Long Focal Length and Speed - Where is it?

[glend (Glen)]

Since I have decided to stay in my present location (and observatory) for another year, I am looking for a new imaging project. I have completed my multi-year emissions nebula (narrowband) program, and have sold off some of the scopes used for that. I am considering a broadband galaxy program for this year, but I need long focal length for that.
So any recommendations on suitable equipment. I already have a well sorted CGX mount on a pier. My preference is to stay away from the budget GSO RCs (having owned one in the past). Criteria are:
Minimal central obstruction
Speed - faster the better
Prefer not having to deal with moving mirror issues.

Suggestions?

[multiweb (Marc)]

What is your budget? If you want long FL and speed you'll need some serious aperture.

[glend (Glen)]

Originally Posted by multiweb

What is your budget? If you want long FL and speed you'll need some serious aperture.

Under $10k.


I can leave it on mount in the observatory, so I don't have to move it (well once to mount it).


Have been looking at ODKs at f6,8 (fl 1700mm). Then there are the usual Newt suspects. I am constrained by swing room in the Obs but think I could get a 12" f5 under the roof ok (and that could be a cheap solution), but only 1500mm fl. A 16" would not fit at all.

[mental4astro (Alexander)]

The Skywatcher 180mm Maks are outstanding. Mirror shift is very small, and eliminated with an external focuser if it's a concern for you.

Better still are Russian Mak optics such as Intes or APM. A 9" Mak does chew up galaxies!

You can go with Celestron or Meade SCTs, but you need to test these individually to find the really good ones. They do exist. My own Meade that I thought was a dud ended up just having the corrector plate retaining ring screwd on too tight which distorted the corrector and stuffed the image. Now it is really blooming good. Hell of a journey to get there. My Intes Mak still muscles it out even though it's a little smaller in aperture. Finding this Intes Mak showed me SCT's have a WIDE range of performance! Few are really good.

[mental4astro (Alexander)]

Newts, Skywatcher makes nice optics, but the mechanics of the OTA will need reworking to get it to where it should be. This is where they skim cents off their production costs, but it leaves the end user with performance headaches that shouldn't be there.

You will also need to carefully consider how to balance the OTA as many people forget that with the focuser being on the side of the OTA, greatly alters the load distribution as the scope is swung around on the mount. It is a very different situation from a scope whose entire optical train is in one line, and not off-set.

[multiweb (Marc)]

Originally Posted by glend

Under $10k.


I can leave it on mount in the observatory, so I don't have to move it (well once to mount it).


Have been looking at ODKs at f6,8 (fl 1700mm). Then there are the usual Newt suspects. I am constrained by swing room in the Obs but think I could get a 12" f5 under the roof ok (and that could be a cheap solution), but only 1500mm fl. A 16" would not fit at all.

With all the horror stories I'd stay clear of Orion Optics. TBH I'd wouldn't touch them with a 10 foot pole but that's just me.
I reckon you can get a good deal on a second hand RC or PlaneWave Dall Kirkham. Classical Cassegrains with no glass in the imaging train are very good too. I'm very happy with my CN-212. If you take your time and do a bit of digging in the second hand market I think you'll find a fit for what you want to do at a good cost. There are a lot of good scopes out there.

[glend (Glen)]

Originally Posted by mental4astro

Newts, Skywatcher makes nice optics, but the mechanics of the OTA will need reworking to get it to where it should be. This is where they skim cents off their production costs, but it leaves the end user with performance headaches that shouldn't be there.

You will also need to carefully consider how to balance the OTA as many people forget that with the focuser being on the side of the OTA, greatly alters the load distribution as the scope is swung around on the mount. It is a very different situation from a scope whose entire optical train is in one line, and not off-set.

If I am imaging with a Newt I rotate the tube to point the focuser down the counter weight bar so it hangs underneath and does not create balance problems. I have done this on my previous newts and Mak-Newt and it gives good balance.

[Atmos (Colin)]

It’s more important to consider aperture and imaging scale rather than focal length.
A 12” F/4 with an ASI183 will give an image scale of 0.4”/pixel. As Alex mentions though, their mechanics are built to a price point and leave a bit to be desired.

[mental4astro (Alexander)]

Found the perfect scope!

https://astromart.com/classifieds/as...ksutov-f30-ota

* Big aperture - TICK, 280mm
* Focal length - TICK, 8400mm
* Tiny central obstruction - TICK, 16%
* Optical quality - TICK
* Mechanical quality - TICK
* Price tag - FAIL, £7000

Bugger, just one short...

[Slawomir (Suavi)]

Originally Posted by mental4astro

Found the perfect scope!

https://astromart.com/classifieds/as...ksutov-f30-ota

* Big aperture - TICK, 280mm
* Focal length - TICK, 8400mm
* Tiny central obstruction - TICK, 16%
* Optical quality - TICK
* Mechanical quality - TICK
* Price tag - FAIL, £7000

Bugger, just one short...

As far as I know, f/30 is not optimal for galaxy imaging.

[AG Hybrid (Adrian)]

A Vixen VMC260L would mount nicely on your CGX. F11.5. Probably really good optics. Central obstruction might be a bit large though? At least there's no large corrector plate to deal with.

Also, within budget with $$$ to spare.

[mental4astro (Alexander)]

Originally Posted by Slawomir

As far as I know, f/30 is not optimal for galaxy imaging.

Can't have it all, eh...

The question Glen asks has been much discussed. Cass vs Newt, slow vs fast, small obstruction vs large, and a secialized application. The final selection is a confounded one. Why not a big SCT with focal reducer? Good compromise I would have thought. Aperture along with a couple of focal ratio options, slow and fast-ish, in the one scope. Yes there is the corrector plate, but not difficult to protect from dew, especially if it's in an obs.

[Wavytone]

So... you have been dreaming of putting one of these in the obs, eh ?

https://www.apm-telescopes.de/en/tel...mc-400-mm.html

Have to win lotto first...

[Camelopardalis (Dunk)]

Glen, small central obstruction is less important for imaging than visual, look at many of the large imaging reflectors, often monstrous secondary...if it was important, they wouldn’t have

I’m with Colin...high QE sensor, fast scope, decent aperture...ultimately the seeing will determine your end result.

[mental4astro (Alexander)]

No argument from me there, Dunk. I understand what you and Colin are saying.

The issue comes from what load capacity the mount can take. The size of a 12" Newt means its weight distribution on a mount is not the same as that of a C11 or 12" SCT, even if their dead weights are similar. Add to this the off-set focuser and camera, and the weight geometery/moments, unless this is properly countered, it will add additional inbalance and strain upon the mount. Particularly with a scope this size. The scope does not move in three axis, but four. It is something that is not thought of enough.

Even if the "weight" of a 12" Newt falls within the stated capacity of the mount, this stated capacity does not take into account the moments generated by the size of the 12" Newt. Think of it in terms of levers. Two levers, one longer than the other, and the same load on the end of each lever. The longer lever will be experiencing a greater load. This is also the reason why the focuser/camera combination needs to be countered as well. As the OTA is moved by the mount, the force exerted by the focuser/camera changes, and the flex this will introduce. This is without even addressing the mechanical issues the OTA may have itself.

This has nothing to do with imaging. It's all mechanics, and looking to do what is mechanically best for mount, scope and ultimately the imaging train.

[Wavytone]

Dunk, with 45 years experience and roughly a dozen scopes over the years, I'd have readily agreed with you - prior to acquiring the MK91.

For most observers their experience is based on the one scope they have, and they think it's fine simply because (a) the star images look like they should according to most sources, and (b) it performs ok compared to other similar scopes. Many many observers are in this category. All fine and dandy, until the day when someone sets up another scope side-by-side with yours, same targets, same conditions, same magnification (same eyepiece even) and you realise what you are seeing is chalk, vs their cheese. Another clue is that whole most cheers consider magnifications around X1.5 per mm to be the practical upper limit for reflectors or X2 for refractors, to reach Dawes limit on double stars requires X3 per mm of aperture and a few scopes can exceed that on the moon and bright planets.

Yet as anyone with an f/10 SCT would know, an eyepiece under 8mm is pointless, never mind 3.5mm.

Several years ago poking around on the intertubes I stumbled on images taken by owners of MK91s and was struck by the the resolution they were routinely achieving with quite ordinary cameras. I'll skip the details as to how I eventually acquired this one.

Soon after acquiring the MK91 I very quickly noticed at club observing nights this scope routinely gave constantly good high power viewing while 8"-11" SCTs a few metres away were struggling to give decent glimpses at 150X on the same targets. In the MK91, even though poor seeing means the image will wobble around, it doesn't dissolve into a blurred soupy mush.

Alex (Mental) had been using his two 8" SCT's for years to make lunar sketches... thinking all was well with them, until one night I took the MK91 to his place and side by side with his scopes. It was very quickly apparent it was in a totally different league.

To his credit, Alex embarked on something which I am sure very few have attempted - he begged, borrowed and in some cases purchased a whole swag of SCT's and maks with apertures 180-200mm and critically compared the lot over several nights, and ultimately put up the best three against the MK91.

The result being an Intes M715D clearly outperformed the SCT's by a significant margin, and that scope also shows a similar response to average/poor seeing as the MK91, ie the image wobbles, but doesn't turn into mush.

A few conclusions were reached:

1. Scopes with really, really good optics are significantly less affected by seeing than scopes with average to poor optics.
2. if the collimation is off even by a tad, while you may think the image looks OK in moments of steady seeing it will be badly degraded by poor seeing, Reduced to blurry mush, basically.

It also suggests premium APO refractors (such as the AP 130GTX) behave similarly and in some respects helps to explain why some are addicted to refractors since they don't have collimation issues (assuming no-one has tinkered with the objective post-assembly).

FWIW in good seeing on Saturn, the MK91 is capable of 700X and shows the Encke division. Never mind Cassini's division, that's a walk in the park. I'm not aware of any SCT under 14" that can do that.

I know many will struggle in disbelief at this.

[glend (Glen)]

Guys, I am sure, as you say, the MK91 is a brilliant visual scope, but it's not something I would want for imaging - which is what the question was originally about.

Secondly, having imaged with Newts and Mak-Newts, the key to good balance lies in having a good focuser and a tube which is rotated so the load aligns with the counterweight bar, it effectively just becomes part of the counterweight axis. The mirror at the rear is a good offset for the imaging gear, meaning less polar movement inequity. The Mak-Newt was even better balanced (neutral) due to the corrector. I just wish there was a longer one.

[mental4astro (Alexander)]

Glen, your experience with balancing a Newt has me intrigued. I'll look at using the same approach with my own asap to see how it works out. So far my own experience has been the OTA wants to rotate and out of balance once the OTA is turn say 90° in RA from vertical. All the same, a big Newt is a different load situation from a Cass, and needs different consideration from a Cass for the mount not to be adversely affected.

My posting of that f/30 Mak was a flippant one... That is a pretty much just a planetary and lunar scope. It's a shame that there are not reducers for Maks (not that I'm aware of). That would be a game changer.

A big SCT with reducer I still see as a plausible option. A "two scopes in one" situation. Difficulty is finding a great SCT. Yes you can barlow a big Newt, I'm also thinking of the ergonomics of such a scope not just for the mount, but your obs too.

I hope you see my posts as looking into this, and not implying Maks are the be all and end all - I have only 2 Cats of the 12 scopes I have, and of those two are fracs, Newts are everything else. I'm thinking only about your application, and being Devil's Advocate to make sure all aspects are investigated

Alex.

[glend (Glen)]

Originally Posted by mental4astro

Glen, your experience with balancing a Newt has me intrigued. I'll look at using the same approach with my own asap to see how it works out. So far my own experience has been the OTA wants to rotate and out of balance once the OTA is turn say 90° in RA from vertical. All the same, a big Newt is a different load situation from a Cass, and needs different consideration from a Cass for the mount not to be adversely affected.

My posting of that f/30 Mak was a flippant one... That is a pretty much just a planetary and lunar scope. It's a shame that there are not reducers for Maks (not that I'm aware of). That would be a game changer.

A big SCT with reducer I still see as a plausible option. A "two scopes in one" situation. Difficulty is finding a great SCT. Yes you can barlow a big Newt, I'm also thinking of the ergonomics of such a scope not just for the mount, but your obs too.

I hope you see my posts as looking into this, and not implying Maks are the be all and end all - I have only 2 Cats of the 12 scopes I have, and of those two are fracs, Newts are everything else. I'm thinking only about your application, and being Devil's Advocate to make sure all aspects are investigated

Alex.

Yeah sure I appreciate the perspective. I am not a big fan of diffraction spikes anyway, but admit Newts have some advantages in this application and in my location they are easier to dew manage than the SCT I have tried. I have noticed that my Mak-Newt was very easy to dew manage and I suspect the difference between it and the SCT comes down to the thickness of the corrector. The MN190 corrector was a pretty thick chunk of glass, and it retained heat very well ( when wrapped with a heater strap around the tube). The SCT corrector (at least on the Edge) was much thinner and harder to get heat into and retain.

[mental4astro (Alexander)]

Dew control is certainly a challenge for a corrector plate. But there is a change of thinking when it comes to dew control.

We seem to think that the only way of using a scope is to wait for it to cool, and then we stick a heater on the thing... I've always thought this was strange because of the thermal problems a heater creates... But that's beside the point here.

The new thinking is NOT to let the scope cool!

The problem of a scope cooling is not the heat per say, but the temperature differential that is created between the warm interior (mirror, baffle, cell), and the cold metal tube, and the heat plumes that are then generated inside the OTA.

So, if the heat differential is not allowed to occur in the first place, then the heat plumes won't be generated, the inside of the OTA is thermally stable, and you can start using the OTA immediately without waiting for the thing to cool.

I am not talking about keeping the OTA warm. Instead I am saying we should slow the rate of cooling. By insulating the OTA, the metal/CF tube is not allowed to get cold, and so the heat differential is reduced to the point where there are no heat currents being generated inside the OTA.

The insulative material is extended beyond the corrector plate of the SCT or Mak, and this acts as both a dew shield and an extension of the insulation to enhance the protection of the corrector. This won't totally prevent dew formation on the corrector in humid conditions, but it will extend the dew-free period, and if a heating strap is used it can be implemented sooner while the OTA is still warm, so to take advantage of the residual heat of the OTA so the strap is not needing to also heat a stone-cold metal tube at the same time.

This is not a new idea of mine. I've adopted it however, and the results so far have been outstanding for me. I've been able to take my 7" Mak outside and rip it straight away up to 400X without needing to wait with the OTA insulated. I used a sheet of Corflute to make mine. Other people have used builders insulation foam, car foil heat shields and other materials. Some folks have also insulated the rear of their OTA, but it is not necessary. The most important part is keeping the tube wrapped up. Below are a few pics of what I've done with my Mak. I've also made a secondary dew shield for the scope that acts as both a dewshield extension and an additional light baffle. And the link below shows how some other people have been insulating their Maks and SCTs.

Dewshield-insulated Maks and SCTS

From this:


To this:
, , ,

Newtonians are a different kettle of fish because their OTA's are open. Newt's require a different approach, and I've written about this here:

Cooling Newtonian Optics

Alex.

ADDITIONAL INFO: I've been looking into focal reducers for Maks. There's a lot of hear-say and assumptions about using existing focal reducers (of all types) with Maks, but there is very little by way of hard in the field experience. There is a 0.6X reducer made by Intes, but it is a rare fish. Some people have tried existing reducers with some Maks. Their reviews are tainted in what they both expected or their applications. Visual is different to photo, and with photo vignetting needs to be expected, but this depends on the camera and the application. If you have a full size chip camera and expect a fully illuminated field, you're kidding yourself. You will need to work with what the scope and reducer can give you. And this depends on your application. I have a couple of 1.25" reducers, I may see for myself what they do with my Mak and small camera.

Alex.