A new scope for Mark?

[MarkS]

I've now come to the end of the road trying to field flatten the C11.  It has become clear that the very weird shaped stars at the edge of a DSLR sized field when using a reducer/flattener are simply a feature of the SCT design.

The Celestron and Alan Gee reducers give similar performance but both produce similar weird shaped stars outside the designed FOV of the scope.  And that is the key - it is clear that the design of the scope is a compromise that has been optimised for a particular FOV.

These are the reducers I have considered:
Celestron
Alan Gee
Optec
Astrophysics

None are designed to cover a DSLR sized field.  In addition all cause serious vignetting - again, a feature of the overall design of the scope.  In sum, the scope was simply never designed to give a flat field over a DSLR sized chip.  

So where should I look?  Here is my thinking so far:


C14 with Hyperstar
  This is far too unwieldy for a transportable set-up
Celestron Edge HD range
  http://www.celestron.com/c3/category.php?CatID=94
  Nicely optimised OTA but very slow at F10 - very narrow FOV.  Focal length too long.
Takahashi Epsilon 180  
  http://www.takahashi-europe.com/en/epsilon-180ED.php
  F2.8 - Very fast for imaging!  Focal length 500mm is a bit short.
Orion Optics AG -  http://www.orionoptics.co.uk/AG/agrange.html
  F3.8 - Still quite fast and off axis spot sizes smaller than the Takahashi
  AG8 Focal length of 760mm is probably almost perfect for general imaging under UK skies
  AG10 focal length is 950mm - still a good all rounder but maybe a bit pricey

Any other suggestions for an astrograph under £4000?  Needles to say, it must produce a flat, well-corrected field large enough for a DSLR.

Mark

[RobertM]

Mark,  whatever you go for is a compromise.  Ideally you want something that is very fast, has a variable f/l and large imaging circle - ok, that's telescope heaven so back in the real world...

You haven't got either a Newtonian/MPCC or a fast refractor on your list !  Personally I think you should look at several high quality instruments to cover your needs.  Of all the scopes you mentioned the Edge HD would be the most flexible and the whole range is farstar compatible.  Like you I found the traditonal SCT is very limited but didn't want any thing too bulky so bought a SW Mak Newtonian (1000mm f/5.3) instead.  It fits into my range of focal lengths well and has enough resolution and field for galaxies too; more importantly it has a very wide flat field suitable for large sensors. 

The Edge range is still very new and there are few reviews and might cover your needs but I feel you should also include a top short f/l refractor for flexibility.  Sky90/TMB 80/480 etc.

You are more than welcome to try my MakNewt if you wish; it has a Vixen dovetail that I can attach.

Hope that helps
Robert

[MarkS]

Ideally you want something that is very fast, has a variable f/l and large imaging circle.

That's it - that's exactly what I want!

The Mak-Newt is something I hadn't considered - looks like excellent value for money.  I may well take you up on your kind offer.

Mark

[RobertM]

Mark, I took some frames with the Canon and 190 last night.  Collimation looks out but the images didn't turn out too bad considering.  Let me know and I'll send them to you.

Cheers Robert

[MarkS]

Robert,

Yes please, I'd love to see them.

Mark

[RobertM]

http://gallery.orpington-astronomy.org.uk/displayimage-812.html

The curvature is because the focuser needs to be centered over the secondary (roughly 10-12mm).  Will repeat with that sorted but it should give you an idea of what it's capable of.

[Rick]

(...and if you get "...does not exist!" then log in to the gallery. )

[MarkS]

Very nice.   Especially if that is an uncollimated example.
Those stars are virtually Tak sharp!

What do you mean by "the focuser needs to be centered over the secondary (roughly 10-12mm)"?  How can the focuser not be centred (English spelling!) over the secondary?

Mark

[RobertM]

The focuser can be adjusted along the tube by about 10mm or so.  Had another go at collimation but didn't have enough clear sky to check out the result.

[Rocket Pooch]

How about a meade ACF, they have a flat field

[MarkS]

A good idea.  But I think I want something with a shorter focal length.  For the long focal lengths I'll keep the C11.

Mark