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# Diagonal size

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### #1 hakann

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Posted 12 April 2019 - 04:29 AM

This topics is debated before.. but my question that came to my knowledege when Mel Bartels program was recently up under ; How To Use The Bartel Diagonal Calculator with a Coma Corrector, and one said ; even if ex program show a very low magdrop at around say 0.15 he decide to use a 4.5” instead of a 4” , but it was from ’look’ manually into focuser hole and he could not see the full primary, so therefore a 4.5”.
I can’t say this is wrong or right, and I never heard that before.
I did check up his combo whit the 4” and it was a low magdrop and he had a short L-distance for combo.

So is the manual way a better deal here to set up a diagonal ?

-Just asking vs I never heard that.

" Keep in mind the I chose the 4 1/2 secondary as I was not able to see entire primary in the secondary - not because of the drop off in illumination."

Edited by hakann, 12 April 2019 - 07:38 AM.

### #2 Asbytec

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Posted 12 April 2019 - 04:45 AM

"...but it was from ’look’ manually into focuser hole and he could not see the full primary, so therefore a 4.5”.

If you cannot see the entire primary, including the clips - i.e., the edge of the primary within the secondary, from some presumably tall focuser position then you are not fully illuminated anywhere in the FOV at that focuser position. You should be able to rack the focuser all the way in and easily see then entire primary within the secondary, but sometimes focusing outward the image of the primary will (visually) expand beyond the edge of the secondary. In the latter case, the secondary is too small for the focuser position when the edge of the secondary becomes no longer visible. The amount of aperture loss is proportional to the amount of the primary mirror you cannot see.

That's probably why he went with a larger 4.5" secondary, to maintain full illumination in some part of the FOV over the full range of focuser movement. Despite his programmed focuser height, he could visually tell he was losing full illumination (full aperture) at some point (because he could not see the entire primary). When using his calculator, it's probably best to use the maximum focuser height in case some of your eyepieces require it to achieve focus. At the maximum focuser height, you should see all of the primary mirror, at least, inside the secondary. Usually that means we'll see the mirror clips as a reference, but it's really the primary mirror edge we are concerned with.

Edited by Asbytec, 12 April 2019 - 05:11 AM.

### #3 hakann

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Posted 12 April 2019 - 07:39 AM

Normally in use a PII and say TV EP, you are almost parafocal.

### #4 tommm

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Posted 12 April 2019 - 10:37 AM

Iircc, he said he chose 4.5" for collimation purposes only - much easier to collimate when he could see the entire primary mirror, which he couldn't with a 4".

### #5 Vic Menard

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Posted 12 April 2019 - 11:20 AM

...it was from ’look’ manually into focuser hole and he could not see the full primary...
I can’t say this is wrong or right, and I never heard that before.

So is the manual way a better deal here to set up a diagonal ?

There is a point on the axis where the primary mirror and the secondary mirror appear equal in apparent size. This is the apex*, and is useful for assessing the precise placement of the secondary mirror with correct offset.

You can determine the position of the apex if you read here:  http://www.vicmenard...rspectives.html  scroll down to, "Notes on matching a sight tube to your 'scope's focal length"

*When the apex is located on the focal plane the apparent size of the primary and secondary mirrors will be equal. Configured this way, the secondary size is considered fully "minimized" (any smaller reduces the full aperture of the primary mirror). In this configuration, the intercept distance divided by the secondary mirror axis equals the focal length of the primary mirror.

### #6 hakann

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Posted 12 April 2019 - 12:33 PM

+, I was not able to see entire primary in the secondary

### #7 Vic Menard

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Posted 12 April 2019 - 01:49 PM

+, I was not able to see entire primary in the secondary

This would mean your/his pupil was outside of the apex.

### #8 BGRE

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Posted 12 April 2019 - 04:19 PM

There is a point on the axis where the primary mirror and the secondary mirror appear equal in apparent size. This is the apex*, and is useful for assessing the precise placement of the secondary mirror with correct offset.

You can determine the position of the apex if you read here:  http://www.vicmenard...rspectives.html  scroll down to, "Notes on matching a sight tube to your 'scope's focal length"

*When the apex is located on the focal plane the apparent size of the primary and secondary mirrors will be equal. Configured this way, the secondary size is considered fully "minimized" (any smaller reduces the full aperture of the primary mirror). In this configuration, the intercept distance divided by the secondary mirror axis equals the focal length of the primary mirror.

apex of what precisely?

### #9 mark cowan

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Posted 12 April 2019 - 05:30 PM

"...but it was from ’look’ manually into focuser hole and he could not see the full primary, so therefore a 4.5”.

If you cannot see the entire primary, including the clips - i.e., the edge of the primary within the secondary, from some presumably tall focuser position then you are not fully illuminated anywhere in the FOV at that focuser position. You should be able to rack the focuser all the way in and easily see then entire primary within the secondary, but sometimes focusing outward the image of the primary will (visually) expand beyond the edge of the secondary. In the latter case, the secondary is too small for the focuser position when the edge of the secondary becomes no longer visible. The amount of aperture loss is proportional to the amount of the primary mirror you cannot see.

That's probably why he went with a larger 4.5" secondary, to maintain full illumination in some part of the FOV over the full range of focuser movement. Despite his programmed focuser height, he could visually tell he was losing full illumination (full aperture) at some point (because he could not see the entire primary). When using his calculator, it's probably best to use the maximum focuser height in case some of your eyepieces require it to achieve focus. At the maximum focuser height, you should see all of the primary mirror, at least, inside the secondary. Usually that means we'll see the mirror clips as a reference, but it's really the primary mirror edge we are concerned with.

You may need to pull an EP out to achieve focus but it doesn't change the focal plane itself, which is where the eyeball goes if you want to check illumination, and that's always the entrance pupil for EPs (field stop).  This is true of either SIPS PII or tunable top PII as the geometry ends up the same.

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### #10 Vic Menard

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Posted 12 April 2019 - 05:32 PM

apex of what precisely?

Apex of the cone where the edges of the primary mirror and secondary mirror converge (as illustrated in the link I provided).

### #11 Asbytec

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Posted 12 April 2019 - 08:16 PM

...and that's always the entrance pupil for EPs (field stop).

Yes, thanks for the clarification. Not so much about focus, it's about illumination. I used the idea of focus simply to set the maximum focuser height somewhere near the apex of the primary light cone.

### #12 BGRE

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Posted 12 April 2019 - 09:29 PM

Apex of the cone where the edges of the primary mirror and secondary mirror converge (as illustrated in the link I provided).

More precisely the apex of the cone tangent to the edges of the reflecting surfaces of the secondary and the primary mirrors. The axial apex position  varies with the separation of the mirrors whilst its lateral position depends on the relative centring of the projected edge of secondary and the primary.

### #13 tommm

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Posted 12 April 2019 - 10:59 PM

More precisely the apex of the cone tangent to the edges of the reflecting surfaces of the secondary and the primary mirrors. The axial apex position  varies with the separation of the mirrors whilst its lateral position depends on the relative centring of the projected edge of secondary and the primary.

From post #5: "There is a point on the axis where the primary mirror and the secondary mirror appear equal in apparent size. This is the apex*, and is useful for assessing the precise placement of the secondary mirror with correct offset."

### #14 BGRE

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Posted 12 April 2019 - 11:08 PM

From post #5: "There is a point on the axis where the primary mirror and the secondary mirror appear equal in apparent size. This is the apex*, and is useful for assessing the precise placement of the secondary mirror with correct offset."

That statement and the diagram on the linked page by themselves are somewhat confusing.

When the apex coincides with the focal plane there is no allowance for a finite field of view.

The resultant fall off in illumination for off axis objects can be detrimental for some applications.

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### #15 Vic Menard

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Posted 13 April 2019 - 08:46 AM

More precisely the apex of the cone tangent to the edges of the reflecting surfaces of the secondary and the primary mirrors. The axial apex position  varies with the separation of the mirrors whilst its lateral position depends on the relative centring of the projected edge of secondary and the primary.

Properly (optically) configured, the apex is on the axis. If the apex is laterally offset, the secondary mirror placement is incorrect. But you are correct in your observation that the position of the apex on the axis varies with the separation of the mirrors.

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### #16 Vic Menard

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Posted 13 April 2019 - 09:00 AM

That statement and the diagram on the linked page by themselves are somewhat confusing.

When the apex coincides with the focal plane there is no allowance for a finite field of view.

The resultant fall off in illumination for off axis objects can be detrimental for some applications.

The apex only coincides with the focal plane when the "fully illuminated image diameter" is a point (the secondary mirror is at its absolute minimum size where the full light cone from the primary mirror can still be redirected to the focal plane). If you look at the diagram on the linked page, you can see the location of the focal plane between the apex and the secondary mirror. This represents the fully illuminated image diameter for the mirror diameters and separation as configured. The larger the secondary mirror, the more distant the apex, and the greater the fully illuminated image diameter.

I'm not sure what you mean by "finite field of view", but the "fall off in illumination" is outside the fully illuminated image diameter at the focal plane. This isn't important for the apex application--which is to place the collimation pupil at a point on the axis where the apparent diameters of the primary and secondary mirrors are similar in size.

Edited by Vic Menard, 13 April 2019 - 09:03 AM.

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### #17 ed_turco

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Posted 13 April 2019 - 12:18 PM

Iircc, he said he chose 4.5" for collimation purposes only - much easier to collimate when he could see the entire primary mirror, which he couldn't with a 4".

I ran into the same problem with my Definitive Newtonian Reflector  (see below).  I could get away with a .75' diagonal but collimation was a nightmare and I changed to a 1" diagonal.  There really wasn't any discernible degradation of the image.

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### #18 Starman1

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Posted 13 April 2019 - 02:52 PM

Well, as I've said before, the size of the secondary is a range:

The smallest possible is with the center point on the focal plane fully illuminated by the two-mirror combination (remember, the secondary cuts some light, so the illumination at the center is never equal to 100%).

That would be a nightmare to collimate, and the light drop-off at the edge would be fairly significant.

The largest possible is with the entire focal plane chosen (usually the field stop of the lowest power eyepiece or camera chip) to be fully-illuminated to the maximum.

This would result in a large secondary shadow, more than necessary light loss, and reduced contrast.

So we typically choose where the light loss at the edge doesn't exceed 30% (about a 0.4 magnitude drop, as Mel uses in the calculator) and the center 8-13mm is fully-illuminated.

Keep in mind, the calculator is calculating the size of the reflective surface on the secondary.  If your secondary mirror holder has a lip around the secondary, this reduces the size of the secondary, as far as the calculator is concerned.  My 2.6" secondary, for example, has 2.45" of exposed surface and it is 2.45", not 2.6" that I enter into the calculator to figure illumination.

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### #19 andreww71

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Posted 14 April 2019 - 06:49 PM

The OP is referring to our discussion under the thread that he mentions. Now that there is a (another) new thread to discuss sizing the secondary I’ll provide a little more detail as well as some additional comments on the topic since my post a few weeks ago.

I’ve built a number of Dob’s and my biggest struggle when collimating comes when there is very little room around the edge of the primary reflection in the secondary. The tighter the distance the more difficult it is to do correctly.

Admittedly, the OP in the previous thread has real concerns regarding secondary size since the price difference between a 4” and 4 ½” is pretty steep. I don’t fault anyone for wanting to really dig deep to determine if it’s worth it or not.

I purchased a 22” F/4.16 primary and 4” secondary as a set, a few years ago. The primary mirror was made by John Lightholder, the secondary was made by Galaxy Optics. I had no reason to doubt the original owner’s ability to size the secondary mirror and so proceeded with specifying the appropriate secondary holder from AstroSystems.

I chose to use the Starlight Instruments SIPS focuser on this telescope. I don’t own a P2 and have only one other telescope that I have no plans on changing the focuser on to use with the SIPS. For me it was the right choice. I also decided to purchase a 31 Nagler 5 to use as my low power eyepiece with this scope. Combined with the SIPS it yields a 6.5 exit pupil but also requires more in-focus than many other eyepieces.

Getting back to the secondary holder, since I was also going to have AstroSystems “build in” the offset to the holder itself I needed to calculate it. Along with the primary mirror specifications, I plugged the FIF value and intercept distance into the Dakin formula to size the mirror and determine the offset and that’s when I noticed that I needed to take a closer look.

For those who are not familiar with using this formula, for visual use it is suggested to use a value of 0.5” for the diameter of the Fully Illuminated Field (FIF). You would use a larger value if sizing the secondary for photographic use, and a smaller value if you’re interested mainly in planetary or other high magnification / narrow field of view work. If you use a FIF of 0” you will calculate the smallest possible secondary you can use in your scope and good luck trying to collimate!

For my initial calculation I used the 0.5” Diameter Fully Illuminated Field value and an intercept distance of 14.75” which resulted in a secondary mirror size of 4.03”. Since the AstroSystems holder has a lip, my clear aperture would really only be about 3.8”. I wouldn’t get the 0.5” FIF value I was looking for with the 4”.

I ran the numbers again, using a secondary size of 3.8” to see what the FIF would be and the result was 0.25”. Since I was building the scope to provide views at all magnifications ranges, very low to very high I was concerned about the diameter of the FIF being smaller than the “tried and true” value.  At this point, I began to sweat. What does the difference between a 0.5” FIF and 0.25” FIF look like? Running the numbers through the Bartels calculator seemed to show that I had nothing to worry about.

I made a quick line drawing in CAD using the 4” mirror and there was still a little room between the edge of the primary and the secondary holder – not a lot, but a bit. Since it came as a package with the primary I decided to proceed with using it in the design. As a backup though I sourced a 4.5” secondary – just in case.

Once the structure was complete I tested the optical system to make sure it focused with all of my eyepieces. I then brought the structure back into the lighted work area and it is at this point that observed that I could not see the entire primary reflection in the secondary mirror. It could have been a case where some modification of the lip that holds the mirror in place - I didn’t measure it – would have been enough. Having less metal around the edge of the secondary holding it in place was not an option that I would feel comfortable with. This aspect, along with having a smaller FIF than I wanted made making the decision to replace it with something bigger a non issue for me.

Needless to say, the 4.5” solved the problem nicely.

After posting a few weeks ago I thought about this a bit more. Neither my CAD drawing nor the Dakin formula result showed that the 4” would not work. But visually it didn’t. Looking at the SIPS installed in the optical system shows the focal plane lying inside of the SIPS body, about 1.75” below the top of the focuser. If you installed a 2” low profile focuser in place of the SIPS, you would bring the top of the focuser 2.25” closer to the mounting board and probably allow the user to see the entire primary mirror reflection in the secondary.

So, if you’re planning on installing the SIPS into a system where you are trying to use the smallest possible secondary mirror you should look at all aspects thoroughly before making a decision.

Andrew

Edited by andreww71, 15 April 2019 - 06:20 AM.

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### #20 hakann

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Posted 15 April 2019 - 12:02 AM

Thanks Andrew.

As I got it both Sips and the PII are the same system.
Sips is fixed and one can say that goes for the PII aswell.
You has a rather short L-distance here to std focus pt at
14.75”.
Mirror is 22” / 2 = 11” + 3.487” ( CC lengts = 88.58 mm or 89 mm from TV )
That is 14.487”, so the 14.75” will not intrude in the primary. That is ok but but no much of a clearance ( 0.263” )
If I run numbers in Bartels program ( Mel mean ’line tracing’ is not the same ) on the data at 0.5 it is ( 3.8” ) it show = 0.07 in mag-drop at 0.6”.
Should be all safe here from all I read on it before.
But then this ’new’ came up by look into it and not see the full primary ( vs this thread )

At the data Bartels show it should be no worry on collimate.

Even if you rack your focuser you has the fixed L-distance here ( or ? )
You just move EP to its focus ot, but ok you move eye distance if you ’remove’ it and look in.

In my scope ( 18” f/4 ) I has a 3.8” (4”) and a L-distance at 13.48”.
So at 0.5” I has 0.055 in mag-drop ( a hair better than your data for the 4” in your scope.

I thought my step up from a 3.5” to 4” made me safe and ex CZ said I overkilled it and mag-drop was far from importanted I had thought.

I also heard few can see a difference ( ex Matt from Oz in his 18” f/3.5 )

But you feelt it was a improvement by step up.

In my ideas ( in theory ) maybe yes if you had a longer L-distance say a clearance at 1”.

In my case I has a mag-drop in Bartels program on low power EP ( E21 ) by 0.18.

-According from Don above 0.3 to 0.4 for visual use should be ok.

Your data on N31 ( 1.6” ) show a mag-drop at 0.25 ( 3.8” ) and the 4.5 ( = 4.3 ) is = 0.12.
Its great data !
( but can it be needed that’s up to operator.

Edited by hakann, 15 April 2019 - 12:20 AM.

### #21 andreww71

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Posted 15 April 2019 - 05:42 AM

hakann,

I'm approaching this discussion from a pure mechanical point of view. I think the Bartels program proves the point that the magnitude drop off for a visual observer is in many cases a non issue due to the lack of sensitivity of the human eye.

I followed the SIPS instructions for locating the focal plane (intercept distance in many formulas). The instructions state to place the focal plane about 64mm above the top of the focuser mounting board and use this distance to calculate the secondary mirror size. In the 22" this equals 12.25" + 2.5" [64mm] = 14.75".

But when you are collimating and are using any of the various collimation tools available on the market today that are designed for your eye (I have the three Tectron tools that were introduced early in the 90's), the location from the secondary mirror to where you place these tools is actually 12.25" + ~4.75" [120mm] = 17". This is where using the smallest possible secondary mirror is a problem and why in my case I was not able to see the entire reflection of the primary mirror in the secondary.

As I stated in the other thread I suggest the ATM use multiple references / tools to assist in their design to avoid problems.

Andrew

Thanks Andrew.

As I got it both Sips and the PII are the same system.
Sips is fixed and one can say that goes for the PII aswell.
You has a rather short L-distance here to std focus pt at
14.75”.
Mirror is 22” / 2 = 11” + 3.487” ( CC lengts = 88.58 mm or 89 mm from TV )
That is 14.487”, so the 14.75” will not intrude in the primary. That is ok but but no much of a clearance ( 0.263” )
If I run numbers in Bartels program ( Mel mean ’line tracing’ is not the same ) on the data at 0.5 it is ( 3.8” ) it show = 0.07 in mag-drop at 0.6”.
Should be all safe here from all I read on it before.
But then this ’new’ came up by look into it and not see the full primary ( vs this thread )

At the data Bartels show it should be no worry on collimate.

Even if you rack your focuser you has the fixed L-distance here ( or ? )
You just move EP to its focus ot, but ok you move eye distance if you ’remove’ it and look in.

In my scope ( 18” f/4 ) I has a 3.8” (4”) and a L-distance at 13.48”.
So at 0.5” I has 0.055 in mag-drop ( a hair better than your data for the 4” in your scope.

I thought my step up from a 3.5” to 4” made me safe and ex CZ said I overkilled it and mag-drop was far from importanted I had thought.

I also heard few can see a difference ( ex Matt from Oz in his 18” f/3.5 )

But you feelt it was a improvement by step up.

In my ideas ( in theory ) maybe yes if you had a longer L-distance say a clearance at 1”.

In my case I has a mag-drop in Bartels program on low power EP ( E21 ) by 0.18.

-According from Don above 0.3 to 0.4 for visual use should be ok.

Your data on N31 ( 1.6” ) show a mag-drop at 0.25 ( 3.8” ) and the 4.5 ( = 4.3 ) is = 0.12.
Its great data !
( but can it be needed that’s up to operator.

Edited by andreww71, 15 April 2019 - 06:29 AM.

### #22 drneilmb

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Posted 15 April 2019 - 07:41 AM

I don't need to butt into a thread with the fancy people, but since the SIPS makes the focal plane/apex inaccessible to the eye, why not use a collimation technique that doesn't depend on seeing the primary edge in the secondary? (Barlowed) laser, cross-haired sight tube, defocused star, etc.

I've never depended on the sight of the primary in the secondary when I collimate and I don't THINK that I'm doing it wrong. If you can afford a 22 and a SIPS, then maybe you can also afford a Glatter laser and tublug?

But, as Mel Bartels also says, it's probably not worth worrying about diagonal size too much.

### #23 Starman1

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Posted 15 April 2019 - 08:30 AM

Andrew,

If the final focal plane of the scope is above the racked in focuser, why can't you position the pupil of the collimation tool at the focal plane?

### #24 hakann

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Posted 15 April 2019 - 08:39 AM

By the mean 'pupil of the collimation tool' - does that mean end of a Glatter laser at telescopes std focus pt - or ?

### #25 Starman1

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Posted 15 April 2019 - 08:45 AM

I was referring to tools you look through.

Andrew mentioned the Tectron collimation tools, which are sight tube, Cheshire, and autocollimator.

But, technically, a laser should have its source at the focal plane in use..

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