TheSkySearchers

Hi All. I'd appreciate more input about the relative merits of commercially available Cats & Casses for visual use, emphases on DSOs but also interested in planets and the moon. Not for solar. Use will be in Bortle 4-5 skies, primarily. Some portability would be nice but not critical, as I may eventually build a small observatory to go with it. High quality and reliability are very much desired. Not very difficult-to-collimate is also preferred. (I know that mounting options are important too, but I'll get to that later on.) I hereby solicit/re-solicit your suggestions and pleasantly-diplomatic commentary about the Optical Tube Assemblies (OTAs) listed below, which is an expanded list (compared to ones I mentioned previously) of currently-manufactured/available OTAs with apertures ranging from 9.25" to 16," based on what I could find documented/offered for sale in the US, and constrained by not exceeding $15K (for the OTA alone). [I apologize that my list is incomplete, failing to include, among others, any OOUK-made telescopes or ToscanOptics telescopes, for example. If you think I should consider those too, please let me know. Finally, I would have included Parallax Instruments' classical Cassegrains in this list, but I have learned that they do not make cassegrains anymore.] Also, I apologize for any errors there may be in the table; I tried to be careful to get it right. I appreciate your thoughts and advice. Click image for larger view!

The Celestron/ Meade Aplanatics are better than the standards, Vixen has probably the quickest equalization and for the price of a top of line Tak or Vixen you could buy a small vehicle to drive to darker skies
Unfortunate that the Vixen sale has just ended.
APM has a nice custom 275mm Mak-Cass from Intes Micro for sale right now at 17.000€

The first question to ask is do you really need an aplanat? The Celestron Edge HDs, Meade ACFs, Vixen VMC and Takahashi Mewlons are all aplanats and therefore pricey. If your eyes can accommodate the curvature of field and associated astigmatism tradeoffs of a plain vanilla SCT you would be saving yourself a lot of money going that route.

Some eyes (like mine) can't cope with the level of residual field curvature and astigmatism of a plain vanilla SCT. I need consistent residual field curvature and astigmatism in order to get crisp clear views. SCTs are like looking through a smoke filled room to me and others with this issue.

Where controversy and acrimony develop it is usually on the part of folks who are perfectly happy with the images provided by a non aplanat SCT. That's a lot of people too. It is difficult for them to imagine what folks who need an aplanat see in them. Everything looks perfectly clear to them in their SCT. So they deny that those who need an aplanat are seeing what they are actually seeing. This leads to an unwholesome discussion that degenerates quickly since the aplanat folks certainly "know what they done sawr". The aplanat folks cannot be talked out of their perfectly valid experiences and the non aplanat folks need to stretch their imagination about individual observers and physiologies.

The discussion gets focused on the hardware which is futile since the difference lies more in the physiology of the individual's eye than in the aberrations of the OTA / eyepiece combination.

I intend to start a separate thread detailing how the differences in use of nonaplanat versus aplanat CATs depends critically on the eye in a way that is too complex for simple radius of curvature measures for OTA and eyepiece to capture the role of field curvature and astigmatism.

So let's keep that discussion for another thread where I will try to explain it more clearly than in the thread that was blown away. For your thread and to help with your decision two important questions for you to answer are:

1) Do you require an aplanat?
2) Have you made observations that lead you to believe that you do?

If the answer is 1) yes and 2) yes then let that be your requirement for the thread. If you've observed with an aplanat and find the experience clearer, sharper, more contrasty than with a nonaplanat then that is all that needs to be said about it. This thread can then proceed serenely (I hope) with that as a staled requirement by you. Hopefully civil discussion of the optics and physiology, aplanats versus nonaplanats, can be contained in the thread I'll start leaving yours in tranquility.

Does that work for you? Do you need an aplanat and is this based on your own observations?

BTW I took delivery today of a VMC200L as part of my risk reduction looking at an eventual VMC260L or 11" Edge.

Lady Fraktor wrote: ↑Sat May 02, 2020 4:00 am The Celestron/ Meade Aplanatics are better than the standards, Vixen has probably the quickest equalization and for the price of a top of line Tak or Vixen you could buy a small vehicle to drive to darker skies
Unfortunate that the Vixen sale has just ended.
APM has a nice custom 275mm Mak-Cass from Intes Micro for sale right now at 17.000€

Here it is: viewtopic.php?f=71&t=9552

If I could I would but.... :shrug:

Thank you. I expect an aplanat would be beneficial for me, although I have not looked through one. My eyes' lenses have become remarkably inflexible compared to when I was younger. I now need tri-focals (but I bought separate prescription full-lens eyeglasses instead, and switch between them for different purposes.) Even so, I have to adjust my eye-to-target distances (books vs. computer screens, etc.) or simply settle for things being a bit out of focus, while swapping eyeglasses when appropriate. My younger self took my ability to focus my eyes for granted. Anwyay, I noticed the effects of field curvature using my Celestron 6" (conventional type) SCT recently. Actually, I had noticed it before but without realizing what it was; I kept thinking that it was simply a telescope-focusing issue (i.e., that I wasn't turning the knob to the right position, or perhaps that the focus was drifting mechanically), since re-adjusting the focus at any given moment tended to correct the problem; yet ultimately I would then have to refocus again when looking at a different direction within the field of the eyepiece. Based on all I've read here and elsewhere, that is the tell-tale sign of field curvature. And then... I learned more about field curvature, and that my inexpensive SCT was likely among the worst for this sort of thing (e.g., see https://starizona.com/tutorial/field-curvature/), and it all made sense.

notFritzArgelander wrote: ↑Sat May 02, 2020 4:28 am

Lady Fraktor wrote: ↑Sat May 02, 2020 4:00 am The Celestron/ Meade Aplanatics are better than the standards, Vixen has probably the quickest equalization and for the price of a top of line Tak or Vixen you could buy a small vehicle to drive to darker skies
Unfortunate that the Vixen sale has just ended.
APM has a nice custom 275mm Mak-Cass from Intes Micro for sale right now at 17.000€

Here it is: viewtopic.php?f=71&t=9552

If I could I would but.... :shrug:

notFritzArgelander wrote: ↑Fri May 01, 2020 4:50 pm

Lady Fraktor wrote: ↑Fri May 01, 2020 4:45 pm At first I thought this was Markus Intes Mak then realized it is a 10", I wonder where he had this hidden!

Looks like a one off. US optical materials, Intes Micro figuring, German mechanics. A special order gone sideways?

I would drool but will restrain myself.

I realize that that may be a great scope, but it's rare, I'm not ready to buy yet, it's expensive (17,000 EU = $18.7k), and frankly, it probably deserves to be owned by someone who knows more about what they are actually doing with a telescope than I do (i.e., someone like one of you folks.)

notFritzArgelander wrote: ↑Sat May 02, 2020 4:26 am BTW I took delivery today of a VMC200L as part of my risk reduction looking at an eventual VMC260L or 11" Edge.

Excellent, and congrats. I am looking forward to learning what you think of it.

AntennaGuy wrote: ↑Sat May 02, 2020 3:50 pm Thank you. I expect an aplanat would be beneficial for me, although I have not looked through one. My eyes' lenses have become remarkably inflexible compared to when I was younger. I now need tri-focals (but I bought separate prescription full-lens eyeglasses instead, and switch between them for different purposes.) Even so, I have to adjust my eye-to-target distances (books vs. computer screens, etc.) or simply settle for things being a bit out of focus, while swapping eyeglasses when appropriate. My younger self took my ability to focus my eyes for granted. Anwyay, I noticed the effects of field curvature using my Celestron 6" (conventional type) SCT recently. Actually, I had noticed it before but without realizing what it was; I kept thinking that it was simply a telescope-focusing issue (i.e., that I wasn't turning the knob to the right position, or perhaps that the focus was drifting mechanically), since re-adjusting the focus at any given moment tended to correct the problem; yet ultimately I would then have to refocus again when looking at a different direction within the field of the eyepiece. Based on all I've read here and elsewhere, that is the tell-tale sign of field curvature. And then... I learned more about field curvature, and that my inexpensive SCT was likely among the worst for this sort of thing (e.g., see https://starizona.com/tutorial/field-curvature/), and it all made sense.

That clarifies to a degree. What you describe sounds a lot like my experience with my eyes and also with the Celestron 9.25 SCT versus the revelatory clarity of the 9.25 Edge HD. I suspect your need for an aplanat is genuine and if you wish to confirm it with an actual test that would build confidence of course.

I'll be reporting in greater detail in another thread on the VMC200. I've unpacked it. My initial impression is that it is really superbly built compared to the Celestron 9.25 Edge which in turn was better built than the plain 9.25 SCT which had a plasticky feel to it. It is a beautiful instrument with smooth focuser action, perfect coatings with no visible defects. The unpacking inspired a great deal of confidence. I'm really looking forward to a night of good seeing. The only downside is that AS ADVERTISED the tube is without accessories needed for first light. So I placed a supplemental order for some mechanical parts. Some of this supplemental order is redundant because I like to keep an optical train together and I have 2 configurations in mind for how I want to use the OTA.

AntennaGuy wrote: ↑Sat May 02, 2020 4:03 pm

notFritzArgelander wrote: ↑Sat May 02, 2020 4:28 am

Lady Fraktor wrote: ↑Sat May 02, 2020 4:00 am The Celestron/ Meade Aplanatics are better than the standards, Vixen has probably the quickest equalization and for the price of a top of line Tak or Vixen you could buy a small vehicle to drive to darker skies
Unfortunate that the Vixen sale has just ended.
APM has a nice custom 275mm Mak-Cass from Intes Micro for sale right now at 17.000€

Here it is: viewtopic.php?f=71&t=9552

If I could I would but.... :shrug:

notFritzArgelander wrote: ↑Fri May 01, 2020 4:50 pm

Lady Fraktor wrote: ↑Fri May 01, 2020 4:45 pm At first I thought this was Markus Intes Mak then realized it is a 10", I wonder where he had this hidden!

Looks like a one off. US optical materials, Intes Micro figuring, German mechanics. A special order gone sideways?

I would drool but will restrain myself.

I realize that that may be a great scope, but it's rare, I'm not ready to buy yet, it's expensive (17,000 EU = $18.7k), and frankly, it probably deserves to be owned by someone who knows more about what they are actually doing with a telescope than I do (i.e., someone like one of you folks.)

My pockets are insufficiently capacious! My experience with closed tube Maks is that the cooling time is large and dewing time is short. There seems to be some provision in this OTA for ameliorating that. I think for my own use that for larger than 8" aperture I'd rather have an open tube design. But I'd be interesting in a test run of this item!

Just saw this one, at the relatively-low price end:
https://agenaastro.com/gso-10in-f12-cla ... -tube.html
Agena Astro says the mirrors are quartz and they (Agena) also do optical tests/checks before shipping.
S&T Review at: https://agenaastro.com/mwdownloads/download/link/id/65/

Hmm. At f/12 in a classical Cassegrain, there will be more coma. Will need to learn more about that. (Evidently, avoiding coma is the main reason that other large classical Cassegrains have higher focal ratios.)

An interesting telescope, what mount are you planning on using?

Lady Fraktor wrote: ↑Sun May 03, 2020 3:01 am An interesting telescope, what mount are you planning on using?

Presently leaning toward a Losmandy mount (G11 something), with the particular choice to depend on it being a good fit to the OTA. Considering others too, however. I'm still undecided on both the OTA and mount.

AntennaGuy wrote: ↑Sun May 03, 2020 2:03 am Just saw this one, at the relatively-low price end:
https://agenaastro.com/gso-10in-f12-cla ... -tube.html
Agena Astro says the mirrors are quartz and they (Agena) also do optical tests/checks before shipping.
S&T Review at: https://agenaastro.com/mwdownloads/download/link/id/65/

That looks nice and has an attractive price point. The weight at 38lbs would require something like a Losmandy G11 with a 60lb carrying capacity. Oh you might get by for visual only with a 40 lb mount like the Atlas but if you are on a high and windy hill.... (a little movie music reference there).

The Classical Cassegrain design has a few aberrations to worry, so let's worry them.

The primary mirror is hyperbolic, like a Newtonian, so just like in a Newtonian that adds coma as an aberration. The secondary is hyperbolic not flat, but that just means that you can't use a coma corrector designed for a Newtonian to fix the coma. Still you have field curvature to contend with like you do with all two curved mirror systems. Now the kicker there is that to have a clue how the field curvature would work, we need to know the prescription of the system: what is the focal length of the primary and the secondary? Or equivalently 1) what is the overall focal ratio of the entire system and 2) what is the magnification provided by the secondary? Unfortunately I cannot find GSO's prescription details.

However there are two things I did find that are helpful. First a pair of reviews:

https://agenaastro.com/mwdownloads/download/link/id/65/

https://astronomytechnologytoday.com/20 ... pe-review/

The second review is of the 8" model but Classical Cassegrain scale linearly with aperture so it's still relevant to the optics.

Secondly the advice in Rutten and Venrooij. It is likely that the design uses a fast f4 parabola and a magnification factor of 3. (We know GSO makes good f4 parabolas.) No guarantees but we are looking at a radius of the curved field of around -760mm for an 8" scope. To compare an SCT's curvature an f10 has -155mm according to R&V. So remembering that the field curvature is the inverse of the radius A Classical Cassegrain has about 1/5th the curvature of an SCT.

Looks good.

AntennaGuy wrote: ↑Sun May 03, 2020 2:47 am Hmm. At f/12 in a classical Cassegrain, there will be more coma. Will need to learn more about that. (Evidently, avoiding coma is the main reason that other large classical Cassegrains have higher focal ratios.)

Comparing the spot diagrams of a 200mm Newtonian with f10 and f15 200 mm Classical Cassegrain designs in R&V I find that the f10 CC is worse and the f15 CC is better. Coma can be ignored in an f12 Newtonian. I wouldn't worry about it.

Thank you very much for that analysis!!

AntennaGuy wrote: ↑Sun May 03, 2020 3:24 am Thank you very much for that analysis!!

I can run a model of an f12 M=4 CC design on the computer where I have my optical design SW. I have my work tools on a different machine.

What about this for a telescope?
https://teleskop-austria.at/SWRMAK300

Lady Fraktor wrote: ↑Sun May 03, 2020 3:34 am What about this for a telescope?
https://teleskop-austria.at/SWRMAK300

You're wicked placing temptation in my path like that! I even have an account there (Lacerta, right?)

That's what I think of it.

Mostly positive discussion of this highly anticipated release....

https://www.cloudynights.com/topic/7039 ... assegrain/

notFritzArgelander wrote: ↑Sun May 03, 2020 3:52 am

Lady Fraktor wrote: ↑Sun May 03, 2020 3:34 am What about this for a telescope?
https://teleskop-austria.at/SWRMAK300

You're wicked placing temptation in my path like that! I even have an account there (Lacerta, right?)

That's what I think of it.

Mostly positive discussion of this highly anticipated release....

https://www.cloudynights.com/topic/7039 ... assegrain/

OK having gotten over my surprise and delight here are somewhat soberer thoughts.

The design appears to be similar to the VMC design. So the competition will be on the price with the advantage there in favor of the SW product versus the machining, fit and finish of the Vixen.

Optically Vixen has chosen the VMC260L to be f11.5 and the VMC330L to be f13. The SW is faster at f10 only, a more aggressive and less forgiving design. So as I ruminate:

1) I think it the reasons for SW announcing it some years back and not delivering any yet becomes understandable. It's an aggressive design.
2) Because of the double pass through the corrector errors in manufacturing are compounded so Vixen might still be a better or competitive choice. The slowness of the Vixen optics helps this issue.