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Old Tuesday 14th March 2017, 11:27   #1
joerg2712
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Introduction and question to the aperture of scopes

Hi all photographers and experts/tinkers/inventors and birders here.

A very big praise to all. A great forum with very familiar terms and a lot of tipps and knowledge and wonderful pictures.

For years, I am a "read-only-user" of this forum, because my English is not really good (SORRY!) Google Translator is my best friend. Now it is time to learn and practice the English language and convert to a "read-and-write-user".


Some informations to my person.
I come from the south of germany near Stuttgart. I'm 55 year old.
I have been photographing the beautiful nature, flowers, mushrooms, animals and landscapes for many years.
I'm active from the old analog Minolta X700 to the modern Pentax K3II with an TS APO 80/480 scope and a lot of old and very old lenses.

I am very interested in all kinds of tinkering around cameras, lenses and accessories.

Now my question:

The sharpness of a good apochromat is perfect for birding photography. My problem is the extremely small depth of focus field.
The eye is sharp, the beak and tail are already in the blurred area.

My questions to the experts/engineers: What is the better solution (image quality) for an aperture?
a) At the front end of the dew cap ((simple construction))
b) In the tube between optic and focuser?
c) At the back end near to the camera (not so simple)

Does anyone have experience with the construction of a diaphragm (perhaps from an old photo lens) at the scope?

Thank you and best regards
Jörg
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Old Tuesday 14th March 2017, 12:06   #2
DanC.Licks
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Servus Jörg,
Welcome! Your English is fine!
A diaphragm SHOULD be as close to the optical node* as possible, which is difficult to do. I have had some luck with putting one on the end of the focus tube, inside the scope.
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Hard to do and not easy to adjust!
The dew cap solution is simple, but by reducing the size of the front element you are actually cutting resolution. There IS a difference between an f/4 lens stopped down to f/8 and a lens that is f/8 to begin with. Putting a diaphragm near the camera is not as effective at increasing the depth of field, and it greatly increases vignetting.
So inside the scope tube is the best place, but the question is how to do it...

Have fun and let us know if you figure out a way.

* The spot in the light path where the image turns up side down.
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Old Tuesday 14th March 2017, 12:45   #3
joerg2712
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Hi Dan, thanks for your reply.

Quote:
Originally Posted by DanC.Licks View Post
Your English is fine!
Google Translator and leo.org and the rest of the knowledge of a lazy student and a lot of time

The mounting in the focuser is not complicated if the diaphragm is not adjustable. Your idea with the adjustment inside (the small lever right in the picture?) is very good. Now I only need a matching eBay offer with a lens with diaphragm that fits in the focuser (2"). An enlarging lens could fit.

Jörg
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Old Tuesday 14th March 2017, 13:41   #4
DanC.Licks
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Actually, the best placement of the diaphragm is not the node, but about half way in between the front element and the focal plane, which would put it well within the tube of the scope. The one pictured is out of some throw away lens, can't even remember what it was, something I found somewhere. Its position in the end of the extension tube is actually too far back and it caused too much "vignetting", in quotes because it was actually just the shadow of the diaphragm cast on the sensor. Vignetting occurs if it is too far forward. It could be adjusted by turning the extension tube. The rubber ring on the lever touched the inside edge of the focusing tube and thus activated the diaphragm. Very iffy, and I ended up ditching the whole idea. A diaphragm on the end of the focusing tube would be better placed, but impossible to adjust, although a good Bastler could for sure make something radio controlled. Not me....
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Old Wednesday 15th March 2017, 14:30   #5
Paul Corfield
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Welcome Jörg.
I keep one a diaphragm from an old lens on the end of the focus tube. I have it set fairly wide open, just use it to stop stray light more than anything. I use another diagram on the 2" camera to scope adapter. If I am using a barlow or teleconverter then I will keep it set to quite a small hole to stop any stray light. Most of the time I have it set up as per these photos. The long tube and the diaphragm really help stop any stray light an keep the contrast high. If I am not using a barlow or teleconverter then I set the diaphragm wide open. I find the depth of field on the f7.5 Skywatcher 80ED is ok. On the rare occasion that I need extra depth of field I just use the dewcap shield that comes with the scope. I find if you get too much depth of field it makes focusing much harder.

Paul
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Old Friday 17th March 2017, 07:44   #6
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There was a discussion some years ago on the subject of iris for the TLAPO804.

Search link: http://www.birdforum.net/showthread....ht=iris&page=2

For instance post #31
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Old Friday 17th March 2017, 11:25   #7
Paul Corfield
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I think it would need to be further into the scope than how Fernando had it though. It would probably vignette more than increase depth of field where it was in the photo. I'd try the lens cap way first, the tiny loss of resolution will be undetectable in the photos, you get the benefit of better contrast and the added depth of field.

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Old Saturday 18th March 2017, 11:29   #8
DanC.Licks
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Lens cap definitely better than the iris so far back!
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Old Saturday 18th March 2017, 16:27   #9
joerg2712
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Hi, thanks for all informations and links.

I also talked to an optics specialist. The position at the back of the focuser is optically not ideal.
According to his opinion,
- the aperture should be fixed at the tube
- just behind the lens.

This fits to Dan's statement "half way in between the front element and the focal plane".
Disadvantages:
- The diaphragm must be relatively large.
- An adjustment of the diaphragm in this position is difficult to implement.

Please do not laugh now! Yesterday I tried an approach with remote control.
From an old Enna-M42-lens I have a aperture, which can be adjusted steplessly and almost with no torque.

I glued a small permanet magnet on the aperture ring. This can be moved with a more powerfull magnet through the aluminium tube.
This "magnetic remote control" adjust the diaphragm from outside without contact. The tube remains uninjured. :-) This actually works quite well.

Unfortunately, this diaphragm from the old lens is too small for a position in front of the focuser (open diameter approx. 20 mm).

In the next step, I will test the aperture with fix hole plates in different diameters between lens and focuser.

Jörg
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Old Sunday 19th March 2017, 08:00   #10
DanC.Licks
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Good idea! Keep trying. You might consider mounting the diaphragm on a piece of thin plywood of plastic or aluminium that could be fit inside the scope tube a little in front of the focus tube so that it would always be in the same position. A lever with a small magnet on the end could then be quite close to the wall of the tube.
I would be a little carefull with magnets though. You wouldn't want to erase your card or your camera's software by accident!
My action finder on my 400/5.6 is held in place with a small magnet from a drive bit holder, but it is way out at the end of the lens, and can't go anywhere. No problems yet.
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Old Tuesday 21st March 2017, 10:20   #11
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Interesting...

The TLAPO804 has a splittable tube which should allow for relatively easy access to work on an iris.

To keep things simple say we place the iris halfway between the front lens and the sensor. The front element has 80mm diameter and in the case of m4/3 the sensor has a 22.5mm diagonal. This means that the iris should have a diameter of (80-22.5)/2 to avoid vignetting meaning 28.5 mm. Am I right in this? (The requirements on the iris diameter would be given by its axial placement inside the tube and also the actual sensor size).
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Old Tuesday 21st March 2017, 11:30   #12
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Not sure, but I found one that has an opening of about 28mm. I will give it a try. I envision a light weight lever with a small magnet attached to the end, hanging straight down with the scope mounted in the gimgal. On the outside I will make a band of neoprene with a hole punched in it for a second magnet. The outside magnet will not only hold the inside magnet in place, by turning the neoprene ring it will adjust the diaphragm. There is a baffle in the tube in just about the right place so I will put it in, mounted in a piece of hard styrofoam, right behind the baffle. That will place the diaphragm about 2 cm. in front of the focus tube when set at infinity.
If it works I will take pictures of it. ;-)
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Old Tuesday 21st March 2017, 13:29   #13
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I was wrong about the size of sensor included in the equation. Apologies for any confusion it may have caused. I have made a sketch to explain. Lens assembly simplified/abstracted as a single lens.
The first is a sketch of the scope focused on infinity and iris placed with this in mind.

If the iris is (ideally) placed halfway in the optical path between front lens and sensor then the iris diameter should be half of the front lens diameter. In you case, Dan, your scope has a 90mm front lens and 600mm focal length. That would require a 45mm iris, which is not so easy to find I guess (and expensive?) The 28mm iris you have should be placed at about 187mm in front of the sensor. Closer to the front lens would mean some stopping down even when iris is open. Closer to the sensor would mean stopping down takes effect first after you have closed the iris to the extent needed.

The second one shows what happens when the focuser is fully extracted for closest distance, with the iris still placed for infinity focus. The third shows iris placement re-calculated for closest focus distance.

If the scope is to be stopped down at closer distances (which is probably the most important use case, gaining DOF for close subjects, then you need to add focuser extracted at maximum. Say the focuser can be extracted 100mm, this brings the focal length to 700mm.
The 28mm iris should then be placed at about 218mm in front of the sensor. Now remember we have added 100mm to the optical path so the iris should be placed about 31mm closer to the sensor, compared to the location for the focus at infinity. Otherwise you would get some stop down without asking for it.


Hopefully I got it right this time.
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Old Tuesday 21st March 2017, 13:34   #14
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By the way - where could one find an iris at reasonable price point? It would be cool to test, thinking about how to take advantage of the TLAPO804 splittable tube. eBay?
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Old Tuesday 21st March 2017, 18:25   #15
DanC.Licks
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Tord,
You forgot that the image gets flipped before it reaches the sensor. How far in front of the sensor that happens I do not know.
I would just go to some photo second hand shop and pick up a couple of junk lenses. The one I will use was out of one I tore apart for the telenegative. Don't remember which one, I think the Canon 100-200.
As I remember when I tried the iris on the end of the focusing tube it did not stop down noticeably when wide open. We will see...by tomorrow I will know more.
The closer you focus any lens the darker it gets, so there is automatic "stopping down". The f-stop is for sure calculated at infinity.

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Old Wednesday 22nd March 2017, 08:48   #16
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Well, the good news is it works! The magnet idea was brilliant! Here is what I came up with:
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Should be pretty much self explanatory. It is placed just inside the lip of the tube so you can see how it works.
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Old Wednesday 22nd March 2017, 09:02   #17
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Here "wide" open, stopped down one stop, and then two stops.
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I had it placed just in front of the focus tube at infinity. That comes to about 320mm in front of the focal plane.
There is no vignetting on the 4/3 sensor. Shots of the gray sky are clean.
The BAD news is that "wide" open it is stopped down roughly one stop, so 28mm is too small. My guess is that about 35-36mm would do it.
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Old Wednesday 22nd March 2017, 11:49   #18
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This one would work perhaps...
http://www.ebay.co.uk/itm/Microscope...oAAOSwiLdWAlXf
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Old Wednesday 22nd March 2017, 14:19   #19
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Quote:
Originally Posted by DanC.Licks View Post
Here "wide" open, stopped down one stop, and then two stops.
Attachment 619481 Attachment 619482 Attachment 619483
I had it placed just in front of the focus tube at infinity. That comes to about 320mm in front of the focal plane.
There is no vignetting on the 4/3 sensor. Shots of the gray sky are clean.
The BAD news is that "wide" open it is stopped down roughly one stop, so 28mm is too small. My guess is that about 35-36mm would do it.
If you get exactly 1 F-stop down with 28mm aperture then you will need a 40mm aperture. Using the formula: F-stop = log2 ((d1^2) / (d2^2))

Measuring with the camera, which has a scale with 1/3 EV resolution is tricky though. To be on the safe side and eliminate risk of any F-stop with iris open you should take the 1/3 EV into consideration which would require a 44mm aperture.

Let me know if i did not manage to explain in understandable form.
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Old Wednesday 22nd March 2017, 17:45   #20
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I just had a long chat with my telescope expert friend, and it seems that I have misunderstood a couple of things about resolution etc.
This is the gist of our conversation:
If a lens is diffraction limited, as our scopes normally are, and as are high quality camera lenses, it means that they reach their highest resolution wide open. There is nothing to be gained by stopping down as is often the case with lenses of less quality. Assuming that a scope is diffraction limited, the only thing gained by stopping down is depth of field, BUT, it comes at the cost of resolution. It doesn't mater where you stop it down (not counting so far back that you are only getting vignetting) the result is the same, more depth of field, less resolution. So the dew cap method is in the end actually just as good as having an iris diaphragm inside the lens, though less practical.
The best place for a diaphragm inside the scope is as far forward as possible. It is naturally not possible to put one right behind the lens block as the iris mechanism takes up some space and it would have to be huge. A compromise would be somewhere in the middle. So a 44mm iris would do about where I put it, but it would naturally have to be larger if it is to be further forward. One like this one
http://www.ebay.com/itm/2-5-44mm-Iri...EAAOSw241YUoMr
or larger
http://www.ebay.com/itm/Iris-Apertur...EAAOSwB4NWwUhI
would do nicely. But I don't think I am going to do it. Have to think about it.... I almost never have enough light for the scope anyway.
At least we have proven that it can be done, and quite easily.
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Old Wednesday 22nd March 2017, 22:11   #21
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I am tempted to give it a try. The splittable tube in the TLAPO804 should make it easier.

Any requirements I should have on the magnets, by the way?
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Old Thursday 23rd March 2017, 08:54   #22
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I just used two little magnets out of cheap bit holders. You can get stronger ones though, and it wouldn't hurt. I would keep the one inside as small as possible. Careful! They are surprisingly easy to lose! I would keep the outside one on the inside of a band of some sort, something that moves easily but also keeps it in place.
The outside diameter of the larger iris is 76.5mm. My tube inside is about 84, so there is not a lot of room to play with. Your tube is no doubt larger.
What I can't tell from the pictures is whether the adjustment is done by sliding or turning the lever. I have looked at a bunch of pictures on ebay, and the larger ones all seem to be turning, so you would have to drill a hole in the tube once you determine the exact position with cutout baffles of the right diameter depending on which one you get. It would in the end be far better as you could adjust it very easily and precisely.
Edit: Looking at more pictures on google images I would say they are more likely to be of the sliding type.

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Old Thursday 23rd March 2017, 11:53   #23
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It just occurred to me there are light meter app for smart phone, so no need to rely on the 1/3EV resolution.
https://play.google.com/store/apps/d...lan.lightmeter
These are for Android, similar apps should probably be found for iOS.
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