Iris Diaphragm Useless in Visual Observations

Iris diaphragm useless in visual observation?

Hello everybody,

there is a plain, simple device being offered at Edmund-Optics, a T-barreled iris diaphragm:

Link:http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1966

It nicely fits into any T-threaded equipment such as diagonals, prisms, eyepiece holders,... but what for ?

I repeatedly observed contributions in discussions groups questioning whether it was useful to apply an iris diaphragm instead of a set of ND-filters, mostly in cases where pictures were found to be to bright such as in observing the moon or outer planets. The Mars Opposition sparked off some of that debate, always to be answered by more senior experts that there was hardly anything to be expected thereof. I also put this idea forward to two of my more experienced friends, both learned scientists. The outcome was almost predictable: „You can´t just put in blinds at deliberate places of a scope“ and a frank „ this is not going to work anyhow“.

Thus encouraged I got me one.

To sell it short: I found it works pretty well. At least with the sort of telescope I have at hand, a 127/1200 Antares FH, a Williams FLT 132/925 and a small Newton.

For the refractor type telescopes I screw the iris diaphragm between the end of tube and the diagonal/prism. With the newton it is just screwed into the focuser under the eyepiece holder.

The following observations where made with eyepieces ranging from 14 mm to 3,2 mm. With eyepieces of longer focal length the inner rim of the closed iris diaphragm would merely become visible as a limitation of the field of view with no gain.

Planets:

Mars and Jupiter were significantly enhanced if it comes to contrast and details visible. Effects from poor seeing were slightly improved. Of course you would not close the diaphragm fully so as to block out all the light but you would only close the diaphragm by 10 % - 25 %. At this range the iris diaphragm takes the job of ND filters with a transmission of 90% - 75 % that would be desirable for planetary observations but are unavailable on the market for astronomical use.

The effect on detail became more rewarding in the 127 FH, the 132 FLT by its nature here always delivering a picture that can hardly be improved. In terms of contrast the performance of both telescopes gained from a small tip on the adjustment lever thus closing the iris to some extend.

With the small newton I found that almost the same applied, but for diameters of less than 50 % of the iris´s opening. At this point or lesser openings the silhouette of the secondary mirror would increasingly rise from the dark background thus finally eliminating any object centered in the eyepiece. But to close it by 10 % - 20 % still works well.

You will of course find with all three types of telescopes that the more you close the diaphragm the more you will have to adjust focus, but only slightly. You will also observe that there is no color shift as with some filters recommended to increase contrast in observing planets.

Moon:

I had the same experience when observing the moon. You may comfortably adjust to the brightness of the area or feature you are addressing but keep in mind, 10 % - 25 % of closing is enough. The diaphragm thus works as an adjustable ND filter adding no extra color or fringe to the picture. If you close it severely by let´s say 50 % or more the FH would produce a more significantly seen color fringe.

Deep sky:

Of course deep sky may be found a rather exotic if not inadequate field of use of an iris diaphragm and a telescope but still lets try this one:

The disturbing experience was: It also works. Closing the iris diaphragm by 10 % to 20% enhanced contrast significantly. This was to be found with all three types of scopes I use. Galaxies would stand out more clearly from a darkened background, especially the tiny ones, details become visible in gaseous objects in the 127 FH and by repeatedly closing and opening the diaphragm on extended homogenous objects like comets the brighter parts may become more distinctively apparent something that what would otherwise elude the eye for lack of contrast. All observations were made from a backyard city light lit up area in the outskirts of a medium sized town.

With appropriate objects the effect is to be compared almost to a conventional UHC filter but adding the use for galaxies where these filters would not work.

Some thoughts on theory:

I am just delivering some ideas on what I have described, so the following is no concise and serious concept of a theoretical approach. The first thing to think of would be stray light. This presumably exists at a much larger scale than would normally be expected in telescopic systems designed to block out any stray light by carefully placed baffles. The usage on planets suggests the iris diaphragm to diminish the cone of light extending from the front lens thus cutting out the core of the lens and resembling a smaller aperture and thereby producing its beneficial effects.With the FH some of the rays passing through the exterior parts of the front lens might be blocked, thus resulting in lesser shift of color Deep sky use may benefit from the eye opening more widely if the field of the eyepiece is darkened up to a useful grade. Presumably the eye´s iris is opening to a greater extent than the closing of the iris diaphragm would require thus more than compensating for the loss. There may be further aspects that I am not aware of.

Conclusion:

The iris diaphragm offers a princely tool to improve performance of your scope. Be reminded you would not end up with a new scope but performance may only expected to be enhanced within the limits I described. It may also be subject to personal judgment whether you may find the actual change of or increase in performance at all to be of significance to you. As to me, I got me a second one adding them to the standard configurations of the scopes that I mostly use.

Ragold Wenz,

Germany