Maksutov-Cass versus Newtonian

Cheers for that John - looks like it could be very interesting.

Dave

How do-able is this? And it the parabolic primary the best method? Would 16" work or should I go to 18 or even 20"? I guess it depends on the imager dimensions (though the filter wheel is even bigger...).

Ah, yes... I forgot about filter wheels and other gubbins, so scratch the prime focus idea! A standard Newt will give you the option of crusing the skies visually when your camera burns out!

Bang for buck, a paraboloid is probably always going to be the winner. Paraboloids are (relatively) cheap to make and correct. If you go for a paraboloid, then a 20" is definitely affordable (and you'll have kids to spare!) but you will need to enclose or protect your scope and mount from the wind. An instrument that big will catch a lot of wind, and even if the mount doesn't budge, the tube might!

A 20" f/4 will yield about a 0.9degree field of view with about a 20mm fully illuminated field of view (though this is with a 96mm secondary mirror = 19% obstruction.) If you want to get a bigger fully illuminated field, then you will need a bigger secondary. If you want a wider field of view then you will need to look at Schmidt cameras (Meade are revamping their old film based Schmidt for CCD.)

Oldham Optical UK currently produce 1/4, 1/8 & 1/10 wave (on the wavefront) certified paraboloids up to 24" and possibly 30". Prices are not as henious as you might fear!

Phil.

Phil - thanks for the info. That's very helpful.

I've spoken with Orion Optics and it seems we're leaning towards an F5 16" Newtonian. The see no issues with tube flexure when hanging 13lbs of imaging train off the focuser. However, I do need a heavier focuser than they supply - so I have to get one for them. It doesn't need to a microfocuser - just rough and ready and lockable. Fine focusing will be done with the FL PDF (temp controlled electronic focuser).

The 20" was tempting but the price isn't

They were confident that their 1/10 wave optics will give pinpoint stars over 97% of the FOV.

Do you know what the fully illumnated field of view would be for this?

Cheers

Dave

Dave To calculate the fully illuminated FOV you need to say what the size of the secondary is.

:oops:

Will get back to you with that...

:oops:

Will get back to you with that...

When talking to Orion about the size of the secondary mirror, you need to have a few figures at your finger-tips:

Firstly, you will need to know the physical size of the CCD chip you will be using (or plan to use in the future.) You will presumably want to fully illuminate the whole chip if you want to avoid vignetting (darkening towards the corners and edges of the image. So, on a piece of paper, draw a rectangle the exact same size as the CCD chip. Find the centre and then use a compass to draw a circle around the rectangle so that it fits inside the circle with a little room to spare (a couple of mm all round.) The *diameter* of this circle is the number you will need to quote for the size of the fully illuminated field you want. (Guys, correct me if I'm wrong please.)

You will also need to know the exact *outside diameter* of the optical tube assembly (presumably supplied by Orion?) and the two heights of your focusser (fully racked in and fully racked out), the height of your temperature controlled focus compensating unit, and the distance to the CCD chip from where the camera draw-tube collar stops up against the fucuser collar.

If you have any notion of using the scope visually (and why not!) then you will need to know the back-focal-distances of the set of eyepieces you will be using. Take these numbers and the back focal distance of the CCD camera and average them. You want your focuser to reach this average focal plane at its mid-point of travel. This will also directly detirmine how far up the tube Orion must cut the hole for the focuser. Do not, under any circumstances, let them fob you off with one of their pre-cut standard tubes. This will be a compromise which will cause you problems later. Orion must cut the tube specifically to your requirements. You will also want the tube to extend beyond the focuser far enough that it fully baffles the focuser hole from stray light.

Orion (or you) then take all these numbers and crunch them to come up with the ideal size of your secondary mirror. Easy!

Or... there are plenty of spreadsheets and applets that will do all of the above for you. Let me know if you can't find one and I'll rummage around for you.

Sorry if this all sounds a bit confusing!

Phil.

PS - It's not the end of the world if you undersize the secondary mirror. You probably won't notice (visually) any vignetting, though it may affect the results from any software you use to analyse the images. If you upgrade your CCD at a future date you can always get a bigger secondary mirror to give you a bigger fully illuminated field of view. Secondaries cost (or used to!) about Stg£1 per mm.