Different collimation for f/5 and lower newt?

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StarLord101
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Different collimation for f/5 and lower newt?

#1

Post by StarLord101 »


I picked up an avx mount with the six inch celestron Newtonian reflector. I've done collimation before but it's been years, so I read some guides watched some videos for a refresher. Came across a few mentions that fast news f/5 and lower, you actually don't have everything all centered when it's properly collimated. I see a few examples that show what looks to me like a very non collimated state with some things centered and others offset. My issue is that with collimation, you have this target. It's correct when everything is round and centered. With that caveat though, I have no idea how to tell what my target is since I'm not necessarily trying to get something in a center mark. I could not find any clarification other than an aside like " oh by the way, fast will look more like this"
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Re: Different collimation for f/5 and lower newt?

#2

Post by notFritzArgelander »


I don't understand the situation you describe..... I've rolled my own Newtonian, a slow one, and have had f4, f5, and f6 scopes. Aside from the question of whether an offset is necessary for the secondary flat I'm not sure what the issue might be. If you could post a link to the collimation advice you refer to in your post, it would help us answer you better?
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: Different collimation for f/5 and lower newt?

#3

Post by John Baars »


If you meant if an offset is necessary, the answer is yes. See post 4 in this link: https://www.cloudynights.com/topic/5266 ... al-offset/
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
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Re: Different collimation for f/5 and lower newt?

#4

Post by Lady Fraktor »


You center everything while collimating, what it looks like after collimation is no concern.
Sometimes the secondary will seem to be offset more so if a spherical mirror but nothing to worry about.

Read the Astro Baby Collimation Guide, honestly the last place I go for good advice is youtube...
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Re: Different collimation for f/5 and lower newt?

#5

Post by yobbo89 »


Only the secondary is offset in the secondary holder and there is a good reason for that, collimation is as usually as for any other Newtonian in the same/simular design.

As show in the image below fig b. , when one is using a centered secondary part of the light cone misses the secondary due to the angle of the fast primary and in general cutting across downwards on a triangle actually slices through a thicker portion while cutting on an angle away slices through a thiner portion,

Fig a is the correct mirror offset and fig c is somewhat doable that you can actually collimate a fast scope with a centered secondary but you would need to tilt the focuser after to avoid of axis tilt.

In addition you will find that visual scopes will have a smaller secondary as full illumination isn't as much in favour visual over imaging and that less obstruction is more ideal which gives you better contrast while viewing and also that the focal point is alot shorter for eyepieces so the fold of the light cone can be made higher towards the apex.
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scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
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Re: Different collimation for f/5 and lower newt?

#6

Post by Piet Le Roux »


StarLord101 wrote: Thu Jun 04, 2020 9:18 pm I picked up an avx mount with the six inch celestron Newtonian reflector. I've done collimation before but it's been years, so I read some guides watched some videos for a refresher. Came across a few mentions that fast news f/5 and lower, you actually don't have everything all centered when it's properly collimated. I see a few examples that show what looks to me like a very non collimated state with some things centered and others offset. My issue is that with collimation, you have this target. It's correct when everything is round and centered. With that caveat though, I have no idea how to tell what my target is since I'm not necessarily trying to get something in a center mark. I could not find any clarification other than an aside like " oh by the way, fast will look more like this"
When I did my first collimation with a short focal length F6 telescope this "centered" misinformation gave me a lot of headache! The important thing that the "experts" and collimation manual should say is : No collimated reflector does appear completely "centred" then looking down a Cheshire Collimating Eyepiece.....it is just more apparent when this is done with a fast reflector.
Here is a example of collimator instructions :
manual.jpg
manual.jpg (11.29 KiB) Viewed 2721 times

Here is a example of A perfectly aligned Newtonian telescope with focal ratio about 1.75. Displaying centered and
non-centered features:
Offset.jpg
The reason is that the oval shape secondary mirror is "distorted" by 45 degrees so that it appears to be round when looking down the focuser tube. The centre of the image is not the centre of the mirror when looking at it at a zero degrees.
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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