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Barlow in Filter Wheel

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#1 Eddgie



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Posted 03 June 2019 - 01:51 PM

There are many people that started using image intensifiers long after I wrote up my filter wheel Barlow mod, and it occurs to me that many may not be familiar with it.


One of the reasons I like my filter wheel is because I added a Barlow lens to one of the positions.  This allows me to simply spin the wheel and refocus to bring increase the magnifications.


I tried to find the old post but could not, but it should be out there for anyone that wants exact details.


Here is the gist of it though.  I removed the lenses from the cell on a 1.25" shorty Barlow and mounted them into an old 1.25" filter cell. It was necessary to use the filter cell because the Barlow cell was to big to fit into the filter wheel. The lens stack was just pressure fitted using an O-ring.  The filter with the Barlow lens is simply mounted into one of the holes on the filter wheel.


When I originally did this, I was using an 80mm refractor, and at the time, I was using the eyepiece holder on the top of the filter wheel.  With the Micro using a 1.25" nose, the spacing was a bit less than it would have been using the original Barlow tube and using visual fields, I estimated the power to be around 1.7x.  This is OK for slower scopes because there is little issue with off axis illumination falloff when a filter is in place and the focuser has to be racked in from the Barlowed position.


With faster scopes though, I use the Micro attached directly to the T2 threads at the top of the filter wheel. This is necessary to ensure that a 1.25"filter mounted in the filter wheel does not vignetting the field (well, it actually does a tiny amount at f/2.8 but only a very tiny amount which does not show other than using a defocused star. For visual use, there is no vignetting seen in the field).  Anyway, at this spacing, the Barlow only adds less magnification.  With this setup, the Barlow elements are only about half the spacing to the photocatode as they would be from the Barlow element to the shoulder of the original housing.  My estimate is that the power is increased only by maybe 1.3x (maybe a little more, but I have not gotten around to measuring it accurately.  Based on just field observation, I think 1.3x is in the ballpark.


With the 1.25" eyepiece holder, the step in magnification is generous and a 1.7x step to me is actually better than the over 2x step one would get using the Barlow with the Mod 3 or Micro, and I did use that a lot on the f/6 scope.


The 1.3x step is of course more subtle, but it is convenient, and when you want a bit extra scale, it really is nice to be able to just dial it in place. I used this on the 130 f/5 last night and the difference it made on M13 and M15 (even at only a 1.3x step) was quite apparent.  The cores opened up just enough with the extra magnification to resolve slightly better.   1.7x might have been better, but I was configured for 1.3x (or whatever it is) and that is where it will probably stay (with the 130, if I put the 1.25" eyepiece holder on to the top of the filter wheel, the focuser has to be racked in so much that the telescope end of the focuser tube extends into the light path and it is enough that it bothers me. On a big telescope, 3/4th inch extension of the 2" focuser tube into the light path is nothing.  On a 130mm telescope, it makes the total obstruction a couple of square inches bigger.  Not that this is easy to see, but I would rather have all of the avialable aperture working for me and am willing to trade off a bit of magnification with the Barlow to keep it that way, so I will use it with the 1.3x (?) configuration.   


Now the downside is that this take the place of a filter and that means that with this in place, you are unfiltered. If just doing long pass observing, it would seem like the best approach would be to use the 1.25" eyepiece holder and just screw the long pass filter on to the front of the 1.25" nose.  Then one would have the native focal ratio, and the 1.7x amplification.   The H-a filter is still in the wheel, and to go to H-a, one would not even have to remove the long pass filter (for a quick look) though again, if the scope were fast, one might see some illumination falloff when in the higher power mode. (and again, for small scopes, there may be some issue of shading the primary if the tube has to be racked in all the way). 


Anyway, it has been over two years since I made the post and for those that have started doing image intensified and that like using filter wheels, it might be something you are interested in doing.  I am sure the post is out there, but I could not find it. It is an easy mod to make, and it can be quite handy to have even a slight bump in magnification from time to time.

Edited by Eddgie, 03 June 2019 - 01:54 PM.

#2 moshen



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Posted 06 June 2019 - 12:42 PM

I had Siebert Optics put one of his 2x Barlows in a small cell I could use in my filter wheel. Quality of the barlow is very good and his price was reasonable. I don't remember the focal length of the barlow but I do get roughly 2x in my filter wheel setup.


It's really great to have 2x magnification with a swipe of the finger. 

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Edited by moshen, 06 June 2019 - 12:42 PM.

#3 daslolo


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Posted 10 April 2020 - 11:51 AM

I had Siebert Optics put one of his 2x Barlows in a small cell I could use in my filter wheel. I don't remember the focal length of the barlow but I do get roughly 2x in my filter wheel setup.

purple, this looks like ED glass
if you remember the barlow model it will be useful as magnification reduces greatly with shortnening.

filter wheel Barlow

what i was looking for!
you could have a second filter wheel just for filter.
i will make a 2” one. what parts did you use to mount the lens?

Edited by daslolo, 10 April 2020 - 11:53 AM.

#4 Stargazer3236


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Posted 10 April 2020 - 09:13 PM

I always envisioned having a filter wheel with a couple of barlows on it as well as a few reducers. Kind of like the Earthwin design where you could have a Barlow on one side and the reducer on the other. The problem being that it would have to be thin enough that it would fit in the filter wheel housing. Or perhaps buy the EAF and leave the cover off and just use the barlows and reducer while the whole thing is motorized!

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