Primary size on original (gold) Skywatcher 180 Maks

[GazOC]

I know this scope has been tested by indirect means in the past and found to have an effective aperture of less than 180mm due to Maks needing an oversized primary, but I thought someone might appreciate an actual measurement of the mirror.  I've just stripped one down and the diameter of the mirror is 200mm

I bought this Mak when they first came out in this country, I'm pretty sure it was the first batch that FLO received from Skywatcher so it looks probable that every 180 from that point forward has an 200mm oversized primary contrary to most of the info I've seen on the internet.

(The Mak I stripped arrived with the primary badly chipped and, after sending Steve photographs to confirm, he decided that it wasn't worth returning to FLO and sent me out another, perfectly fine, scope.straight away. So it's just been sitting under my bed for the last 15 years or so, with time on my hands during furlough I decided to strip it down and measure the primary)

 

[John]

Very interesting Gaz.

So the diameter of the primary has not changed between your scope and the one that is the subject of this thread:

The "stopped down aperture" issue was reported on by Neil English when he owned one for a while.

Could any other factor cause this that has subsequently changed or is the whole thing a myth perhaps ?

 

[GazOC]

I've only measured the primary, nothing else, but its definitely 200mm

I'd read that the later models were oversized but the original gold models weren't and were actually 180mm giving effective aperture of ~170. This doesn't look to be correct 

Edit: just measured the OD of the tube at 215mm, I'd be very surprised if that was the cause of any issues 

Edited June 2, 2020 by GazOC

[Rob]

Interesting indeed Gaz.. love it when you can fact check for yourself!. Hope the strip down and eventual clean & service goes well

Best Rob

[John]

To be honest I was rather skeptical when I heard that the primary mirror size had been increased on these. This would require a primary cell re-design and a number of other manufacturing changes. Normally, when a manufacturer makes these sorts of changes they release the revised scope as the Mk II or SE or something similar with a fanfare of publicity. I can't recall anything like that happening with the mak-cass 180, apart from the switch to the "black diamond" colour scheme

I wonder if something else was causing the apparent reduction in effective aperture that Neil English reported ?

 

 

[GazOC]

TBH John I don't know enough about the validity of the testing used to guess but with the mirror being 200mm, the tube diameter being 215mm and the lens/ meniscus holders (the white bits either end) being 220mm I'd be surprised if the system was stopped down to less than 180mm due to any of those factors?

[andrew s]

5 minutes ago, GazOC said:

TBH John I don't know enough about the validity of the testing used to guess but with the mirror being 200mm, the tube diameter being 215mm and the lens/ meniscus holders (the white bits either end) being 220mm I'd be surprised if the system was stopped down to less than 180mm due to any of those factors?

I have no additional data but badly designed baffles could act as an aperture stop.

Regards Andrew 

[GazOC]

There's no baffles on the tube, Andrew the only "suspect" IMHO would be how high the baffle coming out of the hole in the primary is but it doesn't look excessively high to me given the size of the secondary spot 

[andrew s]

11 minutes ago, GazOC said:

There's no baffles on the tube, Andrew the only "suspect" IMHO would be how high the baffle coming out of the hole in the primary is but it doesn't look excessively high to me given the size of the secondary spot 

Yes it was the inner baffle tube I was thinking about. But, as I say pure speculation on my part! It's  a fine balance to stop sky flooding and let the whole light cone pass.

Regards Andrew 

Edited June 2, 2020 by andrew s