Re: [ATM] More on aperature stops

["John Murray" <onslow@bigpond.net.au>]

Mike Carambat wrote
> I just posted a picture of why I'm so interested in
> the Herschelian (or off-axis newtonian) designs. While
> playing around with the aperature stops (the easiest
> way to make an off-axis telescope) I have become
> completely sold on them and this is why:
>
> http://www.designwizardry.com/aperaturedifference.jpg
>
> NOTE: (These are photoshop artist renditions as I
> can't take a photo through my newt since it's on a dob
> and is a real bugger for astrophotography at high
> magnifcations. But it's very accurate as to what I'm
> really seeing)
>
> I'm pretty sure that the image on left is pretty much
> what most people see with a 10" f/4.7 newtonian at
> 7.5mm with questionable atmospheric conditions with no
> filters. At least that's how I still explain the
> brightness, blurriness, loss of detail and duplication
> and ghosting of image. I see this in other scopes as
> well at the local Observatory.
>
> If anyone disagrees with me about my comments on the
> image at left, please let me know. I've been told it
> could be related to front surface thermals on the
> mirror, but I'm not seeing the "boiling and roiling"
> apparently typical of this in star tests. Just the
> regular, slow moving, horizontal "shower glass"
> wavering you see at high magnication through the
> atmosphere.


G'day Mike,
	Our club has a 10" f4.8 on a dob mount. I've tested the mirror and it is
very good. Regarding your drawing of full aperture unobstructed, I have only
seen that bad an image through it when the scope was either seriously mis
collimated or when using very poor quality/condition eyepieces. We've used
it on Mars using a Takahashi Eyepiece (7.5 mm) LE occasionally using a 2x
barlow. At no stage do we see those problems. Nice clean images with
occasionally superb resolution when seeing permits. I can't recall you
saying anything on the figure/measurements of the mirror. Do you have those
details?

Cheers

John Murray
Whyalla
South Australia

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