Hello Lawrence,

there are no galaxy clusters around M42. When switching on Messier
objects you will get 3 squares (2 entries (?!?) for M42 and 1 for M43.
When switching on the isophotes you will get no squares but the
intensity distribution of the nebulae in grayscales.

When switching on NGC / IC objects at least 11 diffuse nebulae will be
shown. Switching on isophotes only one (IC 420) will remain.

When switching on "Nebulas" all kind of nebulae will be displayed:

LBN from Lynd's catalog of Bright Nebulae,
S from the Sharpless catalog of nebulae,
DG from the catalog of Dorschner and Guertler of reflection nebulas,
vdB from van den Bergh's catalog of reflection nebulae,
HH from the catalog of Herbig and Haro of small emission nebulae and
GN from the Atlas of Galactic Nebulae from Neckel and Vehrenberg.

There are at least 21 yellow squares displayed with isophotes on and
much more with isophotes off.

> I mean the large yellow squares that overlap each other, particularly
> around Orion's M-42 at level 6. Switching off those that you list, has
> no effect on these squares.

I can't repeat this behaviour. When I switch them off the yellow square
disappear.

> I wondered whether they are galaxy clusters
> (Zwicky + Abel) but you cannot switch these off (have both or either
> only).

Clear skies
Bernd

Bernd Brinkmann
Herne, Germany

http://homepage.ruhr-uni-bochum.de/Bernd.A.Brinkmann/index.html