Musing on the secondary size for a Newt

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Kanadalainen
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Musing on the secondary size for a Newt

#1

Post by Kanadalainen »


I have an XT10, and want to put in a high quality secondary (either an Antares or a Protostar) in combination with a single curved spider to reduce diffraction spikes.

Resources on the web:


- Mel Bartel's diagonal Calculator
www.bbastrodesigns.com/diagonal.htm
I like this one.

- Newt for the web:
stellafane.org/tm/newt-web/newt-web.html
This is on the Stellarfane site.

- Mike Lockwood's page:
http://www.loptics.com/ATM/diagonals.html
Gives similar results to Bartels' page.

Its useful to calculate L (distance from the secondary mirror to the focal plane) for your particular scope (this from a Gary Seronik article):

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Screen Shot 2019-05-22 at 10.45.36 AM.png
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My scope yields a number of 10.2" for L. If you know focal ratio (in my case 4.72) you are set. Dividing 10.2/4.72 gives me 2.16" for minimum size of the secondary.

Interestingly, this is backed up by the Newt program online.

[image]
Screen Shot 2019-05-22 at 10.13.02 AM.png
[/image]

Here is a handy ray-trace using Newt:

[image]
Screen Shot 2019-05-22 at 10.08.21 AM.png
[/image]

This configuration is by the numbers....which may be great for strictly planetary work. I am not interested in limiting the scope to planetary views and want a real-life solution.

Bartel's page and Mike Lockwoods' (respectively) provide perspective on this, and using their calculators and tables, give me a number very close to the stock secondary diagonal size! :)

[image]
Screen Shot 2019-05-22 at 10.33.58 AM.png
[/image]

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Screen Shot 2019-05-22 at 10.37.41 AM.png
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2.48" or 63 mm is the stock secondary size for GSO and Synta scopes, and Protostar has them.

Thanks for reading to the bottom,

Ian
Attachments
Screen Shot 2019-05-22 at 10.37.41 AM.png
Screen Shot 2019-05-22 at 10.33.58 AM.png
Screen Shot 2019-05-22 at 10.08.21 AM.png
Screen Shot 2019-05-22 at 10.13.02 AM.png
Screen Shot 2019-05-22 at 10.45.36 AM.png
Ian

Fracs: Stellarvue 70T f6; SW 120mm Esprit f7; "Mark Mk. II" - 60 mm Tasco f6; C80 frac f 11.4
SCT: C8 Edge f10 or f7 with reducer
Dob: 14.5" homebuilt strut dob (f4.5 ZOC mirror), Nexus II, Moonlite focuser
Mounts - Ioptron Skyguider pro, Astro Physics GTO900
Cameras and lenses - ZWO 2600 mc, 290 mm mini, Canon 60D modded with Rokinon 10mm 2.8; Rokinon 135mm f2

Skysafari 6 Pro, Astro Pixel Processor, Pixinsight - using Mac tablet and ASIair pro to run the AP rig.

"Mothers! It is there!" - Rafael Gonzales-Acuna, 2018.
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Kanadalainen
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Re: Musing on the secondary size for a Newt

#2

Post by Kanadalainen »


I will relist Mel Bartels webpage as well as Newt on Stellafane... I forgot to include the http:// prefix, my bad.

Bartels:

https://www.bbastrodesigns.com/diagonal.htm


... and Newt

https://stellafane.org/tm/newt-web/newt-web.html

Also, here is Gary Seronik's great discussion on L and focal ratio, as well as the importance of focuser height in determining secondary minor axis sizing. Lovely info to determine secondary sizing.

https://garyseronik.com/sizing-up-the-n ... secondary/
Ian

Fracs: Stellarvue 70T f6; SW 120mm Esprit f7; "Mark Mk. II" - 60 mm Tasco f6; C80 frac f 11.4
SCT: C8 Edge f10 or f7 with reducer
Dob: 14.5" homebuilt strut dob (f4.5 ZOC mirror), Nexus II, Moonlite focuser
Mounts - Ioptron Skyguider pro, Astro Physics GTO900
Cameras and lenses - ZWO 2600 mc, 290 mm mini, Canon 60D modded with Rokinon 10mm 2.8; Rokinon 135mm f2

Skysafari 6 Pro, Astro Pixel Processor, Pixinsight - using Mac tablet and ASIair pro to run the AP rig.

"Mothers! It is there!" - Rafael Gonzales-Acuna, 2018.
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Re: Musing on the secondary size for a Newt

#3

Post by notFritzArgelander »


Looks like you're covering the bases well. Another amusing web Newtonian design resource is

https://www.catseyecollimation.com/designie5.html

I like to make the secondary as small as possible to preserve low frequency contrast.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Re: Musing on the secondary size for a Newt

#4

Post by Kanadalainen »


Thanks notFritz,

Did you change out the secondary in your 12" GSO?

I
Ian

Fracs: Stellarvue 70T f6; SW 120mm Esprit f7; "Mark Mk. II" - 60 mm Tasco f6; C80 frac f 11.4
SCT: C8 Edge f10 or f7 with reducer
Dob: 14.5" homebuilt strut dob (f4.5 ZOC mirror), Nexus II, Moonlite focuser
Mounts - Ioptron Skyguider pro, Astro Physics GTO900
Cameras and lenses - ZWO 2600 mc, 290 mm mini, Canon 60D modded with Rokinon 10mm 2.8; Rokinon 135mm f2

Skysafari 6 Pro, Astro Pixel Processor, Pixinsight - using Mac tablet and ASIair pro to run the AP rig.

"Mothers! It is there!" - Rafael Gonzales-Acuna, 2018.
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Re: Musing on the secondary size for a Newt

#5

Post by notFritzArgelander »


Kanadalainen wrote: Wed May 22, 2019 6:46 pm Thanks notFritz,

Did you change out the secondary in your 12" GSO?

I
Not..... yet.... but perhaps soon. I'll be posting a new thread about self rewards I'm considering after successfully navigating my move. Remodeling the Z12 is on the list.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
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Rainmaker
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Re: Musing on the secondary size for a Newt

#6

Post by Rainmaker »


Just been looking at your data Ian and noted a couple of things.....

Your eyepiece field diameter of 2" seems a little large. I don't know what eyepieces you have but the largest I have is the 1.8" field stop in the TV 55mm, whereas the N31T5 is about 1.6".
The stock diagonal of 2.48" seems very small with a fully illuminated field of only 0.4" and will result in a reduction of illumination at the edge of about 0.5magnitude.
If your widest field stop eyepiece is something like the Nagler 31T5 then a 2.6" diagonal would give you a FIF of 0.6" and only a 0.4 loss at EOF which is less noticeable.

I don't know what type of secondary holder your scope has but if it is one that covers any of the reflective surface (a lip) then you need to take that into consideration in your calculations...

With your scope using eyepieces longer than 31mm (unless a wider TF is needed for a particular object) will result in exit pupil will likely exceeding your fully dilated pupil and thus your are effectively using less than the whole primary. A Nagler 31T5 will give you 39x with a 6.5mm exit pupil and a True Field of 2.1º which is very nice indeed.

I went through similar 'musings' with my 18" F3.54 scope and after testing it extensively using both 4" and 4.5" secondaries I found the larger secondary gave a better overall view for the more even illumination across the full field.
Matt in Oz
Rainmaker “Excalibur” 18” F3.5 +SIPS @ 1860mm
TMB-LZOS 152mm F7.9 @ 1200mm
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Kanadalainen
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Re: Musing on the secondary size for a Newt

#7

Post by Kanadalainen »


Rainmaker wrote: Thu May 23, 2019 12:07 am Just been looking at your data Ian and noted a couple of things.....

Your eyepiece field diameter of 2" seems a little large. I don't know what eyepieces you have but the largest I have is the 1.8" field stop in the TV 55mm, whereas the N31T5 is about 1.6".
The stock diagonal of 2.48" seems very small with a fully illuminated field of only 0.4" and will result in a reduction of illumination at the edge of about 0.5magnitude.
If your widest field stop eyepiece is something like the Nagler 31T5 then a 2.6" diagonal would give you a FIF of 0.6" and only a 0.4 loss at EOF which is less noticeable.

I don't know what type of secondary holder your scope has but if it is one that covers any of the reflective surface (a lip) then you need to take that into consideration in your calculations...
Thanks Matti - those are very useful comments and I really appreciate your input,

- Field diameter: Yep, you are right! I messed up the field diameter - should be more like 1.6".

- I *think* the 0.4" 100% illuminated field was specific for the test size of 2.14" axis secondary - that data was further down the page, unable to grab it with my utility app.

At the end of the day I will choose a diagonal that is close to the stocker in terms of axis width, but at 1/14 wave P-V. :P

Ian
Ian

Fracs: Stellarvue 70T f6; SW 120mm Esprit f7; "Mark Mk. II" - 60 mm Tasco f6; C80 frac f 11.4
SCT: C8 Edge f10 or f7 with reducer
Dob: 14.5" homebuilt strut dob (f4.5 ZOC mirror), Nexus II, Moonlite focuser
Mounts - Ioptron Skyguider pro, Astro Physics GTO900
Cameras and lenses - ZWO 2600 mc, 290 mm mini, Canon 60D modded with Rokinon 10mm 2.8; Rokinon 135mm f2

Skysafari 6 Pro, Astro Pixel Processor, Pixinsight - using Mac tablet and ASIair pro to run the AP rig.

"Mothers! It is there!" - Rafael Gonzales-Acuna, 2018.
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Kanadalainen
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Re: Musing on the secondary size for a Newt

#8

Post by Kanadalainen »


Rainmaker wrote: Thu May 23, 2019 12:07 am I don't know what type of secondary holder your scope has but if it is one that covers any of the reflective surface (a lip) then you need to take that into consideration in your calculations...
Hello Matti,

Good comment, what I have now is much smaller than the reflective surface and rests completely below the diagonal. It measures about 2" at its base, and the diagonal is fastened to it.

Ian
Ian

Fracs: Stellarvue 70T f6; SW 120mm Esprit f7; "Mark Mk. II" - 60 mm Tasco f6; C80 frac f 11.4
SCT: C8 Edge f10 or f7 with reducer
Dob: 14.5" homebuilt strut dob (f4.5 ZOC mirror), Nexus II, Moonlite focuser
Mounts - Ioptron Skyguider pro, Astro Physics GTO900
Cameras and lenses - ZWO 2600 mc, 290 mm mini, Canon 60D modded with Rokinon 10mm 2.8; Rokinon 135mm f2

Skysafari 6 Pro, Astro Pixel Processor, Pixinsight - using Mac tablet and ASIair pro to run the AP rig.

"Mothers! It is there!" - Rafael Gonzales-Acuna, 2018.
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Re: Musing on the secondary size for a Newt

#9

Post by Bigzmey »


I don't own any DOBs but it is fun to follow your project Ian. Image
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
Mounts: SW: SkyTee2, AzGTi; iOptron: AZMP; ES: Twilight I; Bresser: EXOS2; UA: MicroStar.
Binos: APM: 100-90 APO; Canon: IS 15x50; Orion: Binoviewer, LG II 15x70, WV 10x50, Nikon: AE 16x50, 10x50, 8x40.
EPs: Pentax: XWs & XFs; TeleVue: Delites, Panoptic & Plossls; ES: 68, 62; Vixen: SLVs; Baader: BCOs, Aspherics, Mark IV.
Diagonals: Baader: BBHS mirror, Zeiss Spec T2 prism, Clicklock dielectric; TeleVue: Evebrite dielectric; AltairAstro: 2" prism.
Filters: Lumicon: DeepSky, UHC, OIII, H-beta; Baader: Moon & SkyGlow, Contrast Booster, UHC-S, 6-color set; Astronomik: UHC.

Observing: DSOs: 3106 (Completed: Messier, Herschel 1, 2, 3. In progress: H2,500: 2180, S110: 77). Doubles: 2382, Comets: 34, Asteroids: 255
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