Is the Skywatcher MN190 really 190MM?

[garymck (Gary)]

Hi
Does anyone know if the Skywatcher MN190 has an oversize primary? I understand the corrector is 190mm, but if the primary is not oversize then my understanding is that will suffer reduced effective aperture. I've seen multiple reports that the Skywatcher Mak Cass variants all suffer from this.

Gary

[glend (Glen)]

Based on owning one (MN190) for several years, the tube is pretty much a standard 8" newt diameter (which is typically larger than 8" so it can house a 8" mirror), but with the focuser placed right at the front (with the secondary mounted directly on the corrector). The mirror is a standard 8" primary rtiding on a standard 8" newt mirror cell with standard newt adjustments, and I have had mine apart for cleaning. The corrector diameter is where the 190mm comes from. The corrector has a collar around it that fits over the front of the tube, this collar contains the position/lock adjustments for the corrector so it takes up some space; this is typical of Mak-Newts. I can't comment on Mak-Cass designs, they are different.

[garymck (Gary)]

Thanks Glend
for the full reply!
cheers
Gary

[Wavytone]

The skywatcher maks are faithful to the original design in that the aperture stop is at the corrector - not the primary mirror, and yes the primary mirror diameter is larger than the clear aperture.

There is a simple “flashlight test” you can do in a long hallway or garage - measure the clear aperture of the corrector with a tape measure. Put a bright LED at the focus and adjust carefully so that the scope illuminates a spot on a distant wall of the same diameter. Now walk back to the scope and measure the width of the beam coming out of the corrector. This is the effective aperture of the scope.

An alternative view is that they started with the largest primary mirror that will fit inside the OTA, and the corrector aperture is thus slightly reduced in respect to that.

There is a reason why this is important, optically; too complicated to go into here. The maks from Intes and Questar likewise.

With the mak-newtonians the field of view is larger than it is in the mak Cassegrains so the ratio between primary and corrector varies accordingly.

If you want the full theory obtain a copy of sky & telescope “Gleanings Bulletin C”, it spells out everything you may want to know about maks.

[garymck (Gary)]

Thanks wavytone,
I was curious as to whether the MN190 was made differently to the makcass versions which are all undersized primary mirrors, and therefore not truly what the are labelled as apertur wise.
Gary

[Wavytone]

That’s only true of some MCTs - certainly not all. There’s a thread somewhere on CN listing the results for many scopes of the flashlight test.

There’s another clue too. Shining a torch inside you’ll see the gap between the OTA and mirror which gives a fair idea of its diameter. Compare with the clear aperture of the front or alternatively the difference between the inner wall of the OTA and that of the corrector cell.