracy's in the mount will lead to the need for guiding. This is a bit more tricky as you have field rotation to content on the guidescope. Chuck Shaw has done some work on this and provided a working solution.
http://www.ghg.net/cshaw/guiderot.htm

An alternate method that I saw the math for that works theoretically is using a Alt/Alt/Az mount. Mel Bartel did the calculation on such a mount and found that it would not be affected by FR. However as yet i Dont think a drive system has been created for it.

But practically speaking I think you need to define what you want first. Because there are two many choices and some impractical and very expensive. I would suggest you establish a list of things you want to do.
1. Visual, DSO up to what magnitude.
2. Visual, Planets (image scale).
3. Imaging Planets
4. Imaging DSO Widefield.
5. Imaging faint DSO's , decide limiting magnitude.

And importantly, portability ease of setup and use is also a factor for Scope's that you want to travel with.

Be waned this may lead to more than one scope.

I have my LX90 and considered taking the OTA off it but I learned from my first trip to Kulnura that the power of the LX90 was as a portable visual scope that was quick and easy to setup. It has planet imaging potential, but image scale is limited by pointing accuracy and tracking accuracy. I can go up to F30 beyond that it gets very hard to archive. DSO is doable using a wedge, 5ash (i think he is the one) has done some great shots using this setup with a Canon DSLR.

But eventually by imaging platform will be built around the G-11 mount.



Regards
Fahim

[sejanus]

to my knowledge, stacking is not just for planets it's just a method of avoiding long duration single exposures, so the subject type is not really relevant (as to if you should stack or not stack)

In terms of the 24" SDM - mega respect :D

[okiscopey]

Thanks sejanus.

Fahim, thanks for the list ... this is to be a fixed set-up, I realise it's impossible to have a scope that does everything and be portable as well ... I'd have something else for 'social purposes'.

Planetary imaging is probably off my list, so I suppose 'faint DSO's' are really the limiting factor.

[netwolf]

Mike, earlier i missed your FR question Alt/Az mounts are subject to FR, as such a motor is used to rotate the Camera to compensate for the FR.

Some amount of FR can be gotten rid of in software, if exposures are short.

The longer the exposure the more suseptable it is to
1. FR
2. Tracking errors
3. Atmospherics.
For planets 2 and 3 are important and stacking can help overcome both. As capturing more frames increases the likelihood of getting good ones.

For DSO 1, 2 and 3 are an issue. Stacking lots of shorter exposures helps avoid 1 and overcome some aspects of 2.

But for better dynamic range ( i think thats the right one) you need better compensation for 2 and 1 to achieve longer exposures. Eric recently demonstrated this with his Orion shot. Where he used a varying amounts of exposures to get much better image showing all aspects of the Orion nebulae. A long exposure may give you lots of nebulosity but you may miss the trap stars. Shorter exposures will give you less nebulosity but help you gain the trap stars. The final image is a composition to show all facets of the Orion nebulae.

Regards
Fahim

[netwolf]

Note Mike that I have very little experience in these matters, only what I have learned by example of others work. So I will leave it for the experts to correct me.

[dannat]

you might want to check out this blog for his SDM setup -
http://www.southcelestialpole.org.au/blogs/stu.php
Paul H seems confident about imaging - drop him an email about his setup.
I live 15mins from Peter R, he has a terrific workshop and considers the finest detail

[Alchemy]

ou could always go the meade 20 inch RCX400 on the max mount ... should do ok at f3.3 and only weighs 670lbs.:lol::lol::lol:

[wavelandscott]

I encourage your pursuit and heartily endorse spending your money on a big SDM with Argo Navis, Servo-cat and the other bells and whistles...

As was noted earlier, I think that Rumples Riot is doing some work imaging along the lines that you are thinking about.

While I am not an imager at all, I think that the visual performance of a big SDM that tracks would be glorious even if you could not take a picture with it!...but I think you can enjoy both with the right set up...I just don't know what that is...

Cheers!

[rmcpb]

I really envy you your choice in scopes. A large dob with AN and Servo Cat would be absolutely brilliant for visual and general observing. However, have you thought of another setup for the DSO photography like a small refractor on a GEM? Not that I know that much about this dark art but I would imagine you would get much more enjoyment out of the two setups than trying to get the large, and very beautiful, scope to do everything.

Just my two bobs worth........

[g__day]

Ping Astroboy - Zane has a 24" Obsession at Magellan Observatory with Agro Narvis and servo cat from memory - he could probably give great advice.

[Satchmo]

In breif, the reality is you may need to own a number of instruments as large traditional dobs aren't generally used for deep sky photograpghy ( tracking issues ) or high resolution planetary imaging( due to thermal lag ).

My own check list for retirement `arsenal' would go something like this ( assuming money no object )

* 32" Altaz in housed under dark sky for visual observing at least one weekend a month.
* 14-18" Binocular scope for Deep sky observing that can be transported and used either in suburbs or at dark sky.
* 14-16" Planetary Newtonian on GEM with lightweight fast cooling mirror for high resolution CCD planetary imaging
* 10 or 12" RC Cass for narrower field deep sky astrophotograghy on quality GEM with CCD happy in suburbs.
* 5" F7.5 Apo refractor for wider field CCD imaging
100mm fork mounted binoculars with interchangeable eyepieces.

* Eyes that stay healthy !

Hope this helps.

[MortonH]

Hi Mike, this decision is one that we all dream about!

What about using the 24" SDM for visual only and having a smaller, equatorially mounted scope for imaging? I'm not sure what the servo cat, etc. costs so I don't know what kind of scope you could get for the money saved by not driving the SDM (you could still have Argo Navis for finding stuff).

Whatever you decide, please invite me for the weekend!

Morton

[okiscopey]

Thanks to everyone who answered re the ‘perfect scope’ - you’re all invited!

There’s a lot to look into, which I will, but I can see that the general consensus is to use the SDM for visual and have other gear for imaging.

Oh well, it was worth asking the question, anyway … I learned a lot!

[rumples riot]

Mike,

large dobs on servo cat and argo navis can work for both DSO and Planetary. Yes a derotator is necessary for DSO work. The drives are really accurate in the SDM with servo cat, so much so that with a bit of counter weight work I could image Jupiter at 11000mm +. Thermal dynamics can be gotten around by making a cooling system. Something around 6-8 peltiers would be right for this size scope. You might also want to hand guide it rather than try to use software. But two minutes or so is nothing for guiding.

The advantages for this size scope are really obvious I reckon. DSO images can be done with very short subs and a derotator. In 90 seconds you are going to get heaps of detail. With planetary you can get great detail and huge image scale. The resolving power of a large scope will always be better than a smaller scope. Cats eye collimation kits are very useful for this type of scope. Go for a 1/30 wave+ mirror too. The better the resolution the tighter the stars and the more wow you are gonna get. A mirror like this might cost 8 grand, but worth it all the way.

I fully support your ideas. Dare to be different.

[Satchmo]

Rumples, I wouldn't give any prospective purchaser of a large dob the idea that serious deep sky photograghy with large long focal length scopes is being done with Servo Cats and field derotator. Can you link to any websites of people doing this? I know it is possible to do short 5 minute exposures with expensive commercial equatorial tracking platforms .

Cheers, Mark

[rumples riot]

I believe that Fahim has already provided a links. There are many more out there too. I did a google search and you can even see plenty of links on the obsession site.

The comment where you say you would not give any prospecitve purchaser of a large dob the idea of serious deep sky photography, is quite frankly suggesting that it cannot be done. Trouble is that it is being done and has been done for a long time. Short subs and a field derotator work well. You could also do short subs and derotate the image in photoshop. There is work to be done to keep the scope controlled, but nothing is impossible. Myths abound in this hobby. Like not using a large mirror for planetary because of the thermal lag. Peltier cooling has changed all that. Using alternative ideas is how the hobby progresses and myths are broken.

So I would and will make recommendation because this stuff can be done. Sorry to offend, but I have been around for a long time and heard lots of people say things that can't be done or you are using the wrong scope for that. Positions of ignorance suggest more research needs to be done, by those making the statements.

Paul

[davidpretorius]

I have seen rumples's 18" SDM in action and it is a serious piece of gear. The tracking is very good and for the high level of magnification he does on jupiter, it works very well.

Mel Bartels and those guys have been playing with field derotators for a while and they seem to work.

It must come down to how "serious" you want to do dso photography. Most guys here will image for 5mins, 10 mins, 30 mins and get fantastic results.

I loved the visuals from the large mirror, but would highly recommend you look at Pauls peltier cooling for the sdm. THe cool down time was very quick as opposed to waiting for 8+ hours.

Have fun deciding

[Satchmo]

I wasn't so much suggesting that *some kind of astrophotograpghy* can't be done with an big dob. Okiskopey I'm sure has his I eye on emulating the kind of deep exposures the astrophotographers are doing generally with german equatorial mounts, not just stacking 30 sec exposures.

Again, the question here was about astrophotography with a ServoCat. I know 700 have been sold , I know they are great for visual or CCD planatary stacking, but I've never seen a single serious long exposure picture with a big dob taken through one, thats just a fact. Please post links if you find any.

This thread started because Okyscopey wants advice on how to distribute his retirement funds to do all the astronomaical things he wants. Clearly , and as you achnowledge, alt-az photography is at the cutting edge for bold tinkerers, and to my knowledge it is not being done with Servo Cat systems, otherwise such examples would be plentiful by now. Okiscopey does not necessarily want to embark down tinkering endlessly at the cutting edge . He was looking for sound advice on what is doable now?

On the subject of planatery photograghy, large dobs are generally not so suitable as medium aperture all metal scopes because of thermal issues. The worlds best planetary shots are being taken with 40% obstruction C14's and good seeing which shows how important cooling is rather than aperture.



Mate, you've not offended me, I've been around optics and telescopes for 35 years and I think I've got a fair idea of whats going on. I've 'researched' the idea of DeepSky photograpghy with a Servo Cat and as I've said I can't find any serious results.
Cheers, Mark

[xelasnave]

Get the Meade max... you know you want one. I know I want one but dont want to sell a house to finance one...
If you want perfect have a look at this...

http://www.rcopticalsystems.com/24inch.html

AND...Stacking for dso's will blow you away.

I was on the same line of thinking until I realised that photos will show so much.

I have a 12inch for visual but use only a 6 inch reflector (SN) for photos and the results from stacks of 200 second runs produce I would expect better than anything visually thru even a very large scope.

I really think the most important thing is the darkness of the site and having a pier, a good eq mount, time to get perfect polar align and something to house everything.

If thinking photos concentrate on the mount.. have a look at what an good 80mm can produce on a Paramount Mount for example...

best wishes
alex

[ausastronomer]

Having an 18" Obsession fitted with Servocat, I can say "I" will never own another dob without servocat fitted to it. It adds at least another 10% to 15% to the aperture of the scope by having the object stay perfectly centred while you are studying it.

Deleting the Servocat (or the Argo) is not an option IMO, on such a premium piece of equipment. Buying a small refractor for imaging is the right option IMO, but do not butcher the main scope for the sake of the toy scope.

Cheers,
John B

[ausastronomer]

Hmm,

That's obvious on the basis you bought a beautiful timber truss dob with a 2" thick mirror for planetary imaging and then had to hack holes in the box because of cooling issues.

Personally, I would have ordered the scope with a 1.6" thick mirror from Mark or Carl Zambuto (both are offered as mirror suppliers), if I wanted the scope for planetary imaging. This would have resolved most of your cooling problems. I can tell you from experience that my 2" thick 18"/F4.5 OMI mirror, while superb, takes forever to cool with 4 fans on it.

You may have been around a long time, but you didn't learn much about mirror cooling and large aperture telescopes :)

Cheers
John B

[Satchmo]

Just adding to Johns point here, I'm not casting any aspersions on ServoCats, just to be realistic of what they are capable of. They do what they were designed to do ( visual tracking), brilliantly. I'll have one on my next `big' bino for sure. :)

Mark

[xelasnave]

I really like this one...
http://www.mistisoftware.com/astronomy/Equipment_RC32.htm
alex

[okiscopey]

Now this is getting interesting!

From the first lot of replies I gathered that in a fixed set-up, a big SDM (or equivalent) couldn’t be made to ‘do everything’ when fully optioned-up, despite the optical quality, the aperture and the focal length.

The standard answers suggested it’s best to stick to the usual different-equipment-for-different-jobs scenario - and I may well go down that track.

I thought the thread was dormant, now I find it’s been revived with an interesting - err, debate - on the original question.

I’ve no idea how it’s all going to work out, but I do thank Paul, Mark et al. for the new discussion and hope others are learning as much as I am from the experts. I’d certainly overlooked the ‘thermal’ factor, and probably a few other practical issues as well.

By the way, like most of us here, I've looked at (drooled over?) the RCOS stuff ... but I’m not going to be that rich!

[sejanus]

I think that rather than try to make 1 scope do everything, get a big SDM for visual use but maybe instead of a 24" get a 18 or 20.

With the leftover funds get a GEM and a refractor for planet astro work.

[rumples riot]

Actually this is totally wrong. First I have not cut holes in my SDM. I run a peltier cooling system which sits on top of the mirror box and cools the mirror in 1.5-2 hours. You are clearly wrong here. I suggest you read my blog on the investigation into cooling my SDM. This can also be verified by Bird and Dave who both own peltier cooled systems and have seen my SDM working in this manner. Patronising me when you are wrong will not convince me otherwise.


Mark as a C14 owner and someone who does hi res imaging, not all the great imagers are producing images from SCT's. Birds images this year are from a newtonian. Hi latest sets of images are clearly on par with Damians images. No one else is producing images of this quality. Large newts can be used for imaging, both DSO and Planetary with both GEM and Servo Cat systems.

Other notable imagers are Wes Higgins, he uses a starmaster 18 for both planetary and lunar.

Here is Two of many links.

http://home.houston.rr.com/gschaeffer/astropix/astropix.html
http://www.obsessiontelescopes.com/imaging/index.html

[davidpretorius]

yup, not mods to the body at all, the scope too good a beautiful piece of equipment to be "hacked" so to speak!

I say the jury is still out......Damian with his non actively cooled c14, but with a lot of experience and knowledge and Bird and fully automated Newt, whose inside knowledge of the way in which processing is done is second to none.

I would not like to call, whether an actively cooled 18" sdm will give better results that an actively cooled c14 on high magnification images.

The only way is to throw the scopes in a row, fly Damian P down and let them slug it out (so to speak).

It seems to me that there have been some huge leaps thanks to aussie amateurs that have brought the newts back to the c14's etc in planetary.

At the end of the day, will a "cheaper" 18" mirror set up as a newt be better than a 18" sct (if such a beasty exists).

I know my own eyes saw the quickness of setting up, aligning, accurary, high mag tracking and superb eye ball views of an actively cooled 18" sdm mirror.......great all rounder for mine!:thumbsup: as for DSO's, well, would like to see that in the flesh one day

[Satchmo]

Yep I know about Bird's efforts, in fact I even know the guy who made his optics ;-). To be fair, Bird uses a fast cooling conical mirror and active cooling system your not going to find on a traditional large dob which is the instrument in the subject of this thread.

//Other notable imagers are Wes Higgins, he uses a starmaster 18 for both for
//both planetary and lunar.

Wes uses a fast cooling 1.6" mirror not common on traditional large dobs which allows him to hold his own with the smaller instruments.

///Large newts can be used for imaging, both DSO and Planetary with both GEM
///and Servo Cat systems.

( Stuff deleted here as I'm just repeating myself )

If the 18" dob was the best for planary imaging why then buy a C14?

. Choose the right tool for the particular job I reckon. Earlier in this thread I posted my own wish list for instumentation if money were no object, you may be interested in that as it reflects that mantra.

Cheers, Mark

[rumples riot]

The active cooling system is not standard but it is easy to make Mark. Take a look in my SDM blog. It was dead simple and makes a large scope more adaptable to differing conditions. This is my whole point. You don't need to always abide by old conventions when things are changing. The hard work of the R&D has been done. The larger the aperture the more peltiers you need, that is all there is too it.

I know Wes quite well and you might be surprised to hear that he sits an airconditioner behind his mirror for several hours to get it cold enough. He is most interested in my cooling system and has asked lots of questions relating to it. I hope that he uses the idea. It will make his images even more brilliant.

The reason why I bought the C14 is that I own a unit. The only place to leave the SDM and image with it is out the front which is not secure. I have a pier and and deck out the back which houses the C14. It was the largest and most compact scope I could get into the space. I could not swing the SDM in that space. Also the SDM is undergoing balance tests with all the weight I have on the side. I want to perfect the guiding of the scope. It has nothing to do with using the right tools. I am a firm believer that you can do almost anything you want with large aperture. You just have to ignore the critics and use current technology. Birds idea of peltiers works on large aperture, I don't understand why you guys cannot see that.

Incidently my C14 is peltier cooled too. The very first of its kind and it works a treat. If you go to CN there is a thread on it in the solar system imaging section. Follow the links.

So that is my advice to Mike. It is not ill conceived, you just need to plan a little more and work out methods around certain issues. One large scope can fit the bill. Ultimately the choice is yours mate, but I have tried to save you spending a whole lot of cash. Try not to follow the nah sayers. The technology now exists to use a dob for both types of imaging and get great results.

[Satchmo]

Oh, boy I'm dipping out of this thread , the signal to noise ratio has become way too high.

[Rodstar]

I think this is the point where Nan offers everyone another cup of tea.

[Satchmo]

Someone pass the sugar please ?:)

[rumples riot]

Is that the standard response here? To say your opting out and saying my comments are noise related?

Imagesplus has the facility to derotate images taken with a derotator. You can buy derotators from a number of different sources (at affordable prices) and use this to gain perfectly acceptable images. With practice fine work can be done. Afterall, large observatories have been doing this for some time now.

Incidently I have been involved in astronomy as long as you Mark. I have a different opinion to yours and unlike you I don't see a reason why Mike cannot use an SDM to good DSO imaging. Simply calling me names without supplying evidence to your argument will not make you right. Or have I got this wrong; just because you say so makes it right??????

[okiscopey]

... and a couple of aspirins. :)

[jase]

I'm going to chime in here. I don't know if you've ever use a field de-rotator. They sound attractive, but from my experience the amateur versions are not accurate enough at anything over 1200mm focal length. I used the Meade #1220 for approx 6 months before selling it for half the price I bought it for (had no takers). It was a complete waste of time. If you're carrying a reasonable payload, you also need to watch out for flexure. I have not had the pleasure of using the Pyxis 2" or 3" camera rotator. These are commonly used for remote operation of object compositing/framing, but can also be configured to de-rotate fields. These are considerably different beasts to those used by large professional observatories such as this - http://www.ee.nmt.edu/~teare/fielddr.htm. You could certainly buy one of these professional units, but for the price of two 20" SDM scopes and then some.

Alternatively, you could image using CCD drift scanning techniques, but this is sub optimal for small FOVs. The level of nebulosity collected is based on the time it takes for the object to drift across the CCD registers and is read out. Thus, obtaining those faint wisps or globs become difficult as exposure time is constant.

I'm all ears to see where this debate is going. The cost per inch of aperture with large newts/dobs (14" and up) is appealing. Great visual instruments and for planetary images. However, until we start talking large dobs/newts on GEMs or Eq. fork mounts, I'm going to tune out. We all know that there is no substitute to long exposures when it comes to DSO imaging. To date, long exposures for DSOs are only practical through the use a GEMs or Eq. fork mount. I'm not convinced that a Eq dob platform (i.e a wedge) provides sufficient tracking accuracy (especially at the focal lengths discussed here).

So to this end, the "ultimate" scope does not exist. We all wish it did and some may come close. The old "horses for courses" statement applies. A superb visual instrument doesn't always apply itself well for photographic work (and vice-versa). You need to make an informed decision as to what features/aspects are important to you - there will be compromises. I guess if there is such a thing as the "ultimate" scope, it would be one that offers you the most pleasure and assists you in achieving your astronomy goals - whatever that maybe.

[Bassnut]

Ive been fortunate enough to experience DSO imaging with the cheapest to the best equipment. As Jase said, in the end, theres no substitute for long exposures, and to that end, the rules are well defined. Going "different" is appealing, but theres good reasons why most dont, unless your a tweaking genius (granted, theres a few) it doesnt work.

Long FL DSO imaging is all mount, mount, mount. The appropriate mount required for long exposures on a long FL, large aparture DOB would be a crippling financial experience ;-).

SCTs and RCs are used for long exposure imaging partly for the reduced load they present to the mount over a DOB/newt.

Ive been amazed over my equipment owning experience, just how important the mount is (rather than aperture). Thats where the money should be spent, a good mount and then an OTA to suit.

[rmcpb]

This thread has shown what is possible by some who put in the yards BUT for someone who wants to observe and take some good shots another option exists. Neither is "right", without the people who push the envelope we will never progress, just think about Dobson and his telescope we would not have the large apertures today. But for some the proven ease of the GEM and a smaller scope for imaging is what they need.

Everyone should just take a deep breath and have a cuppa, noone is wrong, we just think differently :)

[mill]

Correct me if i'm wrong but there is no perfect scope on this world.
If there would be one, everyone would have the same scope :P
And that would be that perfect one.

[Satchmo]

I think the 'thinking differently' really boils down to the ability or not to stay focussed on the ,original, subject of the thread.