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BF1800 And Binoviewers

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#1 Eddgie

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Posted 31 July 2019 - 03:08 PM

Well, my math was pointing to an answer that I did not want to hear, but according to Lunt, you only get about 42mm of inward travel when using a binoviewer with the BF1800 (vs the BF1200) before you loose the fully illuminated solar disk.  In fact, my math says that 6mm past this, aperture reduction starts to occur, though to be fair, I do not have exact measurements for the placement and spacing of the blue glass filter, which is the choke point on the system.  With the BF1200, the rear filter is the choke point and this filter allows for only a little bit of inward travel before the solar disk looses 100% illumination.  With the BF1800, the same 18mm window used on both the BF1200 and BF1800 becomes the choke point but by then the rear filter on the BF1200 is causing aperture reduction.    

 

Unless a Televue Powermate or other telecentric unit is used, many binoviewers are going to take more inward travel than 42mm. 

 

I had calculated this myself in recent days, but I was unsure of the outcome, so I asked Lunt for their figure, and this is what I was given, and again, it kind of matches up with my own computations (I did not have exact measurements for the spacing between the focal plane and the blue glass filter, but I used 75mm and that may be a little more than it really is, but I measured as best as possible using a BF1200.)

 

Now going in further than this and Lunt says the disk itself becomes vignetted, and my own calculations say that much past this and you may start to suffer aperture reduction.  I do not have all of the necessary measurements to give an accurate figure, but we know that the front aperture on the BF1800 is more of a limit than the rear aperture because it is the same diameter but further up the light cone, so my figures say that once the BF is pushed more than 128mm forward of the focal plane (that is at the back of the 75mm light path of the BF housing), aperture reduction begins.   This means that if the BF1800 is pushed in more than 53mm from the point where it would be when the system was used with a regular eyepiece, aperture reduction will occur.  Again, I do not have exact figures. These are based on my own measurments of a BF1200, and I did not take it apart to ensure exactness, but I feel that I am within 5mm or so.  

 

 

For people that can't get behind the 53mm forward limit, a 2.5x Powermate may be a good solution.  The Powermate will even allow the BF1200 to work well with binoviewers, and this may be a good alternative  to selling the BF1200 and fork over the difference for a BF1800.   A Powermate is $217, and just about any BV should give a fully or almost fully illuminated disk because the Powermate only takes about 12mm of inward travel to reach focus with a typical binoviewer. 

 

For the BF1800 though, you run out of front aperture in the BF1800 before you run out of rear aperture and if the user has a configuration that requires more than about 2 inches of inward placement of the BF1800, the system might be working with slightly reduced aperture or at the best case, with a disk that has well under half the surface showing with less than 100% illumination (vignetted). 

 

 

 


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#2 Eddgie

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Posted 31 July 2019 - 03:29 PM

And based on this, some might be wondering what I am going to do.   

 

This is an old solution but it occurs to me that it may simply been posted long enough ago that it is not in the current wisdom database.

 

Here are the numbers.   

 

I am using a Burgess binoviewer with a light path of 96mm (measured by me, so this is an accurate figure). The nose will be removed and replaced with the RAF William Optics to T2 adapter. 

 

I will use a Baader 1.7x GPC.  The Baader GPC will reduce the light path lenght of the Baader Maxbright binovewer by 33mm (to 77mm) and this means that it would shorten the Burgess (or William or Celestron) BV to 63mm.

 

To hold the GPC on to the top of the Lunt BF1800, the eyepiece holder is removed, and a 10mm T2 extension is screwed on it its place.   The 10mm is necessary to give room for the GPC. 

 

So, I have 63mm light path length of the binoviewer and the light path through the 10mm extension, and about 2mm for the light path though the RAF Camera adapter. This totals to 75mm.  With 75mm of spacing, the suns image would be  about 16mm at the point it passes though the rear blocking filter element.    With another 48mm (not exactly measured, but approximated) to the blue glass and that is a total of 123mm and this should be able to pass an f/6.83 light cone, though I do not think the disk will be 100% illuminated but it will be close.

 

This is important to me because the Powermate solution (which I may go to) would result in 2.2x or so, and my preferred eyepieces are 21mm zooms.  With the Powermate, the full disk would almost entirely fill my field of view, and I am willing to accept some small amount of illumination falloff in order to get the ability to show a smaller full disk view when zoomed to low power and a very big range of magnification for when I want higher powers.

 

Now, I have not tested this yet, but if it results in over 50mm of inward focuser travel, I will just go to the Powermate.  Seems a waste of the BF1800 though, but I am crossing my fingers that it will come in at close to 42mm of inward travel, which is what Lunt says is the most inner travel that can be used before loosing the fully illuminated disk.  My numbers are close enough to theirs that this sets the limit I want to use.

 

Anyway, here is a diagram of the system I am putting together.. Just waiting on the 1.7x GPC and the BF1800.

 

T2 binoviewer Lunt 80.png

 

My fear is that these figures for the Baader GPC are incorrect.  The 1.7x GPC has been tested in the past and reported to be working at 1.5x, but I have not tested it myself (the 1.25x was quite close to spec) but if the reduction in path length is exaggerated and I wind up with much more than 42mm of forward movement of the BF, I will probably just return it and go with the Powermate.


Edited by Eddgie, 31 July 2019 - 03:45 PM.

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#3 Eddgie

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Posted 31 July 2019 - 03:33 PM

Another benefit of the Baader GPC is that it corrects for spherical aberration in red (spherochromatism for full spectrum)  caused by the prisms in the binoviewer. Generally this is not a lot of error, but why have spherical aberration if I can eliminate it.  I mean the effects for full spectrum are less easily seen, but when the only color you are working with is 654nm then why not have the best result possible, and the Baader GPC differs form all of the other solutions because it was designed to correct spherochromatism in binoviewers.  


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#4 bigdob24

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Posted 31 July 2019 - 03:34 PM

I’ve got a set of Denk II that I’ve had forever and they won’t focus in my Lunt 100 need way more intravel.

I talked with Russ at Denk and have got one of his special OCS units that replace the 2” tube coming out of the 1800 blocking filter and then will let me use my Binos 

Kind of pricy but worth it to bino view

Dan



#5 Eddgie

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Posted 31 July 2019 - 04:14 PM

Well, no binoviewer is going to focus in the Lunts without using some kind of barlow or GPC and even with the very short light path of the Burgess, I am not confident that the 1.7x GPC will get me where I want to be.   There is always the Powermate though.  That will work with almost any telescope and binoviewer. With a BV, it takes about 10mm of inward travel.

 

Did Russ say what the power factor of the OCS would be?   I would expect it to be 2x. 



#6 bigdob24

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Posted 31 July 2019 - 05:00 PM

I did not ask, If I think about it I’ll call tomorrow and find out

Dan



#7 Mike Mc

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Posted 31 July 2019 - 07:47 PM

Hey Eddgie,

 

Have you checked with Harry Siebert to see how his solution works? If I recall correctly, it involves a lens cell in the nose of the diagonal and a second one in the bino nose. I don't know what the effective focal impacts are, but I'm sure he would tell you.

 

Just a thought.  Good luck - binoviewing the sun is definitely the way to go.



#8 Eddgie

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Posted 31 July 2019 - 10:32 PM

I believe I will be OK with the 1.7x GPC and I would prefer it because it will fix the spherochromatism of the binoviewers. 

 

It is going to be very close, and I might not get the full disk 100% illuminated, but If I get the ability to use the 21mm zooms, I can live with a couple of percent of brightness loss on the edge. 

 

Yes, the BVs are not as good as double stack for surface detail, but it sure makes it easy to resolve surface detail.

 

Even with the Burgess 2x barlow, which I calculate reduced the aperture of the scope to 70mm (BF had to be moved 40m forward to reach focus) the crispness of the detail was quite excellent.

 

Again, I am pretty sure I will get full aperture, but the full disk won't be 100% illuminated.  Even if the scope is reduce by a millimeter or two, I will be happy, but for now, looks like I will get full aperture. 

Even with the 7-21mm Nikon zooms, I should be able to go from 40x (the Baader 1.7 has been measured out to only really be 1.5x) to 120x so that would be an ideal range - from a very well framed full disk to more power than seeing often allows.  

 



#9 bigdob24

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Posted 01 August 2019 - 12:48 PM

Well, no binoviewer is going to focus in the Lunts without using some kind of barlow or GPC and even with the very short light path of the Burgess, I am not confident that the 1.7x GPC will get me where I want to be.   There is always the Powermate though.  That will work with almost any telescope and binoviewer. With a BV, it takes about 10mm of inward travel.

 

Did Russ say what the power factor of the OCS would be?   I would expect it to be 2x. 

2.3x



#10 Eddgie

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Posted 01 August 2019 - 01:37 PM

2.3x

Yeah, this is about what I had guessed.

  

Most likely it is the same elements as are used for the regular OCS, but probably mounted in a cell that has a T2 rear thread.

 

If you have an OCS already, you could probably simply adapt it to T2 .  I don't see why that would not work and much less expensive than buying a specialized OCS. 

 

https://agenaastro.c...LBoC9i8QAvD_BwE

 

I am pretty sure the above adatper would work, but you might also want an M48 2" extension to put on the front of the OCS.

 

Even if you don't have a standard OCS, it would probably be better to get the standard OCS and use the M48 male to T2 female and that way you have an OCS that is compatible with standard 2" diagonal.

 

So, unless there is something magic about the Lunt ready OCS, I don't see why you could not use an existing one. 


Edited by Eddgie, 01 August 2019 - 01:37 PM.


#11 bigdob24

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Posted 01 August 2019 - 04:00 PM

I’ve got a 2” OCS that’s used my refractor and dob. 

‘Don’t think it will work with the Lunt and that’s why Russ makes the OCS for Lunt scopes.

That was my understanding .

Dan



#12 George9

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Posted 01 August 2019 - 05:02 PM

Russ used to offer the adapters to connect his regular 2.3x OCS to the Lunt blocking filter. That's what I use. One between the blocking filter and the OCS, and one on the other side so you can pull it further out into the light path.

 

I had bought a 45mm OCS for my Dob, so I leave the 38mm OCS that came with the binoviewer on the Lunt.

 

George



#13 twjs

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Posted 01 August 2019 - 05:26 PM

I wish I understood what you guys were saying. Can you direct me to some appropriate reading please?



#14 George9

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Posted 01 August 2019 - 06:13 PM

I just remeasured.

 

First to point out that I am aiming for no vignetting within the filter's typical sweet spot, which is a little bigger than the Sun. If it vignettes where it is off-band, I don't really mind because I am never observing there.

 

I measured an LS80 DSII, where I keep the 1.5" collar on against Lunt's advice. With the Feathertouch focuser.

 

With the B1800 all the way in, and with the focuser all the way in, the B1800 just makes the full cone of the centered Sun, which means it probably vignettes the prominences. But since you are probably not ALL the way in, it is probably fine.

 

BUT, if you remove the 1.5" collar and your binoviewer forces you to push your focuser all the way in, then, yes, the B1800 will vignette.

 

Because there is no reason to support a 3 degree field of view, rather than buy a B3400, I recommend you use the B1800 with a Barlow or OCS or GPC, which will force you to pull the B1800 away from the objective and eliminate vignetting.

 

George



#15 Eddgie

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Posted 01 August 2019 - 07:04 PM

The adapter in the link above is not the right one. My bad.  It has to be the M48 female to T2 female.  My apologies for the error. 



#16 bigdob24

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Posted 01 August 2019 - 07:24 PM

George

Do you think the one Russ now makes that I’m getting will allow the full light cone thru with no vignetting?

I guess I thought bino viewing with the 1800 was the way to go no problems.

Ill find out soon as it’s in the mail as we speak and I want to get the Binos going.

Dan



#17 George9

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Posted 01 August 2019 - 08:56 PM

George

Do you think the one Russ now makes that I’m getting will allow the full light cone thru with no vignetting?

I guess I thought bino viewing with the 1800 was the way to go no problems.

Ill find out soon as it’s in the mail as we speak and I want to get the Binos going.

Dan

I am reading the description for the first time. I cannot tell the difference between it and a regular 38mm OCS. They look about the same size. This one has a T2 thread to fit the diagonal and comes with a 1.25" nosepiece for a Binotron to fit into the other end of the B1800.

 

So I don't know, but my very strong suspicion is that it will work just fine. With the OCS, you will be pulling the B1800 further away from the objective to a smaller part of the light cone. So much so that as Russ points out, at 3x you won't have enough out-travel and you will need an extension. (I use a DSII unit and against advice I keep the 1.5" Lunt extension collar on. That way I can reach focus in all three PowerSwitch positions.)

 

In fact, I don't bother to remove the OCS for monocular viewing. I just use a lower power eyepiece than I would have.

 

George


Edited by George9, 01 August 2019 - 08:57 PM.


#18 bigdob24

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Posted 02 August 2019 - 08:00 AM

He did ask If I have an extension as it maybe needed for high power.

Its on the way and I’ll be out of town for a few. 

‘It will be #1 on the list when I return

Dan



#19 bigdob24

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Posted 02 August 2019 - 03:55 PM

I was surprised by a package in the mail this morning from Russ and Denk.

I received my OCS mounted in a nice barrel to replace the barrel on my BF1800.

He also thru in a nice extension that I installed also.

Rolled out my scope and put the Denks in with the 24 pans aimed and was greeted with a solar disk , adjusted the power switch to low power and focused WOW , WOW , WOW such a dramatic increase in surface detail, there are a couple filaments or some sort of disruption that were well defined and granulation was contrasty and just there, where just my single eyepiece even stopped down to 80mm was there but not like thru the Binos, only a couple small proms . Ran through all three power settings and everything works great. Found that if I want to use a single eyepiece and leave the OCS for the Binos In , I need an extension to reach focus.

You guys using single eyepieces are missing the show it’s a dramatic increase in contrast and clarity WOW.

Found some paper work on my bino set and it’s the Newtonian Package with a 2” OCS , with the power switch that provides 

1.4x / 2x / 2.5x  and the OCS delivers 2.3 x so I’ve got to redo my mag list, not sure I’ll ever get that figured out

I find Binos so much more comfortable to observe and I’ve got my expandable eyecups on the 24s so it blocks stray light and helps keep your eye placement.

Not sure I’ll need that DS now

Dan


Edited by bigdob24, 02 August 2019 - 03:57 PM.

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#20 Eddgie

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Posted 02 August 2019 - 06:26 PM

Yeah, I would rather do single stack with binoviewer than double stack without.   

 

Doing both together is difficult (for me) because the sun gets pretty dim when I try to use high power with BVs, but in single stack, I can use very high mags. The amount of detail you can resolve on the surface really skyrockets. 

 

I was using the Lunt 80 with the BF1200, and even with the aperture reduced to maybe 70mm, the views were better than full aperture with on eye. 

 

I had bought a "Gently Used" DSII along with the "Gently Used" LS80THa and did not enjoy the DS that much (dim, some reflection, but good surface detail) but when I put in the BV in single stack mode, even with the aperture loss caused by the BF1200, the views were romping good!   Returned the DSII, and Lunt even allowed me to upgrade the BF1200 to the BF1800 for the difference in price between the new units!   Super nice of them.

 

The increase in detail is not new to me.. I have used interviewers for planets for a decade.

The shock with solar was how much better the views were.  With Planets, the views are better, but with the sun, the views with BF are hugely improved. 


Edited by Eddgie, 02 August 2019 - 06:34 PM.

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#21 Volvonium

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Posted 14 January 2020 - 02:12 PM

Thank you for this post.  I very recently obtained an LS60THa PT with BF1800 for a bonkers price and had h-alpha binoviewing in mind, but I've yet to try my binoviewing configuration with the 60mm Lunt.   

 

I've been using a Siebert 1.3X (1.25) large OCA with 38mm clear aperture with great success to binoview with my fast focal ratio reflectors and it can be reconfigured to install onto 2" diagonals for a refractor.  The Siebert 1.3X large format OCA is a really impressive piece of optical hardware and I'm hoping binoviewers can greatly enhance the view.

 

I had been curious about surface details when double stacking etalons vs binoviewing and will share my configuration when I can fully test it this weekend if there is no rain.




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