Re: ATM A long post ...

[epabcc@epa.ericsson.se (epabcc@brsw342)]

Nils wrote :

> I did, I am sorry , I shouldn't, but it was a misunderstanding on my part

there's nothing to be sorry about; I used the wrong word (TWICE!) so no
wonder everyone was confused.

> I believe if you were to see faint stuff near something very bright, like
> hypothetically details in the ashen light near the bright limb of Venus or
> Moon, this differenc would be important, but not really when looking at
> small markings on Mars nearly as bright as its background.

The mid to low frequency region is traditionally worse in obstructed
instruments. That's what makes them somewhat inferior to refractors in
planetary observations. Broad areas of very low contrast - that is
where refractor shows the clean pair of heels to say Newtonian of equal
aperture. The amount of small, high contrast detail will be the same
(obstructed instrument will in fact have a little edge there).

> But does in your experience an APO of say 5" f/9 perform beter than an
> unobstructed 5" f/12 sub-aperture of a Newtonian? (In my admittedly
> limited, it doesn´t.)

Er - yes, it does. I'll again use the expression of a VERY experienced
planetary observer and instrument maker after comparing 4"
Schiefspeigler to 4" (classic) refractor : "Refractor leaves it for
dead" (both instruments have been made by him, both have long f/ratio,
but lack of scatter and tube currents made all the difference). Note
here that we're talking about many decades of experience in top class
instrument making and planetary observation under one's belt. A casual
glance through both of these instruments may not reveal anything. In
other words, difference _is_ there, but it is not quite all that
dramatic (it all comes to a definition of 'dramatic', really. For me,
once I see the difference, it becomes dramatic; must be my digital
professional deformation !   :-)
Note also that I'm NOT advocating absolute superiority of a
refractor : we just need to increase the aperture of our workhorse
Newtonian to catch up, and easily pass the expensive APOs which more or
less don't exist above 12 inch or so (at price of the average home
here), and vast majority is smaller than 8".

> But other confounding factors like
> ...  prices, expectations may be just as important. 

Unfortunately, in most cases that seems to be the case. We can't forget that
seeing through a telescope is VERY subjective experience. As with expensive
HiFi gear, we often 'hear' and 'see' what our mind wants us to.

>>  0.2 obstructed would be VERY sensitive to zonal errors at 70% zone
> You may be right, but I suspect then the Strehl ratio would be markedly
> lowered in that case. 

Point is, we do NOT take obstruction into account at all when we
calculate Strehl ratio. I'm pretty sure that we can pick two zonal
defects that WILL have same effect to 'raw primary' Strehl, only to see
after placing the secondary that damage at the eyepiece is worse in one
case than in the other.

But you're right, these effects would be very, VERY subtle. I guess if
we get to the point of worying about that kind of defects, most of us
would be very happy with performance of our mirrors.

Bratislav