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Equipment Discussions >> Binoviewers

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Sarkikos
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Mark V or Binotron for 10" Dob - Planet/Lunar Only
      #6141862 - 10/16/13 09:39 PM

I have a Burgess Binoviewer. Eventually I'd like to upgrade to a Mark V or Binotron. I only binoview planets and the Moon - no DSO - and only through my Dobs. The largest SCT or Mak I have is a 6", and my largest refractor is a C102. IMO, I don't think it'd be worthwhile to do what needs to be done in order to use any of those three for binoviewing. And the image would not be as sharp or as bright as through my larger Dobs. So I'll continue to only monoview through my Cats and refractors.

But I am interested in a binoviewer upgrade for my Dobs, especially the 10" f/4.8. At this point, I'm leaning toward the Mark V. So far the deciding factor might be the coma correction available for that binoviewer. The Binotron does not provide coma correction. I achieve detailed views of planets with my Burgess Binoviewer, but I cannot stack it with my Paracorr II. I'd like to be able to binoview and have coma correction. I suspect the planet detail would be even better if I could do both.

On the other hand, the Mark V does not have a Power Switch. The Binotron does. I was hoping that the ability to switch among three levels of magnification using one eyepiece would allow me to sell off some of my bino pairs. I have about 22 pairs, 44 eyepieces just for binoviewing! Time to cull the herd ... I hope. After all, selling some of my bino pairs could help fund the new binoviewer.

However, I am concerned that for the Binotron, the OCS Cell plus the Power Switch lenses plus the prisms plus the eyepieces might degrade the image for planet observation compared to merely changing to a shorter focal-length bino pair or a more powerful OCS and doing without the Power Switch. I like the idea of switching immediately among three levels of magnification - even if some refocusing is needed. But I don't like the idea that it might degrade the image. I'd rather not deal with a Power Switch if the image would be even slightly sharper without it. After all, I'm talking planet viewing here!

Also, I still like the idea of binoviewing with coma correction. That's possible with the Mark V. AFAIK, that's impossible with the Binotron.

So, which should be my next binoviewer upgrade: Mark V or Binotron?

Thanks,
Mike


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crow
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6141935 - 10/16/13 10:13 PM

Hi Mike, well I've received a pair of Binotrons recently. The weather here, usually crystal clear this time of year has been pretty dire so I haven't had them out under the stars yet apart from very briefly. I can say this though, build wise they just radiate quality, they have that swiss watch feel to them, pretty light too. The diopter adjustment, eyepiece locking mechanisms are super slick and work very well from my limited experimentation. You can collimate them pretty easily too. Ill be using them with an 1100 Edge and working on keeping the light path nice and short.

Which would be best for your personal situation Ill let others advice, I've never even seen the Mark V's, but as far as build quality and service goes, i.e Russ at Denk is great to work with, the Binotrons are amazing, I'm not expecting anything less optically when I don't have to run between rain showers.


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6141992 - 10/16/13 10:40 PM

Because the binoviewers greatly limit your true field, the coma is a lot less of an issue than you may realize.

Suppose that you use a Denk with D21 eyepieces.

If your scope is 1220mm focal length, then with the low power arm, it will be like the scope is going to be more like 1590mm focal length.

This means that if you were to use a pair of D21s, the power would be around 73x.

And remember, the D21s are only 65 degree apparent field.

Now suppose you were to duplicate this power in your scope at native focal length. That would be like using a 16mm wide field (something like the ES 16/68).

Because you are limited to relatively high powers, most of the bad coma simply is not going to be in the field.

I used the Binotron in a 12" f/4.9 dob with a pair of 20/68 ES eyepeices, and honestly coma was not really much of an issue.

In essence, you are going to be using only the center 50% of the mirror.

The Mark V lets you use a full 24mm eyepiece with a 27mm field stop, so the power is not that much different from the Binotron.

For example, if I used the Bintron/D21, I would get 91X with a 65 degree apparent field. In the Mark with 1.7x coma corrector, I would get about 106x, but with a 68 degree apparent field, so the difference in true field size is not all that great. The Baader though does do an excellent job of correcting coma, and I believe that the field illumination is quite a bit better with the Mark V/Coma corrector. The Binotron will cut off about half of the off considerably more off axis light than the Mark V, but again, you have to factor in that it does have a bit wider field.

I have used both systems and each has its strengths.

The primary strength of the Binotron is the powerswith.. With D21s, you could have low, medium, and high power without changing eyepeices (though you may want a new focuser tube so you don't have to slip the binoviewrs out a bit to reach focus at high powers).

You have to change eyepeices, but the brilliant design of the eyepiece clamps make it very quick and easy with only a 30% turn to clamp and unclamp.

The Mark V also has the quick change ring which makes it very easy to move the BV body. I have quick change rings on all of my telescopes (two pair of Mark Vs) and I like being able to just undo one screw to move the binoviewr back and forth (I have extra diagonals).

The Mark V also has the potention to work at native focal length in some refractors, or with a 1.25GPC in many, and coupled with the 28mm clear aperture, allows you to get a wider true field in those scopes (if you can reach focus with no GPC or with the 1.25 GPC).

The Binotrons have that powerswitch.. Did I mention that?

These are two very different systems and both have very important strengths.

For a Dob only, I think the Binotron is at its very best, and the powerswitch is hard to resist.

But you can get the same low powers with the Mark V.

I have a pair of 35mm Ultimas that I can get 73x with my 12" f/4.9 dob. To get the same power with the Binotron without vignetting, you could use the 26mm Plossls, but then you would be changing eyepeices anyway, so in the end, unless you accept vignetting, you can't really get any lower power in the Bintron than the Mark V.

But did I mention the powerswitch?

Powerswitch is a great benefit. It is hard to deny the appeal of this feature.

Don't worry so much about the coma though. Again, you are only be using the center 50% or so of the mirror, so most of the bad coma is out of sight in both systems, but the Baader is a true coma corrector.


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REC
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6142654 - 10/17/13 10:45 AM

Just a newbie in the world of BV and have been mainly using my Denk2 P/S in my 8" SCT. But recently bought a 10" Dob to see what all the fuss is about on CN with so many members using them.

I bought an OCS for the Denk's and I must say they are pretty neet. I have not looked at a planet yet as Jupiter rises late here, but looking forward to doing so in this scope. The stars in the double star cluster look really sharp and contrasty with the reducer arm in, get both of them in the EP. The moon is dazzingly in the BV in all 3 modes. I'm using a pair of 20mm SWA and a 26mm SP.

With the OCS I'm working at f/1600, f/2800 and f/3800 in the 10" Dob.

Bob


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: REC]
      #6142700 - 10/17/13 11:03 AM

Yes, I think the Binotron is really at its very best in a Dob/Newt. I have used it in all three of my primary scopes (C14, 6" APO, 12" dob) and my favorite for the Binotron was without question the dob.

The 6" APO to me is too small to binoview and you sacrifce the really fantastic ultra-wide fields.

To much power in the C14 and to narrow a field even in low power.

But in the dob, they were pretty amazing.

I did not like having to slide the OCS out to reach focus at high power, but I have since replaced he factory focuser and with the new focuser, I picked a tube that I thought would be long enough to allow me to reach both low and high powers just by using the focuser.

Also, the focuser has about .75" more in travle, so now I should be able to reach focus in low power with the OCS fully in, which I think would improve the off axis illumination. With the old focuser, I had to screw the OCS out about .7" and this could have made a difference.

I think that if you have to use extensions, it may even be a bit worse, so it may be better to go to a lower profile focuser even if the OCS tube sticks into the light path at low power.

In reducer mode, it really does not matter I think if the OCS sticks into the light path, and you have to rack out quite a bit for the striaght though, so I am sure it would be out then, but if not, when you go to high power for planets it is almost 100% sure to be completely out of the light path, and that is the only time it really matters in my opinion is for high power planetary use...

I loved it in the Dob and I think the lower profile focuser would only make it better.


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Paul G
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6142761 - 10/17/13 11:47 AM

Haven't used one, but in the S&T review they said the power switch did introduce a little aberration.

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beatlejuice
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6142931 - 10/17/13 01:12 PM

Quote:

I loved it in the Dob and I think the lower profile focuser would only make it better.




Yes it does. I got the moonlite with the 2.37" tube and attached it with no spacers. This allows me to come to focus with all 3 power switch positions and without having to unscrew the middle newtonian spacer at all.
The only time the tube extends into the light path is with the reducer arm in. For mid and high power and also with all of my eyepieces in mono the tube does not extend into the light path.
I might add that the moon and sun were spectacular in my Omni 120 and I had no problem achieving focus in all 3 positions with the 4.5" drawtube of the R&P focuser that came with the scope.
I can't wait for Jupiter and Saturn with the binotron's.

Eric


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6142962 - 10/17/13 01:31 PM

Eddgie,

Quote:

Because the binoviewers greatly limit your true field, the coma is a lot less of an issue than you may realize.

Suppose that you use a Denk with D21 eyepieces.

If your scope is 1220mm focal length, then with the low power arm, it will be like the scope is going to be more like 1590mm focal length.

This means that if you were to use a pair of D21s, the power would be around 73x.

And remember, the D21s are only 65 degree apparent field.

Now suppose you were to duplicate this power in your scope at native focal length. That would be like using a 16mm wide field (something like the ES 16/68).

Because you are limited to relatively high powers, most of the bad coma simply is not going to be in the field.

I used the Binotron in a 12" f/4.9 dob with a pair of 20/68 ES eyepeices, and honestly coma was not really much of an issue.

In essence, you are going to be using only the center 50% of the mirror.




I am still not settled as to how important coma correction really is to planet/lunar observation. The only times I have been aware of obvious coma in my 10" f/4.8 Dob is with large AFOV's and usually more often at moderate-to-low power. Coma scales linearly with apparent field of view - IIRC that's how the math mavens say it! - so it's not surprising that it should be more obvious the wider the AFOV. However, this is also why advocates of coma correction say it is a good idea at higher magnifications as well, since you can use wider AFOV eyepieces at high power.

But to my eyes coma is much less obvious at high power, even with a wide AFOV. Maybe this is due to the fact that the Diffraction Limited Field takes up more of the AFOV at higher powers, or that the exit pupil is narrower? I'm not sure. I'm just saying what I see.

IME binoviewing has a much greater impact on ability to see fine surface detail and an increase in perceived contrast than using a Paracorr. I've done this experiment a number of times, comparing the image of Jupiter binoviewed w/o Paracorr vs Paracorred w/o binoviewer. To my eyes, the binoviewed image consistently looks sharper and more contrasty. I see the benefit for wide-field deep sky views with a Paracorr, but not really so much - if at all - for moderate-to-high magnification planet/lunar work.

That said, the optical theory and the numbers seem to indicate a Paracorr should make a significant difference for planet/lunar. Ideally the FOV should be entirely within the DLF for a telescope that does not track.

The DLF for my 10" f/4.8 Dob is about 7 arcmin. So for an eyepiece with a 44 degree AFOV - my orthos and XO's - I would need to boost the magnification to 350x to achieve a 7 arcmin TFOV. For a 50 degree eyepiece, I'd need 400x; a 60 degree eyepiece, 500x; an 80 degree eyepiece, 650x. Well then, maybe I should either get tracking so I can keep the planet on-axis or use a Paracorr!

My Paracorr increases the DLF by a factor of about 6x, from 7 arcmin to 42 arcmin. So to have the entire FOV fall within the DLF when I'm using the Paracorr, I'd only need 58x with a 44 degree eyepiece, 67x with a 50 degree, 83x with a 60 degree, and only 108x for an 80 degree eyepiece! All my eyepieces would provide a field entirely within the DLF for magnifications over 110x or so, well below the usual powers for planet/lunar. Even my ES 100 9mm plus Paracorr shows a field completely within the DLF at 153x.

This all sounds good. But like I said, the Paracorr hasn't panned out as I hoped for planet/lunar viewing. I mostly use it for wide-field deep sky.

On the other hand, in hopes that a combination of binoviewer plus coma correction would provide a sharper image than just binoviewer alone, I'm tending to lean toward the Mark V at this time.

Mike


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: REC]
      #6142995 - 10/17/13 01:50 PM

Quote:

Just a newbie in the world of BV and have been mainly using my Denk2 P/S in my 8" SCT. But recently bought a 10" Dob to see what all the fuss is about on CN with so many members using them.

I bought an OCS for the Denk's and I must say they are pretty neet. I have not looked at a planet yet as Jupiter rises late here, but looking forward to doing so in this scope. The stars in the double star cluster look really sharp and contrasty with the reducer arm in, get both of them in the EP. The moon is dazzingly in the BV in all 3 modes. I'm using a pair of 20mm SWA and a 26mm SP.

With the OCS I'm working at f/1600, f/2800 and f/3800 in the 10" Dob.

Bob




IME a binoviewer provides an obvious improvement in fine surface detail seen and enhancement of perceived contrast when observing planets. When I've looked at DSO with my binoviewer, the effect is nice, very pretty, but I don't feel any more info reaches my eye than when I monoview. For DSO, the advantage to binoviewing is more a matter of aesthetics and presentation - IME & IMO, of course.

Mike


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6143002 - 10/17/13 01:52 PM

Quote:

I loved it [the Binotron] in the Dob and I think the lower profile focuser would only make it better.




I recently upgraded to a 1.5" low-profile Moonlite, so I should be good to go.

Mike


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Paul G]
      #6143015 - 10/17/13 01:55 PM

Quote:

Haven't used one, but in the S&T review they said the power switch did introduce a little aberration.




Thanks. I know I had read that somewhere. I'd like the convenience of the Power Switch, but I don't want degradation of the image. Not a good thing when I'm trying to tease out fine surface features in the image of a planet.

IIRC, the Binotron has the middle power setting as straight through, with no additional lenses? If so, at least that setting would have no further aberration of the image.

Mike


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beatlejuice
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6143065 - 10/17/13 02:20 PM

Quote:

I recently upgraded to a 1.5" low-profile Moonlite, so I should be good to go.




I don't think you will be able to reach focus in the high power mode of the binotron with a 1.5" tube without having to pull it out of the focuser at least 1/2".

Eric


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TGModerator
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6143624 - 10/17/13 07:58 PM

Quote:

I am still not settled as to how important coma correction really is to planet/lunar observation.




For planets, coma may not be a factor if you're using a driven scope and can keep the planet centered. For Lunar, I prefer coma correction across the entire field. I don't own a scope with coma but I do have an eyepiece pair which is bad in this department and I can't stand to use them on the moon.

Tanveer.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6143734 - 10/17/13 09:01 PM

Quote:

Quote:

I recently upgraded to a 1.5" low-profile Moonlite, so I should be good to go.




I don't think you will be able to reach focus in the high power mode of the binotron with a 1.5" tube without having to pull it out of the focuser at least 1/2".

Eric




I didn't install the 1.5" Moonlite to come to focus at all settings of a Binotron. I did it so I could come to focus when using filter wheel without having to use an OCS.

Now if I do get a Binotron, maybe I can stack it in the filter wheel and be able to come to focus at all settings. IMO, that would be an excellent outcome!


Mike


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beatlejuice
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6143763 - 10/17/13 09:16 PM

Quote:

Now if I do get a Binotron, maybe I can stack it in the filter wheel and be able to come to focus at all settings. IMO, that would be an excellent outcome!




Indeed that would be an excellent outcome but unfortunately if you do get the binotron the filter wheel will hinder your ability to reach focus at the low power position as well as the mid and high without some kind of adjustment in between.

Before I ordered the moonlite I did all the measurements through every possible combination of eyepieces in mono and with the binotrons with and without a filter wheel. End result was either seemless use of the binos or the filter wheel in mono but not both. I chose the best configuration for the binos and put the filter wheel on hold for now.
Your situation might be slightly different but your scope is very similar to mine.

Eric


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6143855 - 10/17/13 10:09 PM

If the Binotron only needs an inch of out-focus to come to focus at all settings in my 1.5" Moonlite, that is easily remedied by attaching a parfocalizing ring to the neck of the Binotron. I did this in order for my Paracorr II to focus in my Moonlite focuser.

I have the same situation with the PII and filter wheel that you've experienced with the Binotron and filter wheel: one or the other but not both.

Luckily my Burgess Binoviewer now works fine in the filter wheel and Moonlite focuser, though it does still need an OCS. However, in no configuration with any of my Dobs have I ever been able to come to focus with the binoviewer without an OCS or Barlow. Even the 1.5" Moonlite is not low enough to allow this.

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6143933 - 10/17/13 11:01 PM

Quote:

Now if I do get a Binotron, maybe I can stack it in the filter wheel and be able to come to focus at all settings. IMO, that would be an excellent outcome!





I had 1.5" of travel on my 12". I had to extend the OCS barrel abut .75 inches to reach focus in low power and when I racked out, I was still short of focus by about 1/4th inch.

Parfocalizing ring is not going to be a good fix because you would have to remove it to reach low power.

This is the way the Binotron is going to work. You are going to have to rack all the way in to get the low power arm to focus.

When you go straight though, you are going to have to rack out quite a bit, and when you go high, you have to rack out even more.

The solution is simply. Set up the focuser with the lowest profile you can.

Use a 2.1" focuser tube.

Ignore the fact that in low power, the tube is going to extend into the light path. At low power, it really doesn't do enough damage to the image to make it possible to detect. It simply does not matter.

When you go straight through, you are going to pull the focuser back out at least an inch, and at this point, the tube will be out of the light path.

And when you go high, you are going to rack all the way out and the focuser tube will be almost completely inside the focuser housing.

At least that is they way it worked out in my 12" f/4.9 dob. I had a little intrusion in low power, but just a little, and for low power you really can't tell. It is impossible to see the contrast loss from the focuer tube sticking a half inch into the light path.

Medium power, I was clear of the light path, but still about 2/4th inch into the tube, and high power, completely out of the OTA.

So, I went with the longer tube because I did not want to slip the focuser out.

A parfocalizing ring would be a royal pain. Easier to just slip the OCS tube out a bit and use the focuser.


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beatlejuice
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6144042 - 10/18/13 12:27 AM

Quote:

Use a 2.1" focuser tube.




Exactly. That is why I went with the 2.37" focuser tube as well as to be able to reach focus in mono with my 30mm UWA which reaches focus 4 3/16" above the scope. The 3 thumb screw compression ring adds 3/8" to the height and luckily I can just reach focus with the reducer arm in and no extension of the newtonian spacer, which is what I wanted for max FOV in low power.

Eric


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6144092 - 10/18/13 01:11 AM

I spend about an hour on the Moonlight page and did a lot of measuring to make sure that the focuser tube would be the right length.

All of the data is there, but it is not really easy to figure out exactly what length tube you need.

I just had to dig though the pages and get different measurments and the data about how much the tube extends past the bottom and things like that.

Result was perfect, but it did take a lot of measuring and estimating.

For a faster scope though, I would think that less travel would be needed because of the steeper light cone.

For the f/4.9 though, I figured I needed 2.1"to hit top and bottom and the top of the focuser needed to be something like 3.4 inches (as I recall). I rember it was the mounting plate with no shims installed for low power, and the 2.1" travel was required out for out focus to be able to avoid slipping out the OCS to reach high power focus.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6144321 - 10/18/13 07:17 AM

I set up my Moonlite order so that I would be able to use my 2" and 1.25" filter wheels with my eyepieces without having to screw on an OCS. I determined that I would be have to go with the 1.5" drawtube to accomplish this.

So far, all my 1.25" and 2" eyepieces will come to focus in a filter wheel - except for my ES 100 20mm. That eyepiece needs a little more in-focus. If I hadn't gone with a 1.5" drawtube, I'm sure that more eyepieces would not have come to focus in the filter wheels. My solution was the best for my purpose.

On balance, I was able to do what I wanted to do, which was use eyepieces in filter wheels without an OCS. The Binotron was not even on my radar at that time. It still might not be. More and more, I'm tending toward the Mark V and away from the Binotron. Sounds like the Binotron would be more trouble than it's worth for me.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6144343 - 10/18/13 07:51 AM

Quote:

This is the way the Binotron is going to work. You are going to have to rack all the way in to get the low power arm to focus.

When you go straight though, you are going to have to rack out quite a bit, and when you go high, you have to rack out even more.

The solution is simply. Set up the focuser with the lowest profile you can.

Use a 2.1" focuser tube.




No, the lowest profile I can have is a 1.5" focuser tube. That's what I have now. If I went any longer I'd probably not be able to come to focus with the filter wheels in some of my eyepieces - besides the one eyepiece, my ES 100 20, that won't come to focus now in the wheel.

Quote:

Ignore the fact that in low power, the tube is going to extend into the light path. At low power, it really doesn't do enough damage to the image to make it possible to detect. It simply does not matter.




Racked all the way in, my 1.5" drawtube barely encroaches into the light path, so no worries there, even if it did matter.

Mike


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: TG]
      #6144394 - 10/18/13 08:35 AM

Quote:

Quote:

I am still not settled as to how important coma correction really is to planet/lunar observation.




For planets, coma may not be a factor if you're using a driven scope and can keep the planet centered. For Lunar, I prefer coma correction across the entire field. I don't own a scope with coma but I do have an eyepiece pair which is bad in this department and I can't stand to use them on the moon.

Tanveer.




My 10" Dob is not driven, which is why I have some concern about the effect of coma on planet/lunar images. My intention is to eventually upgrade to about a 14" Dob with tracking, but for now I have to nudge.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6144407 - 10/18/13 08:44 AM

One factor that I keep forgetting is that I now have a pair of Baader 8-24mm Zooms for binoviewing. I haven't even first lighted them yet. During dry runs in the house, my 65mm IPD seems adequate for viewing through the Baader Zooms in my Burgess Binoviewer.

With decent Zoom eyepieces in the binoviewer, do I really need a Power Switch? I'll have to compare the image of planets and the Moon through the Baaders with the image through single focal length eyepieces. If the Zooms work out well, they should be reason enough to cull some of my other bino pairs and eliminate the Binotron.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6144447 - 10/18/13 09:02 AM

The only negative of the zooms is the greatly reduced apparent field when zoomed out.

For planetary and general high power, I think a pair of zooms would be fine.

But you will loose a fair amount of true field at the lowest powers.


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6144480 - 10/18/13 09:26 AM

Yes, since I almost always binoview planets and the Moon rather than DSO, the relatively narrow low power would not matter so much to me.

My lowest power, widest AFOV/TFOV bino pair are Meade 5k SWA 24mm's. I deshrouded them for easier binoviewing. With the mandatory 1.9x OCS, they give me a 43 arcmin TFOV at 95x. If I didn't have to use the OCS, they'd yield about 1.4 degrees at 50x.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6144846 - 10/18/13 12:59 PM

Quote:

No, the lowest profile I can have is a 1.5" focuser tube. That's what I have now. If I went any longer I'd probably not be able to come to focus with the filter wheels in some of my eyepieces




But Mike, the length of the focuser tube doesn't effect the profile it just adds extra length for outfocus. Thats why I got the 2.37".

Eric


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6144945 - 10/18/13 01:50 PM

Yes, this is true. Adding or removing spacers from the focuser itself will raise or lower the profile of the focuser. I misunderstood what Eddgie meant by "profile." IIRC my Moonlite has no spacers now.

In any case, when I ordered my Moonlite I wanted to avoid enchroachment of the drawtube into the light path and allow for use of my eyepieces with both my 1.25" and 2" filter wheels. I did not want the drawtube to dip far into the light path. This cannot be a good thing for contrast. Many observers make a big deal about the CO and spider vanes, but seem oblivious to possible degradation of the image from the drawtube.

If I need more out-focus for some eyepieces, I can always add an extension. In fact, I was doing that even before I upgraded to the Moonlite.

Possible future acquisition of a Binotron was not part of the selection process for my Moonlite. At this point I don't plan on returning my Moonlite to install a longer drawtube.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6147372 - 10/19/13 08:01 PM

I am using a Binotron with a 10" home-built f/6.5 newt. I asked Russ if a Paracorr would be useable and he said it was not needed--even at shorter f-ratios. I suspect that this may have to do with the OCS. However, the magnification effect of the OCS alone would not affect coma correction as coma scales linearly with magnification.

Other problems that had to be solved were these:

1. The OCS protrudes into the optical path a lot. I moved my mirror an inch forward and used a taller focuser--not a low profile focuser which will only exacerbate the problem.

2. The weight of the binotron requires a very robust focuser. I ended up with a large format Moonlite focuser and this works quite well.

3. The weight of the binotron (which reportedly is not particularly heavy as binoviewers go) nonetheless required a serious effort at counterbalancing. I made a custom 2.5 Lb counterweight opposite the focuser that is easily removed in case I want to use a mono-view eyepiece.

JimC


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Mirzam]
      #6147783 - 10/20/13 02:01 AM

I certainly agree with you Jim on 2 & 3. The regular moonlite focuser + the counterweights that I already had solved those issues.

As far as a taller focuser is concerned I am not so sure. Since the binotron reaches focus with the reducer arm in at a certain height above the tube it seems that the taller the focuser is the more you would have to extend the newtonian spacer to still achieve focus at low power thus marginally reducing the FOV the more it is extended.

I have found that the views have not been noticeabley effected by the protusion into the light path in low power and is a non-issue at mid and high power as the low profile focuser is racked out enough to clear the light path in these settings. This without unscrewing the newtonian spacer at all, giving a seemless transition between all 3 powers without any added procedures.

Eric


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6147972 - 10/20/13 08:09 AM

Quote:

the more you would have to extend the newtonian spacer to still achieve focus at low power thus marginally reducing the FOV the more it is extended.





I agree. Kind of.. I did not see the field reduced, but rather the off axis light rays were badly cut of. I would say that at f/4.9 in my scope, having to extend the OCS was causing about 50% light cutoff.

I believe that it is better to use the lowest profile focuser possible that will allow the OCS to be used at the shortest length (OCS screwed all the way in).

In low power mode, it is impossible to see the effects of the additional obstruction into the light path because if it extends into the light path by one inch, it still only take 3% of the light from a 12" scope, and at low power, you can't see the contast loss.

And yes, at medium and high power, when you need it to be out of the way, it is.

I recommend the lowest possible focuser height with a longer tube to allow the OCS to be screwed all the way in, even if it projects into the light path at low power. It simply does not do enough damage at low power to be seen.


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6148086 - 10/20/13 09:42 AM

When monoviewed, won't short focal length eyepieces tend to require the drawtube to be racked farther in? In fact, the only times I've needed an extension on a drawtube is when I'm using a long focal length eyepiece. IIRC, in those cases the drawtube was racked all the way out from the telescope. (It must depend, though, on where the field stop is located in each eyepiece.) Why is the situation seemingly reversed in the Binotron with its Power Switch? Why all this talk about the drawtube intruding into the light path at low power but not at high power?

I don't intend to do all my viewing with a binoviewer, even if I upgrade to a Mark V or Binotron, so I need to live in both the monoview and binoview worlds.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Mirzam]
      #6148114 - 10/20/13 10:02 AM

Quote:

I am using a Binotron with a 10" home-built f/6.5 newt. I asked Russ if a Paracorr would be useable and he said it was not needed--even at shorter f-ratios. I suspect that this may have to do with the OCS. However, the magnification effect of the OCS alone would not affect coma correction as coma scales linearly with magnification.




Russ did not answer your question of whether a Paracorr would be useable with the Binotron. I doubt very much if a Paracorr is useable with a Binotron. He said the Paracorr was not needed. Of course, it's not really needed - with your telescope. But that's because your Newt has an f/number of 6.5!

I don't see what the OCS has to do with reducing coma unless is a coma corrector. As you say, coma should not be affected by magnification. I do think, though, that if you magnify to the point where the entire DLF of the primary is contained within the AFOV of the eyepiece, that ought to reduce the perception of coma.

Of course, there would be a greater need for a Paracorr if the f/number is below 5. My 10" Dob is an f/4.8.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6148168 - 10/20/13 10:34 AM

Actually, I have used my binotron (sans Paracorr) with my 20" f/4 Starmaster and did not notice any coma. So something is going on. I'll test it out again because I really was not looking for coma. At f/4 it jumps right out under normal circumstances.

I also have the larger size OCS, which reduces any chance of vignetting.

JimC


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6148356 - 10/20/13 12:38 PM

Quote:

Why all this talk about the drawtube intruding into the light path at low power but not at high power?




Mike, when using the binotron's in a newtonian there is a minimum of 4" between the shoulder of the focuser and the OCS. This consists of the regular 2" shaft + a 2" Newtonian spacer which can itself be unscrewed a further 1/2" to help reaching focus in low power if necessary. The newtonian spacer is removed for use in a refractor or SCT thus making the whole system compatible with any type of scope. In my scope (10" f/5) I rack out more than 1.25" for mid power and about 2" for high power.

I should have picked up on this when you talked about using the binoviewer with a filter wheel. Since you already are able to use the filter wheel in mono, when using the binotron you would have to modify the length of your focuser tube or use an extension tube or pull the whole thing out quite a bit when binoviewing with mid or high power. But using the binotron with the filter wheel is of course not possible.

BTW I still don't know how you are able to reach focus with the filter wheel's extra inch of height + the extra 3/8" in height for the 3 thumbscrew compression ring holder. The math just doesn't seem to compute for my eyepieces.

Eric



Edited by beatlejuice (10/20/13 01:41 PM)


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6148450 - 10/20/13 01:36 PM

I have to screw out the OCS about 3/4 the way out for the low power arm in my 10" Dob. The normal and the barlow arm also will focus ok.

BTW...I had a beautiful view of the Double Star Cluster last week and was able to fit both clusters in the FOV....nice!


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6148925 - 10/20/13 07:09 PM

Quote:

Mike, when using the binotron's in a newtonian there is a minimum of 4" between the shoulder of the focuser and the OCS. This consists of the regular 2" shaft + a 2" Newtonian spacer which can itself be unscrewed a further 1/2" to help reaching focus in low power if necessary. The newtonian spacer is removed for use in a refractor or SCT thus making the whole system compatible with any type of scope. In my scope (10" f/5) I rack out more than 1.25" for mid power and about 2" for high power.




Refocusing for the three levels of the Power Switch is not a big deal. In my setup, lower and mid power might not be a problem. But it sounds like I may have to stick in an extension in order to reach high power with the Binotron.

I don't think I'd want to bother with a Power Switch if I'd need to remove the Binotron and stick in an extension every time I wanted to view at high power. Even having to pull the Binotron out an inch or two would be a kludge. These solutions sort of negate the convenience of the Power Switch. If I could insert the extension into the focuser first and then have the Power Switch focus at all three levels, that would be fine.

Quote:

I should have picked up on this when you talked about using the binoviewer with a filter wheel. Since you already are able to use the filter wheel in mono, when using the binotron you would have to modify the length of your focuser tube or use an extension tube or pull the whole thing out quite a bit when binoviewing with mid or high power. But using the binotron with the filter wheel is of course not possible.




The funny thing is, I have no problem using a filter wheel with my old school Burgess Binoviewer in the Moonlite focuser. Of course, I still have to put an OCS on the neck of the binoviewer. But I needed to do that with my old focuser, too.

Quote:

BTW I still don't know how you are able to reach focus with the filter wheel's extra inch of height + the extra 3/8" in height for the 3 thumbscrew compression ring holder. The math just doesn't seem to compute for my eyepieces.




In the 1.5" Moonlite focuser with compression ring - no spacers - I can use either my 1.25" or 2" filter wheel without an OCS and come to focus with all my eyepieces except the ES 100 20mm.

Maybe if I'd gone for a Moonlite without the compression ring the ES 100 20 would have had enough in-focus for the 2" filter wheel? I'm not sure. But I wanted the extra security of the compression ring for filter wheels, binoviewers and heavy 2" eyepieces. I think that's worth not being able to focus for just one solitary eyepiece in the filter wheel.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6149106 - 10/20/13 09:17 PM

My preferred system for planetary viewing is now my Denk II Supersystem, Denk 21's, the 2" OCS, and an AP Advanced Convertible Barlow on a 18" Zambuto Starmaster with goto. I screw the AP Barlow's optical cell onto the OCS using a Denk-supplied adapter that Russ sent me to accommodate a Lunt blocking filter (the AP cell uses standard 2" filter threads but it requires a spacer between it and the OCS). I could see switching to Denk 14s but I have not bothered yet.

I spent some time playing around with the Paracorr and I forget exactly what configuration finally worked but it didn't add anything so I now skip it. I didn't specifically test for coma, but just got an impression of the view.

This has replaced my monocular planetary viewing, where I used to use a 5.2XL or my orthoscopics plus a Barlow and Paracorr. I only use the Denk on the 18" for planetary and lunar. I go monocular for DSOs.

The biggest advantage of the Denk over the Mark V for me is that I also use it on my solar scopes (Lunt 80, AP130 with wedge, etc.) and I find that the OCS and Powerswitch work very well without major contortions to bring it to focus.

Still, it would be nice to try a Mark V someday.

George


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: George9]
      #6149230 - 10/21/13 02:00 AM

Quote:

I could see switching to Denk 14s but I have not bothered yet.




Thats the thing about the power switch, after you get that first pair of eyepieces every other pair will only get you one more magification that is not close to being a duplicate of what the first pair gives you.

Eric


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: beatlejuice]
      #6157101 - 10/25/13 09:57 AM

At this point, I think I'll pass on both the Mark V and the Binotron.

The Binotron sounded interesting because of the Power Switch. But in practice it appears too fussy to make work easily for all three settings. Also, the image might not be as sharp through the high setting. That wouldn't work for me, because I only binoview planet/lunar. For deep sky, I always monoview. And the Binotron does not allow for coma correction.

The Mark V, on the other hand, does not have a Power Switch. But it does have optional coma correction. I'd like more information on that feature. Theoretically, binoviewing plus coma correction should be an ideal combination for planet/lunar.

For now, I'll continue to use my humble, old Burgess Binoviewer. It works very well for planet/lunar. But no Power Switch and no coma correction. Nothing is perfect.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6157276 - 10/25/13 11:38 AM

I have to tell you that recently I became aware that the Baader coma corrector might induce some spherical aberration.

I had star tested my new dob when I go it, and SA was actually really nicely corrected. This is not a premium quality mirror, but way better than I had prepared for.

Well, I did not really give it much thougth, but last time I used the scope with the binoviewer, I happened to change from the 35mm Ultmas, which take a lot of in travel, to one of my other pairs that don't take as much in travel.

When I did, for the first time, I noticed when I put in the other eyepices and refocused, the secondary shadow seemed a bit larger than it should have.

I am going to qualify this statement before I even make it. I was not using a 33% obstruction, so the test may not be fully accurate.. Anyway, I decided to rack though focus and to my surprise, the secondary shadow ratio was a lot different on the other side of focus!

This is the classic sign of spherical aberration.

Again, it is important to mention that I did not use a 33% obstruction. The problem is perhaps not really serious and maybe is some higher order spherical aberration (and that is why you use a 33% obstruction rather than the native 23%.. It greatly reduces any influence of HSA).

So, for now, I have to say that I have not had a chance to re-test with the 33% obstruction in place.

I plan on doing this the next time I have the scope out, which hopefully will be Sunday night.

Will try to remember to follow up.

Hoping it is just some HSA that would not be a factor at best focus with the native obstruction. The star test is super sensitive to HSA with small or no obstruction, but you can have a totally meaningless amount of HSA and it can give a bad reading.

Scope seems to perform well on the moon though. I would say at least ad good as my 6" APO and I think perhaps a bit better, but I really haven't done a side by side, so just working from memory of detail around the Triesnecker Rille. I know this area well from the C14 and the 6" APO, and I felt that it was showing detail right between the C14 and the 6" APO.

And that is what makes me think that I am seeing some HSA and not true LSA which I think can be more detrimental.

Still, not conclusive. Will know more when I test with the obstruction.

I love the Mark V and coma corrector though.

But the coma corrector extends into the focuser tube 92mm. This is not much different than the OCS on the Binotron.

So, this may not work for you either..


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157371 - 10/25/13 12:28 PM

Quote:

I had star tested my new dob when I go it, and SA was actually really nicely corrected. This is not a premium quality mirror, but way better than I had prepared for.




My 10" f/4.8 Dob is a Celestron C10NGT OTA ported to a Dob mount. (I can't stand any Newt OTA larger than a 6" on a GEM.) The optics are very nice. I see no difference in the diffraction pattern in-focus and out-focus.

Quote:

I am going to qualify this statement before I even make it. I was not using a 33% obstruction, so the test may not be fully accurate.. Anyway, I decided to rack though focus and to my surprise, the secondary shadow ratio was a lot different on the other side of focus!

This is the classic sign of spherical aberration.

Again, it is important to mention that I did not use a 33% obstruction. The problem is perhaps not really serious and maybe is some higher order spherical aberration (and that is why you use a 33% obstruction rather than the native 23%.. It greatly reduces any influence of HSA).




A larger CO reduces HSA? I had not heard of this. Maybe I should go back and reread my Suiter's?

But I would think that any advantage to a larger CO - and I'm really skeptical that there is any practical advantage, except maybe for double stars - would be offset by the diminished contrast for planet/lunar. Did you raise the primary in an attempt to binoview without an OCS, and so had to change to a larger secondary? Otherwise, I can't imagine willingly changing from a 23% CO to a 33%.

By the way, my 10" Dob also has a native 23% CO. I like it that way, because IME it places the scope in a Goldilocks Zone making it good for planet/lunar and deep sky.

Quote:

Will try to remember to follow up.




Please do, even if you have to start another thread to show your binoviewer experiences.

Quote:

Scope seems to perform well on the moon though. I would say at least ad good as my 6" APO and I think perhaps a bit better, but I really haven't done a side by side, so just working from memory of detail around the Triesnecker Rille. I know this area well from the C14 and the 6" APO, and I felt that it was showing detail right between the C14 and the 6" APO.




Sounds very much like my 10" Dob. That's why I'm not in such a hurry to upgrade to a 14" Dob - which I do want to do eventually. I'll just have to continue to ignore the condescending remarks and glances from my fellow observers at the dark site. It seems like for deep sky you're not really a member of the club unless you have at least a 14" Dob.

Quote:

I love the Mark V and coma corrector though.

But the coma corrector extends into the focuser tube 92mm. This is not much different than the OCS on the Binotron.

So, this may not work for you either..




Yep, this definitely needs more thought before I pull the trigger. I might even wait on a binoviewer upgrade until I get that 14" Dob. Looks like I'll have to design the Dob around the binoviewer so everything will work optimally.

I'm also concerned that the Mark V coma correction might not be as good as a TV Paracorr. After all, the Paracorr has built-in tuning to optimise coma correction. AFAIK, there is no accommodation like that for the Mark V corrector OCS. Unless the coma correction is just right, there is a definite possibility of making the image worse rather than better. That may be what you are finding out with your Mark V and Dob.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6157463 - 10/25/13 01:19 PM

Quote:

A larger CO reduces HSA?




No, not what I said. The larger CO does not eliminate or reduce HSA at all.

The problem is that the star test is super sensitive and if you do it with to small (or no) obstruction, it can give the impression that there is lower order spherical aberration.

This is the mistake that Roland Christen made when he condemmed the star test. He noted that his breakout was very unbalanced.

It was clear to me from his essay on the star test that he did not follow Suiter's instructions.

The 33% obstruction is part of the process. The purpose is to desensitize the star test for the presence of HSA.

Unlike LSA, the HSA component will normally not throw light outside of the in-focus Airy Disk until it is quite bad.

It will slightly expand the diameter of the Airy disk, but not usually enough to be disenable visually. Since the light remains in the Airy Disk, contrast is not lowered.

But, if you defocus, you quickly change that dynamic and now the light is going into the ring structure.

The 33% obstruction at 10 wavelengths just gets you so far away from best focus that the effects are not easily seen.

So, obstruction does not fix HSA. Only masks it for the star test.

HSA when present rarely does damage. Even the all spherical f/12.5 MCT only looses .05 Strehl to HSA (best computed Strhel will be .95). This is not enough effect to be seen visually.

So once again, CO does not reduce HSA, only reduces the star test sensitivity to it. All of the HSA is still there, but you simply have to defocus so much that its effects are not enough to influence the breakout.


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157474 - 10/25/13 01:27 PM

Quote:

So, obstruction does not fix HSA. Only masks it for the star test.

HSA when present rarely does damage. Even the all spherical f/12.5 MCT only looses .05 Strehl to HSA (best computed Strhel will be .95). This is not enough effect to be seen visually.

So once again, CO does not reduce HSA, only reduces the star test sensitivity to it. All of the HSA is still there, but you simply have to defocus so much that its effects are not enough to influence the breakout.




OK, thanks. I understand your meaning now.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157475 - 10/25/13 01:27 PM

I have to add one thing though. I was absolute in my answer, and it is really more complicated than that.

It is perhaps conceivable that a big enough obstruction does in fact lower HSA.

One of the forum members (Norme) removed the cone shaped secondary baffle from his MCT because he wanted to see if it improved the contrast.

When he did this, it lowered the obstruction by a meaningful percentage.

He immediately reported that HSA seemed more pronounced in the star test.

But again, the question is "Was there really more CA? Or did the bigger obstruction simply desensitize the test?"

Ant some point, the size of the obstruction takes so much energy out of the Airy disk that a slight change in diameter caused by HSA becomes so moot as to be purely academic because the far greaterr damage is done by the obstruction itself.

And this is why the f/12.5 MCT works OK. The HSA lowers the Strehl a bit, but the scope will star test well, and no one will complain that their scope has excessive LSA.

And I think Norme saw the difference.

Ask him about it in the Cats and Casses forum. He as probably done more with star testing and HSA than anyone else on the forum.

Nomre is his CN user name.

By now, maybe the best star tester on CN. His work with star testing his MCT pushed his understanding of HSA far beyond mine.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157505 - 10/25/13 01:43 PM

Quote:

It is perhaps conceivable that a big enough obstruction does in fact lower HSA.

One of the forum members (Norme) removed the cone shaped secondary baffle from his MCT because he wanted to see if it improved the contrast.

When he did this, it lowered the obstruction by a meaningful percentage.

He immediately reported that HSA seemed more pronounced in the star test.

But again, the question is "Was there really more CA? Or did the bigger obstruction simply desensitize the test?"

Ant some point, the size of the obstruction takes so much energy out of the Airy disk that a slight change in diameter caused by HSA becomes so moot as to be purely academic because the far greaterr damage is done by the obstruction itself.




IIRC, there have been many numerous discussions about larger obstruction benefiting double star observing, all revolving around these questions. I'm not sure which side is correct. But I'm confident that for planet/lunar, no advantage will be gained by a larger obstruction, ceteris paribus.

Quote:

And this is why the f/12.5 MCT works OK. The HSA lowers the Strehl a bit, but the scope will star test well, and no one will complain that their scope has excessive LSA.




Now you're making me question the excellent star test on my C6.

Quote:

And I think Norme saw the difference.

Ask him about it in the Cats and Casses forum. He as probably done more with star testing and HSA than anyone else on the forum.

Nomre is his CN user name.

By now, maybe the best star tester on CN. His work with star testing his MCT pushed his understanding of HSA far beyond mine.




Yep, I know just the guy you're talking about. I've talked with him many times on the Planetary and Lunar Fora. I envy his retirement in the Phillipines. My choice is between Delaware and Florida.


Mike


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6158285 - 10/25/13 09:56 PM

Eddgie,

Quote:

I am going to qualify this statement before I even make it. I was not using a 33% obstruction, so the test may not be fully accurate.. Anyway, I decided to rack though focus and to my surprise, the secondary shadow ratio was a lot different on the other side of focus!

This is the classic sign of spherical aberration.

Again, it is important to mention that I did not use a 33% obstruction. The problem is perhaps not really serious and maybe is some higher order spherical aberration (and that is why you use a 33% obstruction rather than the native 23%.. It greatly reduces any influence of HSA).

So, for now, I have to say that I have not had a chance to re-test with the 33% obstruction in place.




I have to ask you about this: Do observers usually star test their Newts with a 33% obstruction in place? I'd never heard of this before in actual practice. Suiter does mention it a few times in Chapter 10 of his Star Testing. But he doesn't talk as if this needs to be a standard phase of star testing and I've never read about it anywhere else. In fact, I've never heard of anyone actually doing it until now.

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6158385 - 10/25/13 10:54 PM

Ah, good question.

For an instrument with no inherent HSA, (Newtonian) it is less important in the sense that if the pattern is perfectly balanced, the scope is fine.

But what happens if you do see some imbalance?

The 33% obstruction was chosen by Suiter because it is large enough that when used at 10 wavelength of defocus, if there is unbalance, it makes it easier to estimate how much spherical aberration is present.

This is the beauty of Suiter's method. If a 20% obstructed scope is perfect, then you will see it. But if it is not perfect, having the bigger obstruction makes it easier to see the ratio of the difference so you can make a better estimate of the actual SA present.

And again, in designs with HSA, or in APOs that can have HSA and color mixing that is different one either side of focus, the 33% obstruction can be necessary, but again, I believe he chose the 33% simply for the easy of quantifying the amount of SA. If the obstruction is smaller, seeing a difference of 20% is much harder than if the obstruction is a bit larger.

So this I think is why he came up with the 33% obstruction. It minimizes any HSA, and it provides a big enough shadow that it is easier to estimate any SA that is present.

Also, for the 3:1 breakout, it offers the same advantage. A 2 : 1 breakout would be to sensitive, and the 33% obstruction is once again just the right sensitivity. In particular, the breakout can be very imprecise even at 3 : 1 ratio. I have to often do this test several times and average my breakout point because it seems to be hard to find the exact spot where the inner ring comes out. I think at 2:1 it would be hard.

That is why I keep saying to people that the method is very specific, and my own results with all scopes have felt much more accurate when I use 33%.

But for an HSA free design, if the balance looks perfect, it is probably close to it. If it is not though, estimation is harder.

But I cheat. I always use a 33% obstruction but I use Aberrator to model what I see. I find it easier than estimating the secondary shadow size ratio difference. Finding a pattern with aberrator that looks as much like the pattern I see seems to work best.

But after doing it so many times, I have gotten pretty good at taking a wild guess for the initial settings for SA in Aberrator.

I also studied a lot of Rohr's tests where he has Fresnel patterns, and also the Aberrator site (which is where I learned how to do the modeling based on their 40 or so real telescope star tests and Aberrator models for those scopes.. A great web site for starting to learn about star testing.

Anyway, that is why I believe that 33% is best to use. It seems easier to estimate if there is SA present.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6158790 - 10/26/13 09:20 AM

Quote:

I have to tell you that recently I became aware that the Baader coma corrector might induce some spherical aberration.




Yes, I think this is exactly what is happening. A Paracorr can also induce aberrations if everything is not set up optimally. I have heard that unless the optics are closely aligned and that the field stop to Paracorr distance is just right, aberrations can be introduced into the system - SA and even more coma. I take for granted that your telescope optics are well collimated. So the odds are that the source of the SA is the Baader coma corrector.

When I first heard about the Baader coma corrector OCS I doubted the correction would be optimal and was concerned that it might even be harmful to the image. How can it be optimized if there is no mechanism such as the Paracorr's tuning top to finely adjust the corrector lens distance? Even if some sort of tuning ring were introduced into the system, how could the user easily gauge when the Baader OCS was optimally set? "Some coma correction is better than none" does not always work well in practice.

Especially for planet/lunar, we don't want SA induced into the optical system. Isn't this the same reason why the back-focus distance should not be overly extended when binoviewing a Cat?

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6158982 - 10/26/13 11:27 AM

Well, some SA is not going to make much difference, so it is all a matter of how much SA changes, and I have not been able to measure that.

The Baader OCS is designed very specifically for the spacing provided by the Binoviwer and while it is only optinmal at one specific spacing, because the much longer design length (120mm minimum behind the corrector with the BV body in place) small variations will not be as big in terms of percentage as they would for correctors desgind for accepting just an eyepeice.

The Coma correction itself is quite excellent.

I won't know how much SA there is until I can use an obstruction and estimate it. I am not going to angst over 1/6th wave. It is really difficult to see much affect from this small a change.

But if it is larger than this, I may remove it and use a standard barlow for planets.

People forgat about this aspect of coma correctors. You don't have to use them when you don't want them.

For planets, I can just drop the OCA and stick in the BV with a 2" Barlow element screwed into the 2" nose piece.

Of course I have not tested that configuration to see if I can reach focus, but It worked in my 6" APO so I think it may work here as well.

And again, the only time SA really matters all taht much is for planets.

Yes, lots of back focus in an SCT can indeed induce SA. And not just a little.

I tested one configuration taht induced about 1/4th wave. That is getting to the edge of what I would tolerate.

But kept under about 200mm of light path and the change is pretty modest. Again, I don't angst over small changes in SA.

Years ago, I was totallly anal about it, but after much testing, I realized that it took more than 1/6th wave difference to really see, and even then, until it gets to 1/4th wave, it is not all that easy to see, though if there are any other issues like roughness, you start to push into soft view territory.

Anyway, hope to test tomorrow night if the weather clears.


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6160468 - 10/27/13 10:24 AM

Quote:

Well, some SA is not going to make much difference, so it is all a matter of how much SA changes, and I have not been able to measure that.




Yes, as you - and Suiter - have said, degradation of the image from SA depends on whether it's HSA or LSA and how much is present. Also, effect of SA will depend on the object viewed and how it is viewed. I have a 4.5" f/4.4 Ball Newt with a spherical primary. The image is acceptable - to me - for low power views of open clusters, bright bright nebulae and large bright galaxies. It was the first telescope that showed me M78 under my light-polluted suburban skies. But for higher power observation of planet/lunar? Forgetaboutit.

This is why I am concerned about SA induced by the Baader coma correcting OCS. My binoviewing is virtually always for planet/lunar.

Quote:

The Baader OCS is designed very specifically for the spacing provided by the Binoviwer and while it is only optinmal at one specific spacing, because the much longer design length (120mm minimum behind the corrector with the BV body in place) small variations will not be as big in terms of percentage as they would for correctors desgind for accepting just an eyepeice.




Makes sense.

Quote:

People forgat about this aspect of coma correctors. You don't have to use them when you don't want them.




Some observers, though, apparently refuse to do any observing unless a Paracorr is in the focuser. I'm not that type of observer. I'd rather have coma correction at all times. But sometimes you do have to give it up for other advantages. For instance, at this time I cannot binoview or use filter wheels with a Paracorr in the system. So out goes the Paracorr. eh...

Quote:

And again, the only time SA really matters all that much is for planets.




And for lunar, and probably for any DSO that are better seen at higher power. But SA is the big bugaboo for planet observation. AFAIK, SA affects on-axis sharpness more than coma. So according to my understanding, for planet/lunar, correcting SA is more important than coma correction.

Mike


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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6160532 - 10/27/13 11:03 AM

Quote:

for planet/lunar, correcting SA is more important than coma correction




Yes, this is when it really matters.

But even here, a small diffence is not possible to see visually. It really takes a bit more than 1/6th wave of SA before the contrast is lowered enough that it becomes detectable.

That being said, like you, I would prefer to drop the coma corrector and barlow for planets if the corrector induces any meningful SA.

Hope to measure tonight but clouds have been around the last several days (rain here this AM).

I will say again though that I am very happy with the Baader corrector. For general work, the performance is really great.

And what little lunar I did seemed to be pretty sharp, though I have not had very good seeing on the few ocassions I tried.

Will let you know when I have tested though.


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