The perfect scope … advice required!

[okiscopey (Mike)]

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/jo...r/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 (Mike)]

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 (Gavin)]

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

[okiscopey (Mike)]

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 (Daniel)]

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 (Clive)]

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

[wavelandscott (Scott)]

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 (Rob)]

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 (Matthew)]

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 (Mike)]

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]

Originally Posted by 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.

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]

Originally Posted by okiscopey

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.

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