Ultimate planetary scope for a HEM15

[Ags]

8 minutes ago, globular said:

Deviating only marginally from that and things soften - but parfocalising all my EPs makes keeping everything sharp no problem. And that includes with binoviewers too.

That's good information and may explain why I and many others claim the f6.3 reducer sharpens up the scope. Not because of the reducer, but because it changes the mirror separation.

However, the elaborate steps you take to maintain the critical distance between the mirrors seems an argument against the SCT. This issue does not affect the CC8 (mirrors don't move) or the Mewlons (being a Dall-Kirkham design).

[globular]

1 minute ago, Ags said:

the elaborate steps you take to maintain the critical distance between the mirrors seems an argument against the SCT. This issue does not affect the CC8 (mirrors don't move) or the Mewlons (being a Dall-Kirkham design).

Agreed.  Although it's only elaborate once - to parfocalised everything.  Once that's done you just get on and use it without thinking about it.

[Ags]

I suppose one could just get the mirror separation right, then fit an external focusser.

[globular]

That would be nice... but there isn't really enough back focus for one that keeps things optimal, sadly.

[Well, the new Primaluce Lab ESATTO 2" LP Low Profile Robotic Microfocuser might do the trick - but not at £800 !!]
https://www.365astronomy.com/primaluce-lab-esatto-2-lp-low-profile-robotic-microfocuser

Edited January 17 by globular

[Mr Spock]

I can pass on 10 years of experience of using a C9.25. While it was sharp on good nights, more often than not it suffered when seeing wasn't perfect. I had it on Jupiter many times and never saw the detail I can see in my 4"... Not a scope I can honestly recommend for visual. It's great for planetary imaging though - look up Damian Peach's C9.25 images.

[John]

I owned one of the Russian (Intes) 150mm maksutov-newtonians for a while and that was the closest thing to quality refractor performance that I've ever had with a reflecting scope. I reckon it was as good as an apo refractor of around 5 inches aperture. But the mak-newts are heavy and are slower to cool than a refractor. Optically very good indeed though.

I have read great reports on the Russian mak-cassegrains but I've not used one myself. 

[mikeDnight]

14 hours ago, John said:

I owned one of the Russian (Intes) 150mm maksutov-newtonians for a while and that was the closest thing to quality refractor performance that I've ever had with a reflecting scope. I reckon it was as good as an apo refractor of around 5 inches aperture. But the mak-newts are heavy and are slower to cool than a refractor. Optically very good indeed though.

I have read great reports on the Russian mak-cassegrains but I've not used one myself. 

I had chance to observe with what I think was a 7" intes MC around 2006/7 which belonged to paulastro. It really was superb! We had it alongside my FS128 on my drive way and spent much of our time observing the Moon. The Intes had obvious better resolution than the FS128 and was an absolute joy to observe with, but the 128 was sharper, though not by much.  

Edited January 18 by mikeDnight

[Ags]

I think the Mewlon 180 ticks all the boxes, not too big, unquestionably excellent and it has a handle. On the way to Mewlondalay I'll pick up a CC8 travelling companion. When I've saved enough for the baby Mewlon 180, I may very well continue saving for the 210, particularly if the CC8 is good enough.

[Mr Spock]

The Mewlon 180 has a good reputation. I don't think you'll go far wrong with one of those.

[davidc135]

1 hour ago, globular said:

My SCT provides me with lots of sparkle in star clusters.
It's an 8" Edge HD and I use it at fl 2125 (by ensuring back focus is 133mm). Deviating only marginally from that and things soften - but parfocalising all my EPs makes keeping everything sharp no problem. And that includes with binoviewers too.
I suspect most SCT users don't consider their back focus position and regularly push or pull their primary mirror far too far from it's optimal position - hence softening their views and leading to the bad reputation SCTs have.

Scts aren't very sensitive to changes in back focus.

Telescope-Optics.net, page 10.2.2.4.2 says scts of f2/10, regardless of aperture only vary by 1/23 wave correction per inch change in back focus, on axis. Similarly Edges, but their off axis imagery suffers more. I should think CKs would be similar to non ACF scts due to their both having spherical secondaries.

CCs are much more sensitive which is why they don't have mirror focusing.

David

[Ags]

14 hours ago, JeremyS said:

Sparkle in star clusters is the realm of refractors, not SCTs (or CC for that matter)

I don't have large premium refractors to compare to, but I got more sparkle from a Mak 102. The stars are much tighter and that compensates for the lesser aperture on marginal stars like M13 members etc.

[josefk]

15 hours ago, Ags said:

I think it would not be bad to be "stuck" on the CC8, they seem to be rated really highly by their owners, at least those that can forgive the slightly dodgy aperture. It's heavy for its size though.

I have this very scope Ags and really rate it. 

PROs

• It is a very robust physical package (though  heavy it is compactly heavy) and travels well in a car
• on Globulars and Planetary Nebula it is really fantastic and feels like a lot of bang for its buck
• on other DSO that fit its FOV capability it is fine (even OC) and the FOV context for any given DSO i.e surrounding stars to to the edge is also fine (i.e flat and aberration free). I do like nice edges even if i'm looking in the middle 🙂

CONs

• It is not 8". I trust the flashlight test and it is 185mm when checked this way - this also makes the central obstruction bigger by % if that matters to you. This is disappointing because i'm quite anal and quite  like the Interstellar Deep Sky Atlas that groups objects by 4", 8" and "large" aperture visibility probability.
• The secondary mirror supports are quite thick and they create very bright diffraction spikes. I don't mind them on stars (i accept them as par for the course) but i hate hate hate them on planets and they completely ruin the Mars, Jupiter and Venus planetary experience for me. Strangely they are not so disturbing on Saturn which makes no sense. i bought this as a planetary oriented scope so this was a major disappointment for me - i should have done my homework better.
• It gets back heavy fast - all its own weight is in the mirror so when you add stuff it gets harder to balance

I struggle to get super high magnification out of it and wondered for a while if that was about the quality of its mirrors - on balance though i'm convinced its actually about the quality of my skies because on a few nights here and there (over 2-years of ownership) i have had x300+ out of it with perfect airy disks and perfectly etched (black white black white black white) diffraction rings so i think the scope itself is fine.

Would i buy it again?  No - but that's only because i like it so much I wish i had bought the 10" version instead! There's even a very tempting 12" now but its super heavy.

 

Edited January 18 by josefk

[josefk]

... sorry double post

Edited January 18 by josefk

[Ags]

And now I'm thinking of immediately trading in my C6 for a CC6. Diffraction spikes aside, it seems much more the kind of scope I would like, and I could try out the design without spending any cash. The only drawback would be long exposure photography, the SCT seems to support more aggressive reducers.

[Fedele]

VMC200L
Mewlon 180C
C8edgeHD

[johnturley]

On 17/01/2024 at 22:29, Mr Spock said:

I can pass on 10 years of experience of using a C9.25. While it was sharp on good nights, more often than not it suffered when seeing wasn't perfect. I had it on Jupiter many times and never saw the detail I can see in my 4"... Not a scope I can honestly recommend for visual. It's great for planetary imaging though - look up Damian Peach's C9.25 images.

I'm really surprised to hear about your experience with a C 9.25, I had a C 9.25 (CPC version) for nearly 10 years also. I bought it to provide me with something slightly portable, as at the time I was planning to move house, and didn't know whether I would be able to move my observatory shed which contains my 14in Newtonian. In addition at the time, the C9.25 had a reputation for providing superior planetary views due in part to the longer (f2.5) focal ratio of the primary mirror compared to the C8 and the C11. The house move however did not materialise, as I ended up splitting up with my then girlfriend, but I did compare the C9.25 on a number of occasions with my 14in Newtonian, and found that it gave quite good planetary performance, and depending on viewing conditions, it sometimes gave a steadier view than the 14in.

Regrettably I never got round to to comparing the CPC 9.25 with my Esprit 150 after I purchased the latter in 2019, although it had been my intention to do so, partly because my present wife had 'boxed it in' a corner of the conservatory with some large house plants. Following back problems last year, I decided I wanted something lighter and more portable (with hindsight I think would have done better buying a C9.25 OTA and mount separately rather than the CPC version), and sold the CPC 9.25 to part fund the purchase of my Tak 100 DZ, which when finally blessed with clear skies over the last few days, I have been comparing with both my Explore Scientific 127mm, and Esprit 150 Refractors.

I will give further details in another thread, but looking at the Moon and Jupiter over the last few days, I have been finding that 100DZ and ES 127 gave similar performance, but that the view through the 100 DZ, although less bright (due to the smaller aperture) appeared slightly sharper. Nether instrument however came close to the view through the Esprit 150, which on most nights equals, or gives a more pleasing view on planets than the 14in Newtonian.

I realise however that an Esprit 150 would be far too big and heavy (plus expensive) for the OP to consider, but maybe something like the APM 140 FPL53 doublet, which at about 9kg is a similar weight to the C9.25, might be worth considering.

John 

Edited January 19 by johnturley

[Ags]

I'm calculating second-hand prices of CC8s vs resale value of a C6, AZ-GTi and bits, I could go straight to the CC8 in the coming months...

My only regret is that the C6 is good for photography at f6.3 or f4. The CC8 is much more massive, slow and with an enormous focal length. I imagine in practice it would only be viable for visual and solar-system imaging. I see people use a .67 RC corrector with the CC8, which would still mean 1600mm focal length, which is probably too much although worth a try with sub-10-second exposures.

8 hours ago, josefk said:

The secondary mirror supports are quite thick and they create very bright diffraction spikes. I don't mind them on stars (i accept them as par for the course) but i hate hate hate them on planets and they completely ruin the Mars, Jupiter and Venus planetary experience for me. Strangely they are not so disturbing on Saturn which makes no sense. i bought this as a planetary oriented scope so this was a major disappointment for me - i should have done my homework better.

I'm interested in you both loving the scope and hating its rendition of planetary targets. Is the capability on DSOs really that good? I think I can easily explain why Saturn spikes don't bother you - it is much fainter than Mars, Jupiter or Venus. I was playing with calculating diffraction patterns a few years ago and one thing I discovered from my calculations was that diffraction spikes don't affect planet views as such as although there is a lot of scattered light almost all of it is far away from the planetary disc. It should have more impact on the Moon as every point in the FOV will be casting diffraction spikes. The Mewlon 180 seems to have much more gentle spiking.

[josefk]

Hi Ags,

I think it’s contrast is pretty good - it’s well baffled BTW - so for DSO I feel (with little comparative evidence TBF) like I’m getting quite a high quality nicely contrasted view with the benefits of a decent aperture.  I use it for a lot of DSO at x140 and a 1.3mm exit pupil. This gives me a black sky, nice stars and a contrasty target. 

Per diffraction on planets - I have had champion views of Jupiter re planet detail itself (marble like detail and sharpness) but what bothers me is the planet width diffraction “beams” you get shooting off into space (pretty much from the limb of the planet). This really destroys the “in orbit” feeling I enjoy. You are always aware you are using a scope whereas with a refractor (I don’t own a Mak or SCT) the blackness of space to the limb of the planet (on a good dry night) gives me the “just in orbit” sensation I like. 
 

I think I have a sketch or two I could post of the effect - I’ll take a look. 

Cheers

[Mr Spock]

In my 12" I use a variable polariser when observing Jupiter, so I don't see any diffraction spikes. Something to consider.

[Ags]

@Mr Spock that's blown my mind!

[Ags]

2 hours ago, josefk said:

Hi Ags,

I think it’s contrast is pretty good - it’s well baffled BTW - so for DSO I feel (with little comparative evidence TBF) like I’m getting quite a high quality nicely contrasted view with the benefits of a decent aperture.  I use it for a lot of DSO at x140 and a 1.3mm exit pupil. This gives me a black sky, nice stars and a contrasty target. 

Per diffraction on planets - I have had champion views of Jupiter re planet detail itself (marble like detail and sharpness) but what bothers me is the planet width diffraction “beams” you get shooting off into space (pretty much from the limb of the planet). This really destroys the “in orbit” feeling I enjoy. You are always aware you are using a scope whereas with a refractor (I don’t own a Mak or SCT) the blackness of space to the limb of the planet (on a good dry night) gives me the “just in orbit” sensation I like. 
 

I think I have a sketch or two I could post of the effect - I’ll take a look. 

Cheers

Yes I would also be using this scope as my 'light bucket" on DSOs so an aberration-free field of up to one degree is important to me and a consideration vs the Mewlon. Like you I do my DSO observing in the 1-2mm exit pupil range, but I'd go up to 3mm for things like th Double Cluster. The Mewlon spot diagram is perfect in the center of the field but quite appalling further out.

I have seen planet spikes with a 150PDS, and I feel those planetary views were my best ever (very much let down by the scope being horribly undermounted), so I don't know if they will bother me that much. Mr Spock's polarizer tip is interesting but I don't know if the CC8 gather enough light for filtration. It might be an interesting option to try when doing double stars. I wonder if it could help with picking out Sirius's companion?

Edited January 19 by Ags

[Mr Spock]

21 minutes ago, Ags said:

Mr Spock's polarizer tip is interesting but I don't know if the CC8 gather enough light for filtration.

I'd only use it for visual.

If you want a flat field for imaging would an MN190 work on that mount? They are a bit on the heavy side.

[Ags]

9 hours ago, Mr Spock said:

I'd only use it for visual.

Yes it would only make sense for visual. Does it have to be a variable polarising filter or can I get away with a single polarizing filter?

[Earl]

Having owned a 180 and 9.25, id say they both have owning worthy features, but would go 9.25 if I had to Choose 1

[Ags]

Thanks, they are the same price so when I have the cash I will really make the decision. However I think the CC8 can be obtained this year and may convince me with its own optical qualities, at least for a few years.