Aperture Masks

I guess I kinda know the answer to this but would like to throw it around
for some thoughts.

Ok, I have a 8" f/6 newt. I can make an off axis aperture mask of 80mm
and still clear any obstructions (vanes and secondary). This I understand
now turns my 8" f/6 newt into and un-obstructed 80mm f/15 reflector, right?
Would this now perform like or similar to a 80mm f/15 refractor now that
the central obstruction is gone?

Also why cant you use a focal reducer on a newt like you can on a SCT.
Apart from the corrector plate and the fact a SCT fires the photons back
though the centre of it's primary as appose to a newt who fires it sideways.
There isn't a big difference in the basic concept of each. Can you use
a FR on a Mak-Newt? is there any kind of focal reduction you can do on
a newt?

thoughts and discussion appreciated.

Hope your having a great weekend guy's

regards,CS

[ving (David)]

hi rob,

I have a 80mm off axis aperture mask on my 8" and it clears the vanes. it does turn it into a 80mm f15 as you say but weather it looks like a refractor i dont know. get some cardboard and try it is my sugestion

[rmcpb (Rob)]

I use an aperture mask routinely on the Moon and Venus and get better views. Don't know if it is like looking through a long focal length refractor but the views are definitely improved on these bright objects.

As they are so easy to make, I'm with Ving, give it a go

[Merlin66 (Ken)]

Focal reducers can be used on any telescope configuration. Just depends on getting the lens ( an old bino objective works well!!) into the correct position. This is obviously easier on the SC where the main mirror moves to give more back focal length. Think of it as a "positive barlow lens"

[ausastronomer (John Bambury)]

Originally Posted by xrekcor

Ok, I have a 8" f/6 newt. I can make an off axis aperture mask of 80mm
and still clear any obstructions (vanes and secondary). This I understand
now turns my 8" f/6 newt into and un-obstructed 80mm f/15 reflector, right?
Would this now perform like or similar to a 80mm f/15 refractor now that
the central obstruction is gone?

Rob,

Yes you are basically turning your scope into an 80mm/F15 APO with a couple of differences. Being a reflective optic no fasle colour at all, the downside is about 80% light transmission compared to high 90's for a refractor. so maybe a 75mm APO? Can't be bothered doing the numbers.

I occasionally use a 93mm off axis mask on my own 10"/F5, which turns it into a 93mm/F13.4 refractor. Invariably full aperture is a better way to go because in the fleeting moments of good seeing full aperture shows a lot more detail and provides a lot brighter image. However the off -axis mask does make the views "refractor like" in that the stars become tight pinpoints and the effects of diffraction are eliminated, although lunar/planetary images are dimmed considerably. Andrew Murrell occasionally uses a 7" off axis mask on Hector (20" dob) for planetary observation as the larger aperture is sometimes severely affected by poor seeing yet 7" is still plenty of aperture to extract good detail

Originally Posted by xrekcor

Also why cant you use a focal reducer on a newt like you can on a SCT.
Apart from the corrector plate and the fact a SCT fires the photons back
though the centre of it's primary as appose to a newt who fires it sideways.
There isn't a big difference in the basic concept of each. Can you use
a FR on a Mak-Newt? is there any kind of focal reduction you can do on
a newt?

Optically there is no reason you can't use a focal reducer on a Newt or Mak Newt, if you can get it to reach focus.

A SCT/MCT focuses by moving the primary mirror axially along the optical tube, therefore it has an almost infinite amount of focuser travel in comparison to a Newt or a Mak Newt, or even a refractor for that matter.

CS-John B

[gbeal]

Most of the replies have been in line with what I would have written, so I will spare you the repitition.
The question begs though, "Why bother"?
I always believe aperture rules, so why stop it down? Poor optics? get rid of it.
Less affected by seeing? Wait for a decent night.
F/Reducers are another kettle of fish.
On an f10 SCT they are invaluable, but on an f6 Mak/Newt (assuming an 0.5x reducer giving a total of f3, again "Why"?
I have a .5x reducer and may try it just to see, but at f6 it is nearly fast enough anyway.
Good to experiment, and hypothesise though.

Thanks you all the replies, I put together an aperture mask yesterday. Since the
seeing was soso late last night I gave it a whirl on Saturn, there was a definite
sharpening of the planet disc and ring system, and also a definite loss of resolution,
no banding on the planet disc or different shades in the ring system. As opposed to
full aperture where all these feature present them selves. Interestingly though the
cassini gap was slightly easier to observe with the mask. The background sky was
darker with the mask too.

Splitting double stars Castor and Pollux was visually easier too.

John, I'm looking into one of the new 16" GSO dobs, I figure if I can make an 80mm
mask for the 8" then I should be able to (maybe) make one of around 160mm for the
16".

Gary, ideally I would like to reduce it to an f/4 or there abouts. For wide field visual
and possibly imaging. The optics in my scope are fine in respects to why use an
aperture mask. I'm mainly just experimenting on cutting light down. I live in dark skies
and find at time the planets can overwelmingly bright. No question about it aperture
rules here too.

Anyways thanks for your thoughts guy's

regards,CS

Originally Posted by gbeal

I have a .5x reducer and may try it just to see, but at f6 it is nearly fast enough anyway.
Good to experiment, and hypothesise though.

Gary, I would be really interested in hearing the results

regards,CS ...loving it at present, but can someone take the heat away

[ving (David)]

hi rob,
with the ap mask you made try cutting 3 more 80mm holes and see what that does. you could run with 1 to 4 holes depending on what seeing would allow

Originally Posted by ving

hi rob,
with the ap mask you made try cutting 3 more 80mm holes and see what that does. you could run with 1 to 4 holes depending on what seeing would allow

I think you would then be re-introducing a central obstruction. I did think
about increasing area by changing the shape of the mask to fit the area.
Kinda like a trunicated quarter circle. However this is an extreme shape
far removed from being a circle.

regards,CS

[ving (David)]

er... yeah actually you are right. but is the objective of the mask you created to get rid of the central obstruction or were you trying to sharpen bright planets and doubles by reducing the aperture?

Originally Posted by ving

er... yeah actually you are right. but is the objective of the mask you created to get rid of the central obstruction or were you trying to sharpen bright planets and doubles by reducing the aperture?

Pretty much both as it does both. But really I'm just experimanting. From last
night.... well early this morning the results showed a crisper over all Saturn
with a loss of resolution using the mask, kinda like the focus was better.
Where as without the mask the edges of the planet disc and ring system
were slightly more diffused. Which has had me thinking if I could used this to
improve planetary imaging, by mixing the crisp views of the mask with the
colour and resolution of no mask.

I'm basically just playing around here and seeing if I can improve on what I
have. And posting here hopefully hear from others and their experimentation.

regards,CS

[ving (David)]

do you have any flyscreen type mesh?

33south (chris) was telling me he heard that if you put that over the full aperture of the scope it improves views of planets... I odnt understand why its supposed to work but plan on testing this at some stage.

No I haven't but good idea, which has given me another idea. You can get mesh
from Mitre 10 which comes grades 30% 50% 70% in respect to how much light it
lets through, They use it over kids playgrounds and such. The 30% or maybe even
50% might be good for fullmoon observations. And possibly imaging, would be the
same as a cheap moon filter except cheaper lol.


My understanding of why is, planets are generally bright, low contrast, objects by
nature. So by reducing their brightness and taking away some of the glare. Some of
the low contrast features can be easier to observe. Perhaps someone else has a better
answer, but I think that's the general gist of it.

regards,CS

[ving (David)]

heres an excellent explanation of what happens with air cells and aperture....

When one is using a telescope with an aperture smaller than the air cells, the tendency will be for the definition to be good. When the air cells are smaller than the aperture of the telescope, and especially if those cells are moving fast, definition will be poorer. Air cells will have a slight difference of refractive index, and as they pass across the aperture, a displacement to the wavefront occurs. When no displacement is perceived seeing is said to be perfect. The probability of there being air cells larger than an aperture greater than 12-inches is vanishingly small. Couple this with the tendency of large Newtonians to have fast f/ratios, and you combine the worst of both scenarios.

The image of say an 18-inch f/4 Newtonian is far less likely to stay in focus, in Antoniadi seeing III, or even I-II, than a 10-inch f/10 Newtonian. Astronomers sometimes stop down the aperture of their telescope to combat poor or indifferent seeing, in an attempt to match the aperture to the size of the air cells, which range from a mere 1/3-inch to a well over a foot, but are most commonly between 4 and 8-inches. Stopping down the aperture not only matches the aperture to the seeing, it increases the depth of focus thereby enabling a much more comfortable observing session because the observer no longer has to continually refocus the image. It has been our personal experience that seeing does tend to appear better in long f/ratio telescopes.

taken from http://www.brayebrookobservatory.org