Sky-watcher 180mm Maksutov

[Stu]

7 minutes ago, chiltonstar said:

I think you'll find John that modern 180s with o/sized mirrors are actually 180 or very close. I've measured mine at 179mm and others in reviews have found the same.

Chris

Chris, isn't it the fact that the corrector enlarges the light path slightly, so the mirror would need to be oversized to maintain full aperture? I'm sure there is something like that anyway. 

[johninderby]

Simple fact is the 180 is totally outclassed in every area by the CC. Just no contest.

Edited January 9, 2020 by johninderby

[chiltonstar]

46 minutes ago, Stu said:

Chris, isn't it the fact that the corrector enlarges the light path slightly, so the mirror would need to be oversized to maintain full aperture? I'm sure there is something like that anyway. 

Yes, exactly Stu. The full aperture of 180 is maintained by having a slightly oversized mirror., meaning that the aperture really is 180mm.

Chris

[GazOC]

19 hours ago, johninderby said:

 

Ignore that John, I didn't see the subsequent posts. Apologies 

Edited January 9, 2020 by GazOC

[Peter Drew]

I'm not happy with the original suggestions for the 8" CC not operating at full aperture.  If a 8" CC has an 8" mirror it will operate at full aperture provided that the secondary is the correct size.  Where the effective aperture comes into question is when the loss due to secondary obstruction and/or reflectivity is taken into account, both apparently minimal in this case. The diagrammatic explanation also puzzles me as for the two different focal lengths the convex secondary would not occupy the same place in the light path, as far as I can see the diagram would indicate a plane secondary.

[vlaiv]

2 minutes ago, Peter Drew said:

I'm not happy with the original suggestions for the 8" CC not operating at full aperture.  If a 8" CC has an 8" mirror it will operate at full aperture provided that the secondary is the correct size.  Where the effective aperture comes into question is when the loss due to secondary obstruction and/or reflectivity is taken into account, both apparently minimal in this case. The diagrammatic explanation also puzzles me as for the two different focal lengths the convex secondary would not occupy the same place in the light path, as far as I can see the diagram would indicate a plane secondary.

If you are referring to the diagram that I posted - its purpose was to demonstrate that GSO response did not make sense - illumination of secondary by faster primary. It is not part of actual explanation why aperture is obstructed.

I specifically meant to address this claim:

Quote

With the relatively short-focus parabolic primary mirror, a large secondary that would degrade image contrast would not be desirable.  The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches(186.5mm)

Maybe I'm not understanding what is being said properly, but I'm reading that sentence as: faster primary "requires" larger secondary, but larger secondary will impact contrast and we've chosen to use smaller secondary and that is the cause of aperture being stopped down.

I gave that diagram to show that faster primary in fact requires smaller secondary (if both are placed at the same distance).

We can calculate (a bit of guessing is involved) if there will be some stopping down due to secondary size. If we take primary to be F/4 and secondary magnification to be x3 (giving F/12 scope), then primary will have 800mm FL. 33% linear obstruction means that diameter of secondary is ~67mm.

For it to be illuminated by primary without any loss - it needs to sit at distance of: x : 800 = 67 : 203 => x = 67*800 / 203 = ~264mm inward from focal point, or 800 - 264 = 536mm distance between mirrors.

Now we have couple of issues - as different websites give different figures:

- TS website states 33% linear obstruction ( 203 * 0.33 = 66.99 ) and says tube length is around 620mm (not sure if that includes focuser or not). If we assume those figures, then everything checks out - there is enough distance between mirrors (if we account for cells and mirror thickness - ~20 + ~20 + ~20 + ~20 + 540 = 620mm).

- Agena Astro lists different figures all together - secondary mirror obstruction as 58mm (that would be ~28.6% CO), and tube length as 21.1" = 53.594mm (or just about same distance as needed between mirrors).

That is really interesting ...

I have no idea how long the scope is, I suspect it must be longer than RC 8" - which is about 465mm tube + about 90mm. I did some measurements for some people that make bags for telescopes, so I know figures of my 8" RC. Focuser should be the same so it is about 90mm.

This sort reconciles tube lengths given by TS and Agena Astro, as 536 + 90 = 626 - close enough to 620.

If tube is 536mm long, than I would say separation is probably about 80ish mm smaller so 465? If that is true, then secondary mirror size needed to fully illuminate on axis is 335 : 800 = x : 203 => x = 335*203/800 = 85mm

67 / 85 = 0.788 while 7.34/8 = 0.9175

Not sure it matches. Maybe primary is not F/4 but rather F/3 with secondary being x4?

[John]

It is worth bearing in mind that the original review by Alan that kicked off this thread was posted 7 years ago.

I'm sure there are many examples around of scopes and other equipment that were "king of the hill" 7 years ago that have now had their capability exceeded by more recent products.

 

[paulastro]

Over the years I've heard rumours, or read, of quite a few telescopes not to be working at their full aperture.  I'm not sure if any of the rumours were ever substantiated.  My two refractors often don't work to their full apertures - due to poor seeing and cloud getting in the way unfortunately ☹️.

[Pete Presland]

Cant add much to the discussion about whether it uses its full aperture or not.

I really enjoyed using my MAK180 for several years, my best ever Mars image was captured with it.  Mine did suffer with some annoying mirror slop though.

I did get a dew shield eventually 😀  it was a very capable scope on the planets.

 

[Sunshine]

2 minutes ago, Pete Presland said:

Cant add much to the discussion about whether it uses its full aperture or not.

I really enjoyed using my MAK180 for several years, my best ever Mars image was captured with it.  Mine did suffer with some annoying mirror slop though.

I did get a dew shield eventually 😀  it was a very capable scope on the planets.

 

Do you keep it stored in the deep freezer? lol

[Pete Presland]

Just now, Sunshine said:

Do you keep it stored in the deep freezer? lol

I had a very early morning session. Once it got light i went in for breakfast, came back and things had got a little cool!

[widotje]

Hi all, 

I see I missed a whole lot of comments, thank you so much for this valuable information. I haven't made up my mind yet. the SC 8" would be good for planets. But recently I got interested in imaging galaxies and that would require a somewhat lower f ratio, although an edge HD with reducer could do the trick. As an alternative, I see a many Richey Chretien types which may also be good for galaxy hunting...but probably worse on planets due to obstruction. And then I compare those to a large refractor (TS has an 115mm F/7 for 999). I think I'm going loco...:-). Anyway, thanks for sharing all your experiences and knowledge

Wido. 

Edited February 4, 2020 by widotje

[AstroMarco]

Great review of the skywatcher 180mm mak.

though the thread was severely hijacked with the classic “my scope is better, get it instead.”