•

# Sizing of the secondary question:

42 replies to this topic

### #26 N3p

N3p

Surveyor 1

• topic starter
• Posts: 1,833
• Joined: 25 Jun 2018

Posted 23 June 2022 - 11:24 AM

That depends on the intercept distance (secondary to focal plane) and the size of the field in your lowest power eyepiece.

One way to use Mel's calculator is to enter the fieldstop of your lowest power eyepiece in the illuminated field size (e.g. 31mm Nagler = 1.65"), and select the diagonal size that doesn't descend below -0.4 magnitudes at the edge of the field shown. I think it's a good logic to assume that the telescope must take the field stop of the Nagler 31mm without any problems.  Thanks.

As to what the intercept distance is, it is the sum of:

--clearance to inner tube wall

--thickness of tube

--height of base of focuser off the tube

--height of focuser racked all the way in

--1" additional to focal plane to allow things like barlows and coma correctors to come to focus and have nearsighted friends look through the scope.

So, take a 12" scope.  It is hard to see an intercept distance in a properly designed scope will be less than about 114mm.

As a rule of thumb, I always add 4.5" (114mm) to the mirror radius.

Why?

A minimum of 19mm clearance between mirror and tube to keep upper opening from vignetting the incoming light and allow ventilation of the tube without significant air currents in the optical path.

A usual 25mm between the base of the focuser and the I.D. of the upper tube assembly to allow for the focuser board and ring and to keep the drawtube out of the optical path of the scope.

A low profile focuser 45mm tall.  In reality, many are taller.

A focal plane clearance of 25mm over the focuser.

Hence, 114mm.

Thanks for that!

If any of those numbers is larger than that, then you will add length to the intercept distance.  If the scope is used for photography, the secondary is larger (as it is in your example).

A purely visual 12" f/5 scope would be fine with a much smaller 66mm secondary, but that would assume a shorter intercept distance. Right, I'll inspect the numbers of the GSO tube tonight to see about the precise intercept distance with the 70mm and 80mm, just for the fun.

So, plugging in the numbers, and keeping the edge of field light loss to < 0.4 magnitudes,

### #27 N3p

N3p

Surveyor 1

• topic starter
• Posts: 1,833
• Joined: 25 Jun 2018

Posted 23 June 2022 - 11:25 AM

A field diameter of 1.65" is entered if the low power eyepiece is, say, a 31mm Nagler, and that is a distinct possibility on a 12" scope.

I don't have one right now but it's something I would get at some point in life, totally.

### #28 Garyth64

Garyth64

Fly Me to the Moon

• Posts: 6,533
• Joined: 07 May 2015
• Loc: SE Michigan

Posted 23 June 2022 - 06:50 PM

A field diameter of 1.65" is entered if the low power eyepiece is, say, a 31mm Nagler, and that is a distinct possibility on a 12" scope.

Correct.  And then the use of a 3.5" secondary should be used.

### #29 Vic Menard

Vic Menard

Cosmos

• Posts: 9,019
• Joined: 21 Jul 2004

Posted 23 June 2022 - 07:36 PM

Starman1, on 23 Jun 2022 - 11:41 AM, said:

A field diameter of 1.65" is entered if the low power eyepiece is, say, a 31mm Nagler, and that is a distinct possibility on a 12" scope.

N3p responded:

I don't have one right now but it's something I would get at some point in life, totally.

Again, the scope you're using for your "Sizing of the secondary question" is a photo/visual scope. This means a longer intercept and (usually) a wider fully illuminated field (1.6-inch is also 40mm, the field lens diameter for a Paracorr).

Your intercept distance (secondary mirror to focal plane) will probably be around 12.5-inches (using Jon's 11-inch intercept and adding the required 35mm extension for visual use), maybe more. Using 12.5-inches (317.5mm) and switching to metric, the Bartels calculator shows a 70mm secondary delivers "acceptable" performance visually--8mm central field diameter at 100-percent illumination, and 0.37 magnitude drop off at the edge of a 40mm field diameter. The 88mm secondary mirror delivers good performance for imaging--32mm central field diameter at 100-percent illumination, and 0.15 magnitude drop off at the edge of a 40mm field diameter.

IMO, neither secondary mirror is ideal for its suggested application. The 70mm is too small (75mm is better) and the 88mm is too large (82mm is better). And you may have to watch out for vignetting from the bottom edge of the focuser.

The longer intercept will shorten the overall OTA length (if that makes a difference to you), but it also puts additional stress on the focuser/tube stability when using heavy eyepieces (and a coma corrector if you'll be using one).

Do you want to do DSLR photography/imaging or is your primary interest visual observing?

### #30 MeridianStarGazer

MeridianStarGazer

Hubble

• Posts: 12,958
• Joined: 01 Dec 2013
• Loc: USA

Posted 23 June 2022 - 08:04 PM

Mel Bartel's calculator defaults to the focal plain 3-3.5" away from the edge of the on axis light cone. That is optimized for f3.

Probably best to take half the diameter and add the focal ratio capped at 5. So a 12" f5 has the focal plain 11" from the optical axis.

### #31 Jon Isaacs

Jon Isaacs

ISS

• Posts: 101,541
• Joined: 16 Jun 2004
• Loc: San Diego and Boulevard, CA

Posted 23 June 2022 - 09:32 PM

The link in post #17 is for a 12-inch f/5 Newtonian Reflector Telescope.

If you read closer, item 4-Focuser and Accessories, "...comes with a 1.25" eyepiece adapter with a compression ring to match 1.25" eyepieces and accessories, and it includes a 35mm-long 2" extension tube to let you achieve focus with a 1.25" or 2" eyepiece when using the telescope for visual use."

The secondary mirror is configured for photo/visual use (and a longer secondary mirror to focal plane distance to accommodate a camera body). I would not try to optimize the secondary mirror to reduce the central obstruction without addressing the longer intercept distance, which will likely require a longer OTA.

The scope is incorrectly advertised--it should read, 12-inch f/5 Photo/Visual Newtonian Reflector Telescope.

I'm glad someone took the time to read that. That extra 35 mm push the focal plane out to approximately 12 inches.

Looking at Mel's calculator, a 70 mm secondary seems like it still could work.

Jon

### #32 N3p

N3p

Surveyor 1

• topic starter
• Posts: 1,833
• Joined: 25 Jun 2018

Posted 24 June 2022 - 07:07 AM

Again, the scope you're using for your "Sizing of the secondary question" is a photo/visual scope. This means a longer intercept and (usually) a wider fully illuminated field (1.6-inch is also 40mm, the field lens diameter for a Paracorr).

Your intercept distance (secondary mirror to focal plane) will probably be around 12.5-inches (using Jon's 11-inch intercept and adding the required 35mm extension for visual use), maybe more. Using 12.5-inches (317.5mm) and switching to metric, the Bartels calculator shows a 70mm secondary delivers "acceptable" performance visually--8mm central field diameter at 100-percent illumination, and 0.37 magnitude drop off at the edge of a 40mm field diameter. The 88mm secondary mirror delivers good performance for imaging--32mm central field diameter at 100-percent illumination, and 0.15 magnitude drop off at the edge of a 40mm field diameter.

IMO, neither secondary mirror is ideal for its suggested application. The 70mm is too small (75mm is better) and the 88mm is too large (82mm is better). And you may have to watch out for vignetting from the bottom edge of the focuser.

The longer intercept will shorten the overall OTA length (if that makes a difference to you), but it also puts additional stress on the focuser/tube stability when using heavy eyepieces (and a coma corrector if you'll be using one).

Do you want to do DSLR photography/imaging or is your primary interest visual observing?

I'll do visual observation only.

I'm glad someone took the time to read that. That extra 35 mm push the focal plane out to approximately 12 inches.

Looking at Mel's calculator, a 70 mm secondary seems like it still could work.

Jon

I think there was an error in the spec sheet of the GSO 12" F5 telescope, the seller tells me that the original secondary mirror coming with the telescope is the 70mm, the 88mm is an other possible option for it.

### #33 Garyth64

Garyth64

Fly Me to the Moon

• Posts: 6,533
• Joined: 07 May 2015
• Loc: SE Michigan

Posted 24 June 2022 - 07:59 AM

I'll do visual observation only.

. . . as you said in post #1.

### #34 Vic Menard

Vic Menard

Cosmos

• Posts: 9,019
• Joined: 21 Jul 2004

Posted 24 June 2022 - 09:14 AM

. . . as you said in post #1.

I missed that.

Given "visual use only", I don't think this photo/visual scope is a good match for the OP's intended purpose.

• N3p likes this

### #35 Don H

Don H

Surveyor 1

• Posts: 1,682
• Joined: 28 Dec 2006
• Loc: Near Tucson, AZ

Posted 24 June 2022 - 03:25 PM

The Obsession 12.5" f/5 uses a 2.14" secondary. They easily could have gone with a larger flat, but they calculated a 2.14" as ideal. Many owners of these scopes seem satisfied and are not complaining about that choice. I still think a 12" f/5.5 with a low profile focuser and no extension will work just fine for visual only use with a 2.14". The 2.6 is only 66mm, so even the 70 seems a bit big, but better than an 88. That seems more like the diagonal of the Obsession 20 or 22" f/4.

https://www.obsessio...ssion_Specs.pdf

Edited by Don H, 24 June 2022 - 03:26 PM.

### #36 Vic Menard

Vic Menard

Cosmos

• Posts: 9,019
• Joined: 21 Jul 2004

Posted 24 June 2022 - 04:29 PM

The Obsession 12.5" f/5 uses a 2.14" secondary. They easily could have gone with a larger flat, but they calculated a 2.14" as ideal...

Using the Bartels calculator, a 12.5-inch f/5 will require an intercept distance shorter than 10.6-inches to pass the entire light bundle from the primary with a 2.14-inch secondary mirror. At 10.5-inches, the 100-perecnt illuminated field diameter is a point, and the magnitude loss at the edge of the field (1.6-inch diameter) is 0.6 magnitude. While this may be ideal for a dedicated planetary scope, it will likely achieve better performance with an aperture stop to allow the scope to operate as a10-inch f/6.2.

If you can reduce the intercept distance to 8.5-inches, the 12.5-inch f/5 with a 2.14-inch secondary mirror can illuminate a 0.4- (100-percent) to 0.6-inch (95-percent) field diameter with a 0.45-magnitude drop at the edge of a 1.6-inch field diameter. This will require an ultra-low profile focuser mounted on a thin focuser plate positioned 7-inches from the centre of the secondary.

...I still think a 12" f/5.5 with a low profile focuser and no extension will work just fine for visual only use with a 2.14". The 2.6 is only 66mm, so even the 70 seems a bit big, but better than an 88. That seems more like the diagonal of the Obsession 20 or 22" f/4.

A 12-inch f/5.5 with a 10.5-inch intercept distance will work for visual use only with a 2.14-inch minor axis secondary mirror. The fully illuminated field diameter is 0.2-inch and the drop of at the edge of a 1.6-inch filed diameter is a bit more than 0.5-magnitude. Not the best match (IMO), but the numbers indicate the secondary will "work".

The 70mm and 88m secondary mirrors were not intended for use with a 12.5-inch Obsession or a 12-inch f/5.5 Newtonian. They were intended for a photo/visual mixed use 12-inch f/5 Newtonian with an extended intercept to allow a DSLR or other imaging equipment to be used, or, in the place of a camera, a 35mm 2-inch ID extension tube to be used for visual use.

I have a 22-inch f/4 StarStructure Dobsonian with a low-profile FeatherTouch focuser and a 14.5-inch intercept distance. My 4-inch (100mm) secondary fully illuminates a 0.4-inch field diameter with a 0.2 magnitude drop at 1.6-inch diameter (an 88mm is too small). The 4-inch secondary fully illuminates the field lens of a Paracorr 2, which is always in the focuser when observing.

Edited by Vic Menard, 25 June 2022 - 09:57 AM.

### #37 Starman1

Starman1

Stargeezer

• Posts: 56,861
• Joined: 23 Jun 2003
• Loc: Los Angeles

Posted 24 June 2022 - 07:00 PM

The 12.5" Classic Obsession used a 13.5" I.D. UTA and had an intercept distance short enough many users had to cut the poles on to use a Paracorr.

I think it was designed for 1.25" eyepieces and a very short focuser with drawtube projecting into the light path.

I'm afraid Obsession simply used the wrong size secondary mirror.  Also, 2.14" in an Astrosystems holder has only a 1.99" exposed reflective surface.

As I calculate a classic built, the intercept distance is around 6.25 + 4.5 = 10.75" to allow use of a Paracorr and large 2" eyepieces in a 45mm tall focuser.

and keep a short drawtube out of the light path.  The calculator says the minimum secondary is 2.4" for a 0.4 magnitude loss, but since an Astrosystems holder covers at least 0.15" of the edge,

the minimum secondary is really a 2.55".  Translating to commercially-available secondary sizes, it says a 12.5" f/5 should have a 2.6" (66mm) secondary.

A 2.14" secondary is actually stopping the scope down significantly.

• Vic Menard likes this

### #38 Starman1

Starman1

Stargeezer

• Posts: 56,861
• Joined: 23 Jun 2003
• Loc: Los Angeles

Posted 24 June 2022 - 07:04 PM

Using the Bartels calculator, a 12.5-inch f/5 will require an intercept distance no shorter than 10.6-inches to pass the entire light bundle from the primary with a 2.14-inch secondary mirror. At 10.5-inches, the 100-perecnt illuminated field diameter is a point, and the magnitude loss at the edge of the field (1.6-inch diameter) is 0.6 magnitude. While this may be ideal for a dedicated planetary scope, it will likely achieve better performance with an aperture stop to allow the scope to operate as a10-inch f/6.2.

If you can reduce the intercept distance to 8.5-inches, the 12.5-inch f/5 with a 2.14-inch secondary mirror can illuminate a 0.4- (100-percent) to 0.6-inch (95-percent) field diameter with a 0.45-magnitude drop at the edge of a 1.6-inch field diameter. This will require an ultra-low profile focuser mounted on a thin focuser plate positioned 7-inches from the centre of the secondary.

A 12-inch f/5.5 with a 10.5-inch intercept distance will work for visual use only with a 2.14-inch minor axis secondary mirror. The fully illuminated field diameter is 0.2-inch and the drop of at the edge of a 1.6-inch filed diameter is a bit more than 0.5-magnitude. Not the best match (IMO), but the numbers indicate the secondary will "work".

The 70mm and 88m secondary mirrors were not intended for use with a 12.5-inch Obsession or a 12-inch f/5.5 Newtonian. They were intended for a photo/visual mixed use 12-inch f/5 Newtonian with an extended intercept to allow a DSLR or other imaging equipment to be used, or, in the place of a camera, a 35mm 2-inch ID extension tube to be used for visual use.

I have a 22-inch f/4 StarStructure Dobsonian with a low-profile FeatherTouch focuser and a 14.5-inch intercept distance. My 4-inch (100mm) secondary fully illuminates a 0.4-inch field diameter with a 0.2 magnitude drop at 1.6-inch diameter (an 88mm is too small). The 4-inch secondary fully illuminates the field lens of a Paracorr 2, which is always in the focuser when observing.

How did you get the intercept distance down to only a 3.5" plus the mirror radius?  The focuser height + extra clearance to focal plane is going to be 2.75"  Add focuser board thickness, and space to the mirror, and, even with tight clearances, I still see 4.25".

### #39 N3p

N3p

Surveyor 1

• topic starter
• Posts: 1,833
• Joined: 25 Jun 2018

Posted 24 June 2022 - 08:17 PM

The Obsession 12.5" f/5 uses a 2.14" secondary. They easily could have gone with a larger flat, but they calculated a 2.14" as ideal. Many owners of these scopes seem satisfied and are not complaining about that choice. I still think a 12" f/5.5 with a low profile focuser and no extension will work just fine for visual only use with a 2.14". The 2.6 is only 66mm, so even the 70 seems a bit big, but better than an 88. That seems more like the diagonal of the Obsession 20 or 22" f/4.

https://www.obsessio...ssion_Specs.pdf

There was a slight subject shift, the secondary mirror was about a GSO 12" F5 tube this time and not the 12" F5.5 project. The GSO tube is just another possibility in front of me.

### #40 Vic Menard

Vic Menard

Cosmos

• Posts: 9,019
• Joined: 21 Jul 2004

Posted 25 June 2022 - 07:15 AM

How did you get the intercept distance down to only a 3.5" plus the mirror radius?  The focuser height + extra clearance to focal plane is going to be 2.75"  Add focuser board thickness, and space to the mirror, and, even with tight clearances, I still see 4.25".

It was shorter than 3.5-inches. I was reminiscing about an old 12.5-inch f/7 we had optimized for a 1.25-inch minor axis secondary mirror (10-percent CO by diameter, 1-percent by area) using an ultra-low profile helical focuser that was actually mounted inside the OTA. As I recall, we finally settled on a 1.83-inch secondary and a very-low profile JMI 2-speed focuser (DX1 or DX2?) that was a better match for more "general" use (that was likely closer to an 8.5- or 9-inch intercept). I also remember a very low profile AstroSystems focuser that operated with an o-ring drive system...

### #41 Starman1

Starman1

Stargeezer

• Posts: 56,861
• Joined: 23 Jun 2003
• Loc: Los Angeles

Posted 25 June 2022 - 08:36 AM

That old Phase 4 focuser was indeed quite short.

But if you decrease the intercept distance too much, the drawtube is in the light path and results in additional diffraction spikes on stars.

### #42 Vic Menard

Vic Menard

Cosmos

• Posts: 9,019
• Joined: 21 Jul 2004

Posted 25 June 2022 - 09:36 AM

That old Phase 4 focuser was indeed quite short.

But if you decrease the intercept distance too much, the drawtube is in the light path and results in additional diffraction spikes on stars.

With the helical focuser mounted inside the OTA (and the focal plane barely outside the OTA), there's no doubt there was some intrusion into the light path. We could have easily stopped the front aperture, but the views were really quite good using the full aperture (and back then, for high magnification work, Clavé eyepieces).

### #43 tommm

tommm

Apollo

• Posts: 1,292
• Joined: 16 Dec 2015

Posted 25 June 2022 - 04:46 PM

...I also remember a very low profile AstroSystems focuser that operated with an o-ring drive system...

I have one of those.  I used it on the first large K&B type Dob I made about 20 years ago.  The inner barrel can be lowered to 1/4" below the top of the focuser due to a cutout in the side for the ep retainer screw.  Lowered that way it can be racked into just 1 1/8" (28.7mm) above the focuser mounting board.  It has a 1/8" thick o-ring the focuser sits on, which adds 1/8" for 1 1/4" (31.8mm) racked in height if you use it.  Very low profile.

## Recent Topics

 Cloudy Nights LLC Cloudy Nights Sponsor: Astronomics