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having problems and not impressed with binoviewer

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#26 johnnyha

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Posted 12 January 2013 - 05:16 PM

The new Denkmeier Bino27 is collimatible, looks like a real winner.

#27 Eddgie

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Posted 13 January 2013 - 10:17 AM

No, not complicated.

You need a good ruler and a bright light source like a laser or single emitter LED flashlight.

Then next time you observe, at the end of the session, use your binoviewer to focus on a bright star.

After focusing, do not touch the focuser until you complete the test. Of course you can do the test the next day or the next week, but don't touch the focuser until after you have done the test. You can remove the binoviewer if you like. It is not needed for the test. The only important thing is to not touch the focuser.

When you are ready to do the test, put your mount as scope near a wall and face the telescope to the wall as square as you can get it. The distance is not critical, but keep it close as you can but still reach between the scope and the wall.

Use a mid-power eyepeice (not critical as to which kind or focal length, but a smaller eye lens makes it easier to hit the center of the focal plane). If you have a laser collimator, this is even better. The eyepecie can be put in the diagonal or directly into the visual back. Makes no difference.

Now, hold your laser or light source about a foot or so (not critical) above the eyepiece and shine it in. It will take a bit of fiddling, but when the light source is perfectly aligned to the optical axis, you will see the image of your aperture projected on to the wall.

You might need and assistant, but now you only need to take your ruler and measure across at the widest part.

This measurment will tell you the effective aperture of the system.

I usually put a little piece of tape on either side so that I can mark with a pencil on the tape while holding the light source. Then, when I have marked both sides, I can use both hands to measure.

Easy test, and you get a very accurate reading of your aperture.

I do not know if your scope will have any aperture loss or not, but I know my C5 does, and this is using a light path that is about 200mm (2" SCT adapter, Maxbright prism diagonal, Mark V binoviewer).

The light collection of a C5 is only about 18.5 square inches (adjusted for secondary obstruction).

Even a 5mm light loss will bring the total light collection down to about 17 square inches. This by itself is almost a 10% loss of brightness. Barely enough to notice, but it all adds up. Combined with the brightness loss of the binoviewrs, you get a dimmer view.

But I am not at all sure that you are loosing any apeture, so this is just a data point for future posters to know.

But it also explains why I went to a bigger aperture binoveiwer. I needed some of those lower powers to get back some exit pupil to make the views brighter. For me, it was not so much true field as it was image brightness.

I am even considering going to 32mm Plossls over the 24mm Hyperions.

Binoviewers for me have been a constant evolution. Forcing me to try a lot of diffent things and accept a lot of limitations. I love them for planets, but still not 100% sold for deep sky.

#28 nemesis256

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Posted 13 January 2013 - 07:24 PM

So I did the test, using an 18mm eyepiece, the same ones I use for binoviewing. I take it from your instructions that I was supposed to make measurements without the binoviewers. So I focused without the binoviewer (for comparison), measured, focused with the binoviewer, but then removed it and used only the eyepiece, and measured again. Both measurements were around 150mm, which is the aperture of my scope. Then I measured with the binoviewer for curiosity, and the measurement was 140mm, about 1 cm less. Does this tell us anything? Is the loss of aperture from the 22mm binoviewer?

Oh, btw, I focused on a far away light, not on a star. I'm pretty sure the focus was close to infinity anyway, since the light was already mostly in focus from the last time I used my scope.

#29 Eddgie

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Posted 14 January 2013 - 01:13 AM

It doesn't matter if you use the binoviewers or not as long as you don't change the focuser.

As for not being at true infinity, it might make a little difference. Once the system is in in aperture loss, even a small movement of the focuser can make a difference.

The point was to try to determine if aperture loss is contributing to your dim view. 3 Or 4 millimeters will go unnoticed, but 10mm is enought to dim the view.

I know that this was the case the first time I used a 2" based binoviewer in my C14. The aperture was cut down quite a bit.

This is why keeping the back focus as short as possible is important in most SCTs.

So, now you have dimming from the binoviewer (always) and perhaps some dimming due to some aperture loss.

I would repeat the test the next time you have the scope at true infinity focus though.. My bet is that you see a slight increase in aperture. Again, when you are in the aperture loss configuration, a millimeter of mirror movement translates to a couple of millimeters of aperture loss.

You know how to check it now.

Binoviewers are complicated. If you have followed my posts in the past, you would see that I think keeping the light path as short as possible is one of the great keys to getting the best performance while using binoviewers. This is why I am such a fan of the Baader approach.

I see that Siebert is now offering a similar diagona/bino interface.

#30 GlennLeDrew

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Posted 14 January 2013 - 12:56 PM

When doing the flashlight test for aperture, it's actually very important to keep the configuration unchanged after setting the focus. If a binoviewer was installed, leave it in place. Here's why...

When the instrument is focused with the BV installed, the physical location of the focal surface is moved farther back. If you remove the BV, your eyepiece's field stop will be now located *inside* the focus. Not good.

The flashlight aperture test is simply 'raytracing' the system in reverse. The telescope forms an image of a star as a point in the plane if the eyepiece field stop. Shining a flashlight into the eyepiece (from a distance of no less than about 10 eyepiece focal lengths) forms a tiny image of the light in the plane of the field stop. This acts like the image of a star, but now shining through the system in reverse.

As long as the image of the light is located at the scope's focal surface, one can rely on the envelope of light passing backwards through the system being identical to the envelope accepted from a distant point target and being brought to focus.

If you set the focus with a BV installed, and then remove it so that the eyepiece is now inside the focus as set and hence closer to the OTA's back end, the light source image is located artificially closer to the objective, and an angularly wider light cone can pass through, thus very potentially leading to an artificially too-large working aperture.

Bottom line: For best accuracy, always leave the full system intact as it was when setting the focus.

#31 Eddgie

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Posted 14 January 2013 - 01:29 PM

Thanks for the correction to my instructions. I did not think it mattered.

#32 gcfboulder

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Posted 25 January 2013 - 11:55 PM

My Siebert BN25 works quite well with my C8 and my 2" Stellarvue diagonal. But now you have me wondering if I should try it with a 1.25" diagonal. But the only 1.25" diagonal I have is the cheapie one that came with the scope, so I don't think that would be a good test.

#33 faackanders2

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Posted 26 January 2013 - 10:29 AM

Is it a proble with not having enough infocus at the limit all the way; or not having enough outfocus at the limit all the way out?

Higher powers are always less clear than lower powers, just like a single eyepiece.

Did you focus each individual eye to their sharpest point, pulling out eyepiece if more outfocus needed?

Did you mover IPD in and out to ensure you get on clear outer circle edge?

If you did all the above, and used your lowest power eyepieces, and you still can't merge (but others can) then you are just not fit for binoviewers. If others can't get it to merge either after doing all the above steps, then there is someting wrong with the collimation (or you are using too high a power eyepieces, since images in both eyes must be less than 10% difference to get brain to merge, and the less they are different the less likely you will bet tired or headaches since your brain doesn't have to work so hard to get both eyes to merge.

#34 EdZ

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Posted 26 January 2013 - 11:10 AM

C6 witth diagonal and binoviewer is operating at about a reduced aperture of 118-120mmm.

Extensive testing notes in the SCT forum for C5, C6 annd C8 showing aperture tests aand plotttiing operating focal length
http://www.cloudynig...ber/5102760/...

also, C6 with binoviewer is operating nearr F=1800, so F=1800 and D=120 (my setup) = approx f/15.

Not surprised you're not impressed with image. that is FAR outside the operating design range of the 6" SCT mirror.

For operating focal length see
http://www.cloudynig...ber/5092859/...

#35 EdZ

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Posted 26 January 2013 - 11:19 AM

a C5 with standard diagonal and binoviiewer is operating near F=1700. That's f/13.6 before even taking into consideration the aperture losss. I don't recall, but I'm thinking aperture is cut down to about 100mm, so operating foocal ratio is near f/17.

Regardless what you read and hear elsewhere, because of the vast change in operating focal length and the dramatic reduction in aperture, small SCT scopes were not designed to be used with binoviewers. Can you do it? Sure, you can do anything.

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#36 faackanders2

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Posted 26 January 2013 - 02:32 PM

Ed, i don't have an SCT but would the "starsweeper OCS" help or worsen the SCT binoviewer issue. I have always been entrigued with the starsweepers low power, but disapointed it would not work for Newtonians,

Ken

#37 EdZ

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Posted 26 January 2013 - 03:09 PM

Ed, i don't have an SCT but would the "starsweeper OCS" help or worsen the SCT binoviewer issue. I have always been entrigued with the starsweepers low power, but disapointed it would not work for Newtonians,

Ken


As you will note by reading the results of the testing done in the thread refferencced above, the startsweeeper worsens thhe situation.

It does reduce the effective power at the end of the train, but what a reducer does is causes the need to move the mirror such that it substantially reduces the operating aperture. Using a star sweeper reducer results in a 6" SCT operating at less than 4".

#38 faackanders2

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Posted 26 January 2013 - 03:26 PM

Ed, i don't have an SCT but would the "starsweeper OCS" help or worsen the SCT binoviewer issue. I have always been entrigued with the starsweepers low power, but disapointed it would not work for Newtonians,

Ken


As you will note by reading the results of the testing done in the thread refferencced above, the startsweeeper worsens thhe situation.

It does reduce the effective power at the end of the train, but what a reducer does is causes the need to move the mirror such that it substantially reduces the operating aperture. Using a star sweeper reducer results in a 6" SCT operating at less than 4".


Thanks for the simple explanation. No wonder why SCT owners want to binoview with the mirror as far back as posible.

Would it be possible to design an SCT with a focuser that moves the eyepiece in and out keeping the mirror at the ideal focal length position? If this was optimized for binoviewing, would it also work for single eyepiece mide (with 3" less effective focal length? If so there may be SCT binoviewing customer demand.

Ken

#39 EdZ

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Posted 27 January 2013 - 07:38 AM

Would it be possible to design an SCT with a focuser that moves the eyepiece



But, by design thatt is nnot how an SCT works. It is unfortunate that many people who own SCTs really do not understand how an SCT works. Very few really understand that the operating focal length of an SCT changes for every configuration you put on the back end of it. It even changes according to the eyepieces you use. The act of focusing an SCT changes the operating focal length, so it's slightly different for every eyepiece.

f/10 is nothhing more than nominal. There is actually only one single position when an SCT is actually operating at f/10, and particularly for the small SCTs that is NOT the normal operating focal length.

I addded an SCT focuser onto the back end of myy C6. It allowed the mirror too stay fix and allowed me to focus by moving the eyepiece. But it added so much length to the back endd of the C6 that it significantly reduced the aperture, by a lot. I considered it a poor option and quit using it.

If you want to get the most out of an SCT, keep the length of the accessories behind the backplate as short as possible. A diagonal plus binoviewer, especially if using a reducer, is about the worsst configuration you can put on the back end of a small SCT. It doesn't have as big an effect on the large SCTs 8" and over. But on the small SCTs, it has a huge detrimental effect. For more info, I'd suggest reading the entire thread i linked to above.

FWIW, after several years of options, I've returned to using my 6" SCT with nothing more than a 1.25" diagonal.

#40 Eddgie

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Posted 27 January 2013 - 02:54 PM

This is why I was such a big fan of the Baader Maxbright and Baader T2 Prism diagoanl.

This setup offers the absolute shortest light path that I know of.

When I went from the 2" based Denkmeier Supersystem to the Maxbrights, the differecnce in performance was huge.

I had estimated that the C14 was working at around 11 inches of aperture with the Supersystem low power arm in place. The effect on the image was profound. It was far far to much to attribute to bino induced brightness loss alone.

I have since went to a Mark V, but I kept the Baader standard prism because it still offers the shortest possible light path.

Even in the C5, I measured 122mm of apeture with the Mark V and the Baader Prism. That is not bad at all.

Keeping the back focus down on any SCT is a good idea.

And like you, I went back to 1.25" diagonals on my C5 quite a while ago.

I also continue to see people say that using a 2" diagonal behind a focal reducer doesn't harm anything.

How can that not see the loss of aperture? It is so o glaring that the view is dimmer than it should be at exit pupils they are using!


Small SCTs and MCTs are much better performers with the 1.25" diagonals they were shipped with.

Binoviewers are going to cause apeture loss I think in all C5s and C6s. I don't think there is a configuration that one can use to avoid it, though I think I could come close with the Baader short SCT adapter (10mm) but that makes it hard to turn the diagonal. Might be OK for an Alt-ax situation though...

#41 faackanders2

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Posted 27 January 2013 - 04:53 PM

What about if they designed and optimized an SCT with a 2" diagonal for since eypieces, and a shorter 1.25" diagonal for binoviewers. This would reduce the increase in focal length for the binoviewer.

P.S. I can clearly see that if I ever get an SCT, my binoviewer will not work as well as it does on my 17.5" f4.1 Newtonian Dob.

#42 GlennLeDrew

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Posted 27 January 2013 - 06:31 PM

An SCT could be designed so that the nominal location for the focus is farther outside the back end. But to do this does require a somewhat larger secondary mirror.






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