5 replies to this topic

[GTom]

I was wondering, is there a point of going exotic with the Mak-Newtonian design, other than the hatred of diffraction spikes?

 

I am thinking of getting an approx 1000mm FL scope to my collection, preferably around f/5. The camera it needs to serve is APS-H, sensor diameter 35mm - the actual reason, why I don't push f/4 with a steeper light cone causing vignetting with the most available 2" correctors already at APS-C (diameter: 28mm) size.

Edited by GTom, 01 October 2023 - 07:32 PM.

[licho52]

Maybe if you want to avoid spikes at all cost.  But they are heavy, large, unwieldy with the glass corrector and relatively expensive.  On top of that, their star reproduction is inferior to APOs.  Some make a case for their use but I find it unconvincing.

[GTom]

Also reading that Mak-Newt's can get into steep collimation troubles if you e.g. upgrade the let-down stock focuser.

[John Huntley]

I owned an Intes 150 F/6 mak-newt for a while but I don't image. The views were as close to an apochromat refractor as I've had from a non-refractor. Colimation was a little challenging but once set it stayed that way. Mechanically the scope was very well made, if a little spartan. Optically it was excellent. 

 

The Skywatcher 190mn has a secondary that is proportionately somewhat larger than the Russian ones use, presumably for imaging purposes.  

[Dan_I]

A Mak-Newton is not coma free but the coma is much less than a Newtonian of identical f/ratio. The amount of leftover coma depends on the specific design of the telescope. To get rid of the leftover coma, as well as of field curvature, it is apparently rather simple to design a corrector (because there isn't a lot to correct), the problem being that none is available commercially (the market is too tiny, and it would need to be taylored to a specific MN model, unlike coma correctors for newts).

 

So if one compares a MN with a newt+good coma corrector, the latter probably wins in terms of field correction but one needs to compare spot diagrams to be sure.

 

The skywatcher MN190 theoretical analysis is there: https://www.telescop...scopes.htm#most The conclusion being that coma is equivalent to an f/9 newt. Televue says of the Paracorr type II: "This new Paracorr Type-2 inserted into an f/3 Dobsonian made the stars appear as coma-free as a native f/12 mirror."

 

A MN has some practical drawbacks, it is harder to collimate (two active optical surfaces rather than two), sensitive to dew and takes longer to equlibrate.

 

But being spike-less is priceless for some

Edited by Dan_I, 02 October 2023 - 11:25 AM.

[GTom]

Speaking of spot diagrams, etc: my pixel size is 6μm, binned 12μm, I do not think I'd be in trouble with the mak-Newtonian. Vignetting is a question though, I don't know the exact sizing/focal plane distance from the secondary to estimate vignetting. A full frame flat sample would be great...

Large sensor, anything larger than APS-c is a pain even @f5 for both MN and Newt.

 

Looking around, good, larger than 2" coma correctors are expensive and rare to find. A decent 2nd hand MN can be found £6-700, focuser upgrade brings it to ~1000. A 8" Newt. is cheap but will need a larger focuser AND a 2.5-3" corrector, that costs more than the OTA.

 

A Mak-Newton is not coma free but the coma is much less than a Newtonian of identical f/ratio. The amount of leftover coma depends on the specific design of the telescope. To get rid of the leftover coma, as well as of field curvature, it is apparently rather simple to design a corrector (because there isn't a lot to correct), the problem being that none is available commercially (the market is too tiny, and it would need to be taylored to a specific MN model, unlike coma correctors for newts).

 

So if one compares a MN with a newt+good coma corrector, the latter probably wins in terms of field correction but one needs to compare spot diagrams to be sure.

 

The skywatcher MN190 theoretical analysis is there: https://www.telescop...scopes.htm#most The conclusion being that coma is equivalent to an f/9 newt. Televue says of the Paracorr type II: "This new Paracorr Type-2 inserted into an f/3 Dobsonian made the stars appear as coma-free as a native f/12 mirror."

 

A MN has some practical drawbacks, it is harder to collimate (two active optical surfaces rather than two), sensitive to dew and takes longer to equlibrate.

 

But being spike-less is priceless for some

Edited by GTom, 03 October 2023 - 07:15 AM.