[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
[ATM] Betr.: RTV or not.
Hello MArco,
I also have a telescope withj a 12,5inch mirror, thickness 50mm.
It is calculated in PLOP by several PLOP-experts (see below): their
opinion was: three-pointcell is sufficient, only refocus should be
needed .
I actually built the three point cell according to the
PLOP-calculations. The mirror is not glued. . It works wonderful and is
very stable. And in reality I find their is no need to refocus when the
telescop is aimed from horizontal tovertical position. (possibly the
secondary also shifts downwards a bit and compensates for the focus).
If I understand correctly, your mirror is extremely fast. That requires
a very precise and thight collimation. The more simple the cell-desgin,
the better.
I have no experience with glued mirrors. Don't know if that works
better.
Good Luck
Harro Treur
Here is some text from the email discussion on the ATM -forum (november
2004)
You can also find it in the archives by the name: [ATM] Rebuilding
the mirrorcell
**************
1) Three points should be adequate for that mirror. You could
certainly
try that first. Support radius of between 0.413R to 0.470R. That's
2.58 to 2.94 inches (65.5 to 74.6 mm).
Jeff Anderson-Lee
Sacramento, CA, USA
********************
Treur, Harro wrote:
>Dear Mr Anderson-Lee (and Mr te Marvelde),
>
>A few days ago you sent me an answer on my mirrorcell-question (see
>below) for wich I am grateful. But I have had a discussion with Mr.
Arjan te Marvelde. He finds other values with PLOP: (Arjan finds
r-0,395.)
>
>I confronted him with your email, but he does not understand your
values of r=0,41 to 0,47. He runs Automatic Cell design, fills in the
next parameters: diam = 310mm thick = 50mm focus = 1550 mm sec diam = 60
mm (?)
>cell: 3 points -> Start -> RMS error is 3.4 nm (1/150 lambda).
>Arjan te Marvelde assumes that you switched off the Refocus
optimalisation. Is that true?
>
>As a matter of fact: the mentioned parameters are assumptions of
Arjan. The real parameters of my instrument are: Diam 317mm >Thick
50mm >Focus 1517mm >Secondary 63mm
>By the way: I never used PLOP myself. Have not even downloaded it I
will do this in future. For now I rather trust on the knowledge of
experienced users.
>
>Best regards, Harro Treur
**************************
Van: Jeff Anderson-Lee [mailto:jonah@eecs.berkeley.edu]
Verzonden: dinsdag 16 november 2004 18:39
Aan: Treur, Harro
CC: 'Arjan te Marvelde'
Onderwerp: Re: [ATM] Rebuilding the mirrorcell
No, I did not turn off Refocus optimization. Instead, I did the
following:
First I ran Automatic Cell Design. The parameters I used were D=317.5
T=50.8 F=1524 S=0.
For small secondary mirrors, I usually set the secondary diameter to
zero when evaluating, since viewing near the edges of the field of view
at low power will use most of the center anyway, and the contribution
of
the fully shadowed area to the RMS error is minimal anyway. For larger
secondary mirrors, I use the secondary diameter minus the focuser
width. For a 60 mm secondary with a 2 in (50.8 mm) focuser, the fully
shadowed area would be just 9.2 mm, which is not worth bothering to
exclude.
Next I made sure that that the material was set to Pyrex and went into
the Cell Design Menu. From experience with Plop, I know that the
typical optimized 3-point refocusing radius will be around 41% and the
optimized radius without refocusing will be around 70%. However many
different radii will work. So what I did next is select the Optimize
setting on the Radius, and change it from Optimize to Scan Range. I
set
>From to 0.3, To to 0.7 and Num Steps to 40. This will test radii from
0.30 to 0.70 in steps of 0.01.
Next I went to the Cell Type page and set the Number of Rings to 20.
This will create a finer mesh for the analysis and thus produce more
accurate results. Then I went to the Run Controls window and selected
Save Output In and entered a filename for a .txt file. Only then did I
select Start.
After Plop finishes running, open the text file and look at the results
from the multiple runs. The value listed for "err" in this file is the
RMS error. You can see that the smallest value occurs at around 0.41R
with the RMS error increasing in either direction away from that
value. Once again, from experience, I know that the refocus factor
(not
listed here) will increase as you move away from around 0.7R, being
negative in one direction and positive in the other. Thus, for better
performance you are better to stay on the part of the curve between the
optimal refocus value and the optimal no-refocus value.
Next I look at the range of RMS error values. As a rule of thumb, I
like to keep this under 4.25e-6 which is roughly 1/120th wave on the
surface or 1/60th wave on the wavefront. From the list of values, I
can see that a support radius between 0.36R and around 0.47R will
satisfy this. You can also switch back to Optimize mode for the radius
in the Cell Design tab to see what the recommended optimal radius is.
In this case it is around 0.413R plus some more meaningless decimal
places. (Turn off the Save Output at this point or you will overwrite
the previous file.)
If you compare the results for the "optimal" radius, with the results
for 0.47R, you will see that the RMS value differs by less than 10%,
which would roughly be the difference between 1/60th wavefront and
1/66th wavefront RMS. This is not worth struggling for when you
consider that the mirror is likely more like 1/10 wave (surface) or
1/5th wave on the wavefront (or probably less). Plus, we had already
set our design goals on better than 4.25e-6 RMS. If I had found that
the no-refocus support points satisfied this criteria, I would probably
use them anyway, even if the refocus optimal points gave "better" RMS,
since they have the benefit of not contributing to focus shift as you
slew the telescope and the difference in support is likely to
contribute
insignificantly to the image quality.
Thus, allow yourself some flexibility in where you place the supports,
and since you are going to have some inaccuracy, I feel it is better to
lean towards the no-refocus value than away from it.
Does that clarify things?
Jeff
_____________________________
ATM mailing list http://www.atmlist.net/
Met vriendelijke groet,
Harro A. Treur
020 - 5536 931
Team Voorbereiding en Toezicht district 1
Afdeling Infra Leidingnet
Sector Drinkwater
DWR is per 1 januari 2006 gefuseerd met Waterleidingbedrijf Amsterdam tot Waternet.
De adressen en telefoonnummers blijven voorlopig gelijk.
E-mails die u stuurt naar de oude adressen van DWR en WLB komen bij de juiste mensen terecht.
**********************************************************************
De informatie verzonden in dit E-mail bericht is uitsluitend bestemd voor de geadresseerde. Gebruik van deze informatie door anderen dan de geadresseerde is verboden. Openbaarmaking, vermenigvuldiging, verspreiding en/of verstrekking van deze informatie aan derden is niet toegestaan. Waternet staat niet in voor de juiste overbrenging van de inhoud van een verzonden E-mail, noch voor tijdige ontvangst daarvan.
The information contained in this communication is confidential and may be legally privileged. It is intended solely for the use of the individual or entity to whom it is addressed and others authorised to receive it. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution or taking any action in reliance of the contents of this information is strictly prohibited and may be unlawful. Waternet is neither liable for the proper and complete transmission of the information contained in this communication nor for any delay in its receipt.
**********************************************************************
_______________________________________________
ATM mailing list http://www.atmlist.net/