Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

How To Use The Bartel Diagonal Calculator with a Coma Corrector

  • Please log in to reply
183 replies to this topic

#51 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 26 March 2019 - 06:01 PM

BTW, my arithmetic was off by a millimeter in my earlier post, though 134.16mm is the length of the Paracorr when set to have the focal plane at the lip of the entry tube.

Your optical cone radius of 10.5" only allows the illumination of a 1" field stop.  I would expand that to 0.70" (41mm ÷ 1.15 ÷ 2) to allow for an eyepiece with a field stop the size of the 25mm 100° ES.

That makes the effective secondary-to-focal plane distance = 406mm = 15.98"

This is bottom of Paracorr at edge of light cone at edge of field, and eyepiece w/focal plane at Paracorr shoulder and allowing 0mm for infocusing for near-sighted viewers

(though, to be honest, that tiny amount of intrusion won't matter).

I don't disagree with your conclusion.  However you figure it, you'll be fine.



#52 hakann

hakann

    Surveyor 1

  • -----
  • Posts: 1743
  • Joined: 06 Aug 2015

Posted 26 March 2019 - 06:56 PM

As I see it its a matter just where you like to has your
end of the paracorr ( ex filter or incl filter )
Say incl filter and that is a extra 4.5 mm plus the corrector plus the 9 mm, that is 88.58 mm plus 4.5 mm ( filter )
Note, this is CAD from TV info. ( se my CAD above )

Where focuser is and how much to bottom, its just mechanical only. ( I used a Stralight 2” but could be any 1.5” focuser )

Say one has the EP of ex of 41 mm FS, and that mean 20.5 mm clearance.
A 20” diameter /2 is 254 mm + 20.5 mm + 88.58 mm + 4.5 mm.
-That get a total L-distance at 367.58 mm or 14.47”.

Why go further away out as this will not intrude even whit a filter on ?
If a 4” diagonal is ok vs mag-drop, why go 4.5” ?
( can’t get that, enlight me !

Mag-drop here on this L-distance ( according from Bartels web ) for a 3.9” ( 4” ) is 0.3.
( Bartels say not over 0.4 )

CZ told me the mag--drop is really not that critical anyway.

Edited by hakann, 26 March 2019 - 07:01 PM.


#53 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 26 March 2019 - 07:50 PM

As long as the drop in the center of the field is no more than, say, 0.1 magnitude or less.



#54 careysub

careysub

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2517
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 26 March 2019 - 08:57 PM

As I see it its a matter just where you like to has your
end of the paracorr ( ex filter or incl filter )
Say incl filter and that is a extra 4.5 mm plus the corrector plus the 9 mm, that is 88.58 mm plus 4.5 mm ( filter )
Note, this is CAD from TV info. ( se my CAD above )

Where focuser is and how much to bottom, its just mechanical only. ( I used a Stralight 2” but could be any 1.5” focuser )

Say one has the EP of ex of 41 mm FS, and that mean 20.5 mm clearance.
A 20” diameter /2 is 254 mm + 20.5 mm + 88.58 mm + 4.5 mm.
-That get a total L-distance at 367.58 mm or 14.47”.

Why go further away out as this will not intrude even whit a filter on ?
If a 4” diagonal is ok vs mag-drop, why go 4.5” ?
( can’t get that, enlight me !

Mag-drop here on this L-distance ( according from Bartels web ) for a 3.9” ( 4” ) is 0.3.
( Bartels say not over 0.4 )

CZ told me the mag--drop is really not that critical anyway.

The guidance I have been following includes that of Mike Lockwood that for general use you should aim for a 0.5" fully illuminated center of field.

 

Also note that once you exceed a distance where the fully illuminated field drops to zero in Mel Bartel's calculator, it won't even give you an answer but tells you "No diagonal met the criteria. Please pick another size."

 

Although I entirely accept the claim that "an individual star's light loss of less than 0.3 magnitude is barely noticeable", across the center of the field of view the guidance as that some substantial width (like 0.5") should have even illumination or at most a very slight loss (Don suggests 0.1 mag).

 

Is Lockwood being too generous when calling for 0.5" fully illuminated? I don't know, but I wouldn't dismiss his opinion lightly. But let's say this is a rule of thumb that provides for a safety margin that can be reduced with careful design (which is entirely plausible I think).

 

According to the calculator with a 4" diagonal at 14.1" the fully illuminated field is 0.2", but with a drop at the edge of 0.5" of slightly less than 0.1 mag, so this seems to be the limit for this size.

 

Now my math:

10.7" light cone (I think this is a little generous by 0.1" or a little less, but lets use it)

0.28" Diagonal offset towards primary mirror and away from focuser (haven't been including this)

1.5" Focuser tube (same as used by SIPS, so this is probably OK if the build gets all distances right)

1.94" Paracorr adjustment

0.18" Filter

 

This is 14.6" which is 0.5" beyond the field illumination limit mentioned above, and does allow for any distance above the Paracorr/focuser/filter assembly. At this point the fully illuminated field is zero, and there is a drop-off at 0.5" of slightly more than 0.05 mag (relative to center of field) and a drop-off at the edge of field of 0.2 mag (relative).

 

That sounds just barely within the desirable range (no visible dimming), but if you rack the focuser out even 0.25" the calculator tells you that the secondary is too small.

 

So it seems that 4" cannot cut it.

 

OTOH, if a 4.5" is used then even with the tube racked all the way out an improbable 1.5" the illumination is still good. 4.25" might split the difference but the benefit in cost or CO savings is very small.


Edited by careysub, 26 March 2019 - 09:01 PM.


#55 hakann

hakann

    Surveyor 1

  • -----
  • Posts: 1743
  • Joined: 06 Aug 2015

Posted 27 March 2019 - 03:48 AM

Ok, I never heard of the 0.5” fully illuminated field.
When I tested this L-distance I had the PII whit filter at the clearance at 20.5 mm ( 1.6” FS )
Bartels program showed a mag-drop under 0.3.

-Here is what noted on his web ;
” I find an individual star's light loss of less than 0.3 magnitude is barely noticeable ”

I note on that L-distance if I got you correct its actually at 0.1 vs ( - 0.25” + 0.25” = 0.5” ) on Bartles diagram.

Whit that in mind CZ also told me as I moved up on my 18” f/4 from a Pyrex 3.5” to a Quartz 4” diagonal, as he mean it was not needed as mag-drop is not that critial visually used, so my 4” was a overkill.
I has a mag-drop of 0.12 at at FS at 1.4 ( E21 ) on Bartels program. ( so I has under 0.1 at 0.5” ) but I always thought I overkilled that - now I hear its might is needed ?

From what I did hear, going whit a mag-drop that low around 0.1 or 0.15 was NOT needed ( low power EP )

But if a mag-drop at near 0.3 is not ok, I need overlock from what I learned.

Ideas here ?

Did I get this right - or ?

Edited by hakann, 27 March 2019 - 04:55 AM.


#56 careysub

careysub

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2517
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 27 March 2019 - 08:03 AM

Perhaps others can weigh in on the issue of the fully illuminated field, I noted the authorities whose advice I am taking and my rationales.

 

But I will point out that if the fully illuminated field (FIF) shrinks to zero, although the resulting change in magnitude across the complete FOV might not be directly detectable visually you are at the point of throwing away photons from your primary mirror -- with less than full illumination at the center it is acting like your mirror is smaller than it is because no where in the field are you capturing all of its light. That cannot be a good design choice. So it seems reasonable that you would want to back away a bit from that inflection point and have a non-zero FIF. And if its non-zero, then how big?



#57 hakann

hakann

    Surveyor 1

  • -----
  • Posts: 1743
  • Joined: 06 Aug 2015

Posted 27 March 2019 - 08:21 AM

I did try to put in a picture ( no luck ) as whit the L-distance I used it is a mag-drop on a 0.5” FS at = 0.1.

-Can't really be a issue.

After all, why use a bigger diagonal and a longer L-distance needed ?
A bigger diagonal is ;
Harder to do.
Higher in price.
Weight more.
More obstruction.

One thing I also can’t see why.
Why is most get a longer L-distance than my calc I notice ?
Add half primary, add half low power EP and add 88.58 mm and maybe add upp filter ( vs not intrude in light parth )
Rest is just mechanical where focuser, focuser board is at.


Edited by hakann, 27 March 2019 - 10:28 AM.


#58 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 March 2019 - 10:31 AM

Perhaps others can weigh in on the issue of the fully illuminated field, I noted the authorities whose advice I am taking and my rationales.

 

But I will point out that if the fully illuminated field (FIF) shrinks to zero, although the resulting change in magnitude across the complete FOV might not be directly detectable visually you are at the point of throwing away photons from your primary mirror -- with less than full illumination at the center it is acting like your mirror is smaller than it is because no where in the field are you capturing all of its light. That cannot be a good design choice. So it seems reasonable that you would want to back away a bit from that inflection point and have a non-zero FIF. And if its non-zero, then how big?

The illumination at the center is never zero and it is never 100%.

 

It cannot be 100%, because the shadow of the secondary subtracts, say, 0.05-0.1 magnitude.

It cannot be zero because the secondary would have to cover the entire opening of the scope for that to occur.

So what you really mean is that you do not want to use a secondary mirror so small that the center point loses brightness not because of the secondary shadow, but because

the secondary is too small to even use the entire primary mirror to illuminate the center point.

 

Between that and a secondary so large the entire field has the highest illumination possible (noting that a larger secondary will drop the maximum illumination possible), there is a compromise in size.

Illuminating the center 0.5" of field maximally is one possibility (this was chosen back in the '50s so the full moon would be maximally illuminated).  Another possibility is to not let the light drop off at the edge exceed 30% (about a 0.4 magnitude loss), and, so long as this does not reduce the brightness of the axial ray, is the minimum size normally chosen.

 

The perfect size might fall between two commercially-available sizes.  In that case, pick the larger size.

 

And, secondary holders often have a lip covering the edge of the secondary, so the size of the glass does not equal the size of the secondary for calculation purposes.

As one example, my secondary is a 2.6" mirror, but only 2.45" of mirror is exposed.  It is the 2.45" size I use in the calculator to figure illumination, not the 2.6" obstruction.

I would only enter 2.6" to see how much magnitude I lose in the center of the field (down from 100%, or zero loss), but it is 2.45" I use to figure the illumination in my largest field.


  • Oberon likes this

#59 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 March 2019 - 10:40 AM

I did try to put in a picture ( no luck ) as whit the L-distance I used it is a mag-drop on a 0.5” FS at = 0.1.

-Can't really be a issue.

After all, why use a bigger diagonal and a longer L-distance needed ?
A bigger diagonal is ;
Harder to do.
Higher in price.
Weight more.
More obstruction.

One thing I also can’t see why.
Why is most get a longer L-distance than my calc I notice ?
Add half primary, add half low power EP and add 88.58 mm and maybe add upp filter ( vs not intrude in light parth )
Rest is just mechanical where focuser, focuser board is at.

Add length for near-sighted observers (5mm?).  Add length for secondary mirror offset (6mm?).  Add length of Paracorr when focal plane is at the outer lip (134.16mm)

And, add a few millimeters for eyepieces needing infocus from the in-most Paracorr setting (1-2mm?)

So, on your 18", that makes 228.6mm + 18.1mm + 5 + 6 + 134.16 + 2 = 375.8mm = 14.8"  But that assumes "correct to the millimeter" construction, so you need to add a factor to allow for

construction errors like the UTA being slightly offset from the mirror box.  And maybe some more in-travel of the focuser for accessories like Barlows, filters, etc.

It's rarely completely simple.



#60 hakann

hakann

    Surveyor 1

  • -----
  • Posts: 1743
  • Joined: 06 Aug 2015

Posted 27 March 2019 - 10:45 AM

In this example on a 20" and f/3.7 ( 74" FL ) but add the PII and use a Nagler 9 or a Ethos 8 as FS is around 0.5".

We are around 270X on the Ethos and we has almost the full moon at 0.4 degree.

( Just as Don talking about above )

 

My example showed whit a 4" ( really more a 3.9" ) a mag-drop there at 0.5" at 0.1.

As Don say, we can't has zero as the diagonal will cut some light.

 

I see no problem in this case to use a 4.5", as in reality it has a C-A at maybe 4.3" or 4.4".

 

My wonders is if anyone by visual use can see more vs the 4.5" over a 4" out in the field.

 

I always thought it was the low power that 'mighyt' hurt, going to small or a mag-drop under 0.3-0.4.

 

And in this case the 4.5" will be harder get a good tolerance, weight more ( vs sag ) and a higher price and even if it might not matter much it's more obstruction.


Edited by hakann, 27 March 2019 - 01:42 PM.


#61 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 March 2019 - 10:54 AM

The debate is whether or not to choose a secondary size for te lowest power, largest field stop, eyepiece, or whether to accept vignetting there and get full illumination

only on higher power eyepieces with smaller field stops.

 

We may never agree on that.

 

If I were a commercial builder, though, I'd figure out the largest field stop possible to be used in the scope and illuminate that correctly, even if some people

felt it was too large a secondary.  The edge of a secondary is usually the worst part of the flat, so that means higher power eyepieces would not use the edge of the mirror

for their axial images anyway.  I could satisfy the planetary observer and deep-sky observer at the same time.

 

I think we just worry too much about secondary size.  I've seen some very nice deep sky images in scopes with 30%+ secondaries.

Plus, what big scope user ever spends ALL of his time only observing planets.

If you can't decide between two sizes of secondary, get the larger one and forget about it from then on.



#62 careysub

careysub

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2517
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 27 March 2019 - 12:29 PM

The illumination at the center is never zero and it is never 100%.

 

It cannot be 100%, because the shadow of the secondary subtracts, say, 0.05-0.1 magnitude.

It cannot be zero because the secondary would have to cover the entire opening of the scope for that to occur.

So what you really mean is that you do not want to use a secondary mirror so small that the center point loses brightness not because of the secondary shadow, but because

the secondary is too small to even use the entire primary mirror to illuminate the center point.

I thought that is exactly what "fully illuminated field (FIF) shrinks to zero" meant. That is exactly what I intended to convey. Of course only the light that actually gets to the primary mirror can even be considered for illumination, so the light blocking by the secondary is a given.

 

While I agree that there can be too much fussing about secondaries, in this case the large cost in going up from 4" makes worrying about it very reasonable. I would like to be persuaded that 4" is sufficient, it would save me money, and improve the quality of the mirror I can get, but the analysis here seems to be going in the other direction.

 

The cost more than doubles going from 4" to even 4.25", and I can only get 1/10 wave if I do (on regular offerings). I am not worried at all about CO obstruction, even 4.5" is quite good, but that is not what causes me to wish to avoid it.


Edited by careysub, 27 March 2019 - 12:50 PM.


#63 tommm

tommm

    Messenger

  • -----
  • Posts: 466
  • Joined: 16 Dec 2015

Posted 27 March 2019 - 12:43 PM

The guidance I have been following includes that of Mike Lockwood that for general use you should aim for a 0.5" fully illuminated center of field.

 

Also note that once you exceed a distance where the fully illuminated field drops to zero in Mel Bartel's calculator, it won't even give you an answer but tells you "No diagonal met the criteria. Please pick another size."

 

Although I entirely accept the claim that "an individual star's light loss of less than 0.3 magnitude is barely noticeable", across the center of the field of view the guidance as that some substantial width (like 0.5") should have even illumination or at most a very slight loss (Don suggests 0.1 mag).

 

Is Lockwood being too generous when calling for 0.5" fully illuminated? I don't know, but I wouldn't dismiss his opinion lightly. But let's say this is a rule of thumb that provides for a safety margin that can be reduced with careful design (which is entirely plausible I think).

 

According to the calculator with a 4" diagonal at 14.1" the fully illuminated field is 0.2", but with a drop at the edge of 0.5" of slightly less than 0.1 mag, so this seems to be the limit for this size.

 

Now my math:

10.7" light cone (I think this is a little generous by 0.1" or a little less, but lets use it)

0.28" Diagonal offset towards primary mirror and away from focuser (haven't been including this)

1.5" Focuser tube (same as used by SIPS, so this is probably OK if the build gets all distances right)

1.94" Paracorr adjustment

0.18" Filter

 

This is 14.6" which is 0.5" beyond the field illumination limit mentioned above, and does allow for any distance above the Paracorr/focuser/filter assembly. At this point the fully illuminated field is zero, and there is a drop-off at 0.5" of slightly more than 0.05 mag (relative to center of field) and a drop-off at the edge of field of 0.2 mag (relative).

 

That sounds just barely within the desirable range (no visible dimming), but if you rack the focuser out even 0.25" the calculator tells you that the secondary is too small.

 

So it seems that 4" cannot cut it.

 

OTOH, if a 4.5" is used then even with the tube racked all the way out an improbable 1.5" the illumination is still good. 4.25" might split the difference but the benefit in cost or CO savings is very small.

Not clear to me why you use 10.7” “light cone” but anyway it gives you plenty clearance from the mirror radius. Edit: nevermind, I see Don's suggestion based on FS.

 

I guess 1.5" focuser tube is the focuser draw tube length? Not clear to me why you use this rather than the 75mm length of the P2 barrel below the shoulder since it will protrude 1.5” beyond a 1.5” long draw tube.

 

Is the 1.94” Paracorr adjustment the difference in prime focus and the focus with the P2?  If so, the figure below from the Televue website indicates it is 46 mm or 1.81” (from: http://www.televue.c...d=61&Tab=_ttop)

Paracorr2.JPG

 

I guess you are now using the distance to the focal plane with P2 in Mel's calculator rather than the distance to prime focus?  I' having difficulty following all the twists and turns of this so could you please explain why?

 

You can also use Harold Suiter's calculation of secondary size to solve for the fully illuminated fov for a given secondary size:

 

L = (ma*f - D*T)/(f - T)

 

ma is secondary minor axis, f and D are focal length and diameter of the primary mirror, and T is the secondary to prime focus distance. 

 

Using T = 14.6", D = 20", f = 74" and ma = 4" gives L = 0.07" or 1.8mm.

A 4.5" secondary size gives 0.69" or 17.5mm. 

Using 1.81 rather than 1.94" gives T = 14.47" and L = 0.83", 18.6mm


Edited by tommm, 27 March 2019 - 12:49 PM.


#64 careysub

careysub

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2517
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 27 March 2019 - 12:59 PM

Regarding the 75mm for the Paracorr - you are probably right, I did not physically look at the Paracorr and was just assuming the draw tube was the limiting length, sound like the the Parcorr can be considered in isolation. In which case my math in post #54 is probably in error? Don, could you check whether I have the Paracorr intercept math correct - I do not know the correct way to take it into account, hence my motivation for starting this thread.

 

No I really cannot explain the Paracorr math, I am simply presenting my current attempt to get it correct. I am still not certain that it is.

 

Regarding Suiter, thans for reminding me, I will have to consult him. But right now I am traveling and will have to go back home to get to the book.


Edited by careysub, 27 March 2019 - 01:00 PM.


#65 tommm

tommm

    Messenger

  • -----
  • Posts: 466
  • Joined: 16 Dec 2015

Posted 27 March 2019 - 01:50 PM

Regarding the 75mm for the Paracorr - you are probably right, I did not physically look at the Paracorr and was just assuming the draw tube was the limiting length, sound like the the Parcorr can be considered in isolation...

That's how we started and got the original estimate (Don's post #42).

 

Regarding the difference is prime focus and focus with P2. I see Mel uses 13/16" = 1.81". Likely based off the same Televue figure.

 

I just set my scope up following Televue's figure and placing prime focus 0.8" or 20mm above the racked in focuser giving a margin beyond the 14mm in the figure.  I assumed fully illuminated fov with the P2 would be 1.15 less than what I calculated from Suiter's equation without the P2. I would like to know if that procedure is correct - bet you would too. lol.gif  Seems to agree with what Mel suggests. Edit: I think Mike L. sets prime focus according to the Televue figure also.  

 

In Don's post #42 the bottom of the P2 was placed at the mirror radius. Now you are adding half the reduced (by 1.15) FS, or about 18mm or 0.7". I guess that plus the 0.18" offset is now why you get 14.6" instead of the earlier 13.7". 

 

You don't have to be that stringent.  Mirror radius plus FS/2 is what the UTA I.D. should be, and if you want no obstruction of marginal rays then the bottom of the P2 must be outside this.  But you can choose to let it protrude a bit into the marginal rays.  I think Mitch set his up to be just outside the parallel rays (so the mirror radius) so that on-axis objects would be unaffected by diffraction from the focuser tube/P2 but off-axis objects would.  I set mine up so the focuser bottom is about 0.2" outside the mirror radius. I have a "standard" (no compression ring) 2" travel Moonlite focuser with 3.35" draw tube length, so that is the limiting parameter, not the 75mm of the P2 barrel.


Edited by tommm, 27 March 2019 - 02:35 PM.

  • careysub likes this

#66 hakann

hakann

    Surveyor 1

  • -----
  • Posts: 1743
  • Joined: 06 Aug 2015

Posted 27 March 2019 - 02:00 PM

The paracorr 2 tube is 74.82 mm long ( at least mine )

The total lenghts the coma corrector build is including the distance of 9 mm outboard lens is = 88.58 mm.

That is a fact.

The std telescope focus pt is where the 9 mm is.

 

Tommm's picture from TV show 75 mm + 14 mm = 89 mm ( it's a round up from TV, but it is actually 88.58 mm ).

 

So decide what low power EP you need, take the FS by half and add up the 88.58 mm and half primary ( radius ) and maybe add up a filter ( 4.5 mm ).

 

-There is where the std focus pt is as we can call the L-distance.

 

I has no clue why some like to add 5 or 10 mm more on that distance, as the EP from ex TV is around max +/- 1 mm and settings is on the rolled top, if you has the PII.

What I say is the focuser would not even get changed in position ( when change EP ) as it more or less parafocal here.

 

For some resons I can't add up a CAD Picture now from my computor.


Edited by hakann, 27 March 2019 - 02:45 PM.


#67 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 March 2019 - 02:35 PM

Regarding the 75mm for the Paracorr - you are probably right, I did not physically look at the Paracorr and was just assuming the draw tube was the limiting length, sound like the the Parcorr can be considered in isolation. In which case my math in post #54 is probably in error? Don, could you check whether I have the Paracorr intercept math correct - I do not know the correct way to take it into account, hence my motivation for starting this thread.

 

No I really cannot explain the Paracorr math, I am simply presenting my current attempt to get it correct. I am still not certain that it is.

 

Regarding Suiter, thans for reminding me, I will have to consult him. But right now I am traveling and will have to go back home to get to the book.

The 75mm length of the inserted Paracorr is longer than a drawtube with 1.5" travel, but shorter than a drawtube with 2" travel.

Since the Paracorr significantly reduces the necessity for focuser travel, a 1.5" travel drawtube is just fine.

[this also yields the ability to reach inside the tube and thread a filter onto the Paracorr without removing anything from the focuser.]

 

The upper assembly is 49mm long.  The Paracorr is raised from setting A to setting E to put the focal plane of the Paracorred scope at the lip of the Paracorr, or 0.4" = 10.16mm.

That means the length of the Paracorr, when the focal plane is exactly at the opening, is 134.16mm without an adapter.  TeleVue rounds it off to 135mm.

Incorporated into the 134.16mm Paracorr length is the focuser board inset, the focuser board thickness, the focuser height.

 

The Paracorr's bottom, in focus, should not intrude into the light path, which is the diameter of the mirror plus (approximately) the field stop of the lowest power eyepiece.

With an eyepiece of 41mm field stop, that would be a 20.5mm clearance around the mirror, with the bottom of the Paracorr at the edge of that (ignoring offset or myopia adjustments).

I think the point of earlier posts in the thread was that the 41mm field stop effectively becomes a 35.65mm field stop after the Paracorr, meaning the clearance between 

Paracorr and mirror can be reduced by 5.35mm.  Note how close that is to the allowance for myopia, and I think it's safer to just continue with the 20.5mm clearance between mirror and UTA inner diameter.

 

That makes the distance to the focal plane at:

mirror radius + 20.5mm + 134.16mm.  For the 20" mirror, that's 408.66mm (16.09")

You might want to allow for a trace more for the offset of the secondary mirror (assuming you'll offset it), and a "fudge factor" for any sloppiness in construction.

 

Now, if we look at it a different way, and put the focal plane of the scope WITHOUT Paracorr at, say, 20mm above the racked in focuser (to allow for infocus of the Paracorr and myopia adjustment), then the intercept distance is:

mirror radius + 20.5mm clearance + inset of focuser board + focuser board thickness + focuser height + 20mm.

I estimate 358mm (roughly), which would indicate a smaller secondary.

 

It is the Paracorr which moves the focal plane outward, so the second set of dimensions will still yield the first set when the Paracorr is inserted.

However, as Tommm pointed out, it is using the new focal plane distance that is goofing you up.  There is nothing wrong with using the second set of dimensions to figure the size of the secondary.

And if that says 3.8" is enough (the size of a 4" in a holder), then, by all means, go with the 4" secondary.


Edited by Starman1, 27 March 2019 - 02:39 PM.


#68 tommm

tommm

    Messenger

  • -----
  • Posts: 466
  • Joined: 16 Dec 2015

Posted 27 March 2019 - 02:50 PM

The 75mm length of the inserted Paracorr is longer than a drawtube with 1.5" travel, but shorter than a drawtube with 2" travel.

Since the Paracorr significantly reduces the necessity for focuser travel, a 1.5" travel drawtube is just fine.

[this also yields the ability to reach inside the tube and thread a filter onto the Paracorr without removing anything from the focuser.]

 

The upper assembly is 49mm long.  The Paracorr is raised from setting A to setting E to put the focal plane of the Paracorred scope at the lip of the Paracorr, or 0.4" = 10.16mm.

That means the length of the Paracorr, when the focal plane is exactly at the opening, is 134.16mm without an adapter.  TeleVue rounds it off to 135mm.

Incorporated into the 134.16mm Paracorr length is the focuser board inset, the focuser board thickness, the focuser height.

 

The Paracorr's bottom, in focus, should not intrude into the light path, which is the diameter of the mirror plus (approximately) the field stop of the lowest power eyepiece.

With an eyepiece of 41mm field stop, that would be a 20.5mm clearance around the mirror, with the bottom of the Paracorr at the edge of that (ignoring offset or myopia adjustments).

I think the point of earlier posts in the thread was that the 41mm field stop effectively becomes a 35.65mm field stop after the Paracorr, meaning the clearance between 

Paracorr and mirror can be reduced by 5.35mm.  Note how close that is to the allowance for myopia, and I think it's safer to just continue with the 20.5mm clearance between mirror and UTA inner diameter.

 

That makes the distance to the focal plane at:

mirror radius + 20.5mm + 134.16mm.  For the 20" mirror, that's 408.66mm (16.09")

You might want to allow for a trace more for the offset of the secondary mirror (assuming you'll offset it), and a "fudge factor" for any sloppiness in construction...

 

I agree with all that. I think I was wrong though about how careysub arrived at 14.6" (so I deleted that post and made another one).  I think he added 1/2 the reduce FS diameter, 18mm or 0.7" to his 0.18" secondary offset to go from the original 13.7" to 14.6". 



#69 hakann

hakann

    Surveyor 1

  • -----
  • Posts: 1743
  • Joined: 06 Aug 2015

Posted 27 March 2019 - 02:56 PM

Don,

 

As I got it ;

To set the telescopes std focus pt ( for setting up the diagonal ) has nothing to do whit the paracorr ( more than one need to decide where tube ends )

 

See ;

http://televue.com/e...id=61&Tab=_phot

 

From lens to the correctors focus pt is outboard = 56 mm.

But the std focus pt is 9 mm outboard lens, so its 47 mm extra to get the corrector in focus ( pt ).

This 47 is set by the EP focal pt and the settings of the rolled top.

 

It simple can't be as you say around 134.16 mm ( TV 135 mm long ) in the calculations.

 

To set the L-distance it's half mirror, and the decided 'clearance' and add upp 88.58 mm or say as TV = 89 mm and then add up whit a filter lenghts ( if one like that )



#70 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 March 2019 - 03:01 PM

Well, I think we established that the Paracorr, if always in the focuser, reduces the effective field stop diameters.

And that could reduce the Paracorr-to-primary distance.

But if you add back in the myopia adjustment and/or the secondary offset, you might as well figure the secondary size based on no Paracorr and simply know you will get the necessary illumination.



#71 MitchAlsup

MitchAlsup

    Soyuz

  • -----
  • Posts: 3817
  • Joined: 31 Aug 2009

Posted 27 March 2019 - 03:25 PM

The perfect size might fall between two commercially-available sizes.  In that case, pick the larger size.

Or have a one-off custom diagonal made specifically for that scope.

 

My 20" F/4 has a 3.85" secondary--on this one I paid $~700 and it is in zerodur.

My 20" F/3 has a 4.85" secondary--on this one I paid $200 for the tooling to cut the glass.



#72 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 March 2019 - 03:58 PM

Mitch,

You made your own scopes, too?



#73 tommm

tommm

    Messenger

  • -----
  • Posts: 466
  • Joined: 16 Dec 2015

Posted 27 March 2019 - 03:59 PM

Go back to Don's post #42.  There he stated:

"Let's see:
254mm mirror radius.
18mm tube clearance
(ignoring focuser board thickness, position thereof, focuser height, etc.) Paracorr length 75mm
5mm to allow for nearsighted people to focus
10.16mm height above bottom of Paracorr with Paracorr top at focus position (setting E)
49mm length of P-II tunable top.
That puts the minimum intercept distance at 411mm (16.19") with Paracorr II in place to keep the P-II from intruding into the tube.
"

 

In post #43 I came to a similar conclusion of 412mm, because I used 135mm from the Televue figure rather than Don's 134.16mm.  In that same post I then found 13.7" secondary to prime focus distance: "So again referring to the Televue figure, with the bottom of the P2 barrel located at the primary mirror radius, prime focus is at: 412 - 18 - 60 + 14 = 348mm, or 13.7" by placing prime focus at 14mm above the shoulder of the P2 per the Televue figure. (that's where the 134.16 comes in hakann, but I agree you don't have to do it that way)

 

Note that this already includes the 18mm tube clearance factor, which is half the reduced FS: 41/(2*1.15) = 17.8mm, and it includes 5mm margin "for nearsighted people".  It neglected the 0.18" secondary offset. Adding that gives 13.7 + 0.18 = 13.88". Using 1/2 the full FS, 20.5mm adds about 0.1" to give 13.98".  I think that is what it should be, not 14.6".


Edited by tommm, 27 March 2019 - 04:00 PM.


#74 hakann

hakann

    Surveyor 1

  • -----
  • Posts: 1743
  • Joined: 06 Aug 2015

Posted 27 March 2019 - 04:22 PM

Here is a CAD on my set-up.

 

Paracorr set-up from edge to std focus pt is = 88.58 mm. ( TV = 89 mm )

From there we add 47 mm to come to the correctors focus pt.

http://televue.com/e...id=61&Tab=_phot

 

Ethos ex 13 mm has the focus pt 6.85 mm below the shoulder.

 

Paracorr setting on the PII is setting H for E13.

 

I has set the EP to the correctors focus pt in CAD. 

 

The picture of distance from edge to top on the PII do show it not mach the true numbers - but its more or less ok from the CAD.

CAD = 67.61 mm.

Reality =  68.26 mm. ( Note the top might not be exaclty as where thread starts.

 

It's simple can't be 135 mm long to std focus pt = it is 89 mm..from where we set the L-distance to get the size of the diagonal.

 

Input's. 

Attached Thumbnails

  • CN1.jpeg
  • CN2.jpeg

Edited by hakann, 27 March 2019 - 04:37 PM.


#75 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42074
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 27 March 2019 - 04:28 PM

It's only 134.16mm from the bottom to the focal plane with the Paracorr in the "E" setting (which is where the Paracorr's focal plane is at the opening plane.

The Ethos 13mm has its focal plane lower in the eyepiece, so requires an additional 0.3" of out travel of the tunable top to bring its focal plane to the same place.

The earlier discussion was where the new focal plane of the scope would be with the Parcorr in place, which is 7.6mm lower than your measurement.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics