[ATM] Coma Corrector / Corrected Newtonian

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Jim,

I thought I saw an original post from you describing whether this system 
would be for visual or imaging but I can't find that post.  I'll give some 
information for either use.

I've investigated the use of full aperture correctors for systems designed 
for imaging.  There are a few main designs.  These include but are not 
limited to: Lurie-Houghton (two thin correctors), Schmidt (one thin 
corrector) and Maksutov (one thick corrector).  The one thing they all have 
in common is that it involves a very large piece(s) of glass, in addition 
to the primary mirror.

Such full aperture systems add mass.  For both visual and imaging the mass 
of the OTA will influence the performance of the mount.  It also greatly 
influences the cost of the required mount.  For imaging use, an additional 
20-30 lbs of mass can easily double the price of the mount required.

The added mass in optics creates greater thermal challenges.  It is the 
same reason large observatories use cellular instead of conventional glass 
designs.  Cellular is about 1/4 the mass of conventional designs.  The 
lower the mass, the easier it is to get that mass to equalize and for that 
mass to follow changing temperatures.  The lower the mass, the easier it is 
to make the OTA structurally stiff enough, especially at faster focal 
ratios.  Lower mass in the optics usually translates to a reduced mass of 
the OTA and possibly the mount.  The OTA (structure) and mount both have 
thermal issues of their own.  Many professional observatories vent heat 
produced by drive motors, electronics and any other sources of heat that 
reside inside the telescope area.

For visual use, it's a lot easier lugging around an OTA that is 20-30% 
lighter.  That is unless we are talking about 8-10" aperture here.

Some argue that a closed system keeps the optics clean.  This is true but 
at the detriment of thermal equalization of the glass/system.  A 4" 
anti-static photo brush costs less than $30 and takes less than one minute 
to clean even a 20" system.  CO2 "snow" is often used at observatories to 
clean/dust their optics (normally the primary) on a weekly, monthly or 
quarterly basis, depending on the environment.  Proper use of a lens brush 
is more than adequate to clean smaller optics, ~20" and smaller.

One disadvantage to full aperture corrected systems is that it becomes 
difficult to use such full aperture correctors past 20" in diameter because 
of self-deflection.

But the biggest issue I found with them is the additional weight they added 
to the entire system.  Focal plane correctors are fairly numerous now and 
offer performance that ranges from moderate to excellent correction.  If 
you want visual-use correction, use a TeleVue Paracorr.  For under $300 
it's hard to beat.  Also use their Powermate 2x and/or 4x as opposed to a 
run of the mill barlow.

For the ultimate in imaging performance, I have found Philipp Keller's 
correctors to be far superior to anything else on the market.  I currently 
have one of his 2" focal reducer/correctors in house.  I will be receiving 
about a half a dozen of his 3" coma correctors later this month.  You can 
see the 2" corrector here:
http://www.dreamscopes.com/pages/cells/y-BP%26cell-010-04-2in.htm

You can see Philipp's page for the 2" corrector/reducer here:
http://www.astrooptik.com/sonstiges/NewtRedNew/ReducerNew.htm

You can see his 3" coma corrector information here:
http://www.astrooptik.com/sonstiges/Newton3Inch/NewtWynne.htm

A few months ago I worked with Philipp to compile information comparing the 
spot sizes of a 16" f8.4 RC Cassegrain to a 16" f3.5 corrected 
Newtonian.  These are wildly different systems from each other but the 
point was to compare the same location on the focal plane, the corners of a 
KAI-11000 (24.7mm x 36mm) CCD chip.  The difference is substantial.  The RC 
has a RMS radius of 24.6 microns while the corrected Newtonian has a RMS 
radius of 3.3 microns.  You can see the spot diagrams here:
http://www.dreamscopes.com/pages/06/AstrographN-03.htm

If the goal is to build something, then a full aperture corrector may be a 
challenge someone takes on for the joy of doing it.  But if final 
performance is the goal, focal plane correctors are the way to 
go.  Everyone has their own goals so my opinions are just that, my 
own.  They are based on evaluation of systems for the sole purpose of 
imaging.  My requirements currently start at 16" and go up to 24" in 
diameter.  They take into account all other related factors: thermal, 
structural & mechanical, optical performance, etc..  There are many other 
factors but I've tried to touch on the major issues that influenced my 
decisions.  This post is long enough...

Thank you.

Sincerely,
Shane Santi - Owner
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833


At 04:19 AM 4/2/2006, you wrote:
>I have a nice F3.5 parabola that would break my heart to change....and
>so I was wondering of an overly simplistic approach regarding
>correcting curvature and coma. Now, tell me is I am way off base with
>my thinking here:
>
>A corrector plate that matched the mirror curvature could theoretically
>correct for curvature because it would cause the rays to arrive at
>focus at the same time? I was thinking a friend of mine who does glass
>work could make a impression of the mirror and pour me a lens....if
>this was possible would it really correct anything in practice or is my
>thinking on this problem all wrong?

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