JustinJ
member
Reged: 08/07/08
Posts: 69
Loc: Central Pennsylvania
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I am considering changing the secondary in my Discovery PDHQ 10 F6. It has the stock 2.14 secondary. What type of changes in contrast and view would I see going to a smaller secondary 1.83 or larger secondary 2.6. I was looking at Antares 1/20 or 1/30 or Protostar.
Is it worth upgrading the secondary to a 1/30 wave at the same size as current secondary. Will I really be able to tell the difference in views?
I mostly view DSO and planets at the end of the night.
Thanks,
Justin
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Discovery PDHQ 10 f6
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1951
Loc: California
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I suggest you stay with the stock mirror size. If you use 1.83" then you run the risk of vignetting the primary and reducing the effective aperture of your scope.
As far as the difference between 1/20 and 1/30, I suggest the 1/20. I do not believe the 1/30 is worth the money difference but this is my personal opinion.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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JustinJ
member
Reged: 08/07/08
Posts: 69
Loc: Central Pennsylvania
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Hello Jason,
Would I be able to see a difference in views with a diagonal 1/20 as opposed to the Discovery secondary? Discovery guarantees their secondaries 1/8 or better. I am just contemplating if the money spent on a new secondary will give me much improvement.
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Discovery PDHQ 10 f6
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1951
Loc: California
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In my opinion, you will not be able to discern the difference between 1/8 and 1/20 secondary. Those who are highly experienced observers might be able to discern the difference on excellent nights at high magnification but the rest of us will not especially on average nights.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2054
Loc: Massachusetts
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A smaller secondary will reduce the size of the fully illumnated field at the EP field stop, not reduce aperture. The stock secondary for a 17.5 f/5 Discovery is too big and I specified a 3.1" secondary when I odered mine. I could have gone even smaller. Check out the free program Newt to calculate the effect of different size secondaries.
IMO, the surface finish of the stock secondary is fine and there will be no discernable difference going to a higher spec secondary.
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Jason D
Carpal Tunnel
Reged: 10/21/06
Posts: 1951
Loc: California
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Quote:
A smaller secondary will reduce the size of the fully illumnated field at the EP field stop, not reduce aperture
If the secondary is too small to intercept the entire reflected light cone then you are effectively reducing aperture.
Jason
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XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1531
Loc: Prescott, AZ
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Quote:
I am considering changing the secondary in my Discovery PDHQ 10 F6. It has the stock 2.14 secondary. What type of changes in contrast and view would I see going to a smaller secondary 1.83 or larger secondary 2.6. I was looking at Antares 1/20 or 1/30 or Protostar.
Is it worth upgrading the secondary to a 1/30 wave at the same size as current secondary. Will I really be able to tell the difference in views?
I mostly view DSO and planets at the end of the night.
Thanks,
Justin
Here is a link for an easy and intuitive method for determining diagonal size. Opinions will vary (greatly) but unless you are doing lots of variable star estimates or imaging, 1.86" would be ample for your scope. The 2.6" diagonal you mentioned would excessive.
As to the 1/30th wave marketing claim, I would be a bit skeptical on that. That kind of surface accuracy is pushing the limits of both Pyrex and measurement. OTOH, it will probably be better than a (claimed) 1/8 wave mirror, and under star testing will show a better test of whatever your primary mirror is throwing at it. All things considered there are many areas to spend excessively, the cost of a fine quality diagonal is pretty minor in the scheme of things. Enjoy.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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JustinJ
member
Reged: 08/07/08
Posts: 69
Loc: Central Pennsylvania
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I am thinking of perhaps keeping two secondaries and exchanging them out depending on my viewing. I really enjoy DSO viewing but like to observe the planets when the sky is not that great. It sounds like the 2.14 may be the best for keeping the wider field.
My Question:
Can I go down to 1.52 or 1.3 secondary for planetary viewing only? I know that the field of view would be cut but not sure by how much. It should allow me the ability to have better contrast for planetary work. Am I correct in my assumption?
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Discovery PDHQ 10 f6
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
It is a good idea to have two secondaries.
For DSO keep the stock for now.
It is worth going up to a 1/16th wave secondary IF you have a 1/8th wave mirror for planets and detail in DSO objects.
You need to do a Foucault test of your primary mirror, use a Ronchi grating or do a star test to see how good it is first.
A local group can help you do this. A Foucault tester is not hard to make.
My friend moved to a good 12" mirror and a Clave' 1/16th wave flat on my advice and he gets wonderful detail on Jupiter.
I have a 10" which has a good parabola with a bit of micro-ripple and a Clave' flat as well and had good views of Saturn on first light which was a rare good night for a city.
Of course optical quality is the 6th most important parameter but it is really needed IMHO.
Cheers. Andrew.
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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1531
Loc: Prescott, AZ
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Quote:
My Question:
Can I go down to 1.52 or 1.3 secondary for planetary viewing only? I know that the field of view would be cut but not sure by how much. It should allow me the ability to have better contrast for planetary work. Am I correct in my assumption?
There was a great pair of articles years ago in Sky & Tel by a noted optician on this subject. I can't recall his name at the moment (help someone?), but he looked at several aspects of Newtonian performance and MTF functions compared to unobstrutced designs. His conclusion was that once the central obstruction falls below 20% there was little meaningful improvement in planetary performance (and performance was still pretty good in the low 20's too). The 20% figure also seems to be the conventional wisdom.
Should you choose to go smaller, the issue you have is that the area of full illumination drops down to a very small disk. Keeping a planet centered in this area would be a challenge. And beyond that, you are effectively loosing aperture. Bottom line is the 1.86" would be well sized for planetary work, and the 2.14" wouldn't be bad either. You would be wise not to get too worked up about this decision.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 10471
Loc: Amargosa Valley, NV, USA
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I'll go with the others, 2.14" is an ideal size for a 10" f/6. As far as the surface quality, The quality of the primary and secondary combine to create the final waveform, with the primary being the biggest contributor. A 1/30 wave secondary won't help if you have a barely 1/4 wave primary. The 1/20 wave secondary, if that's truly what it is, will not produce any noticeable image degradation.
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"S.O.E." (Sauron's Other Eye) 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Next Project: The "Eye of Sauron" Observatory!
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Zoomit
super member
Reged: 12/04/06
Posts: 125
Loc: Tehachapi, CA
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Justin,
There's been some great advice given to you on this thread.
There is one thing I'll add about secondary size vs accuracy. Usually the worst part of a secondary mirror's figure is on the periphery. A 1/20 2.14" mirror might be have a large central portion that could test to 1/30 or better. That central area is what you'd be using when looking at a planet in the center of the FOV.
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Brandon
AstroSystems TeleKit 18" f/4.6; Orion XT8 7.4" f/6.4
TeleVue 26Nt5, 13E, 3-6NZ; Antares 2" 1.6x
Astro-Physics 15x70; Nikon Action EX 12x50
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JustinJ
member
Reged: 08/07/08
Posts: 69
Loc: Central Pennsylvania
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It sounds like the best thing is to upgrade the Secondary to 1/20 and leave the size the same. I think that Discovery PDHQ guarantees a wave RMS of 1/15 -1/20 or better on the primary. Since the secondary is only at 1/8 or better, it may help upgrade the secondary to 1/20.
Am I assuming correctly here that a 1/20 should show a little better images during good seeing?
Are there any one other secondary manufactures besides Protostar and Antares that I should look at?
I appreciate everyones help here. I am satisfied with my scope but like to make modifications and fine tune it. One day, I would not mind building a telescope but figure it best to learn modifying my current one.
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Discovery PDHQ 10 f6
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 2054
Loc: Massachusetts
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Keep track of whether you're talking RMS or peak to peak. Usually, when a surface is described as 1/8 wave, that's peak to peak. An 1/8 wave peak to peak may already be better than 1/20 wave RMS.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT, Eon-120ED
Lunt 60mm single etalon HA
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
One each generic rescue Greyhound (pictured)
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jpcannavo
sage
Reged: 02/21/05
Posts: 324
Loc: Long Island New York
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Justin
This link is a must.
http://www.bbastrodesigns.com/diagonal.htm
WRT the general goal of effective optimization, I try to keep these two complimentary priniciples in mind:
1)Dont maximize one parameter at the expense of another. (For ex. smallest possible secondary introducing a turned edge into the light path etc.)
2)Several seemingly insignificant modifications can add up to a net gain.
Joe
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Joseph Cannavo
16" F5 Zambuto, front collimating, scope nearly complete - WSP here we come!
16" F5 Royce, conical blank, front collimating (Mirror Sold)
10" F5 Dob
Mid 70's RV-6
4" Orion 100mm ED
Mr Keeyoots (My Cat)
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davidpitre
Pooh-Bah
Reged: 05/10/05
Posts: 1410
Loc: Central Texas
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Quote:
Are there any one other secondary manufactures besides Protostar and Antares that I should look at?
Galaxy sells them retail.
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David
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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You can squeeze more brightness out of DSO's at your lowest mags by going larger. I know this is counter to what you would think, but it's true. 
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JustinJ
member
Reged: 08/07/08
Posts: 69
Loc: Central Pennsylvania
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I thought that going larger would improve my viewing for DSO. From what I've read, you can go up to 25% obstruction without too much loss. Would I loose that much contrast going from a 2.14 to 2.6 mirror?
Secondary Mirror Companies:
I have noticed that some companies say that you can not test a mirror above 1/20 . How does Antares mirror guarantee 1/30 mirrors?
Has anyone purchased a Galaxy secondary mirror?
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Discovery PDHQ 10 f6
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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The secondary is a tough one. I am still of the opinion, your basic decent secondary is enough.
Galaxy is a reputable company, I'd bank he can make a flat as well as anybody. The guy has been doing it for decades now.
As a reflective surface gets closer to the focal point, it's errors have less of an impact.
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HandyAndy
sage
Reged: 01/11/08
Posts: 412
Loc: West Midlands and around
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Hi,
If the primary really is that good it is worth going to a better secondary for those moments of good seeing.
Cheers. Andrew.
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Monarch 8x42, Zeiss 10x50 WA
10mm F2, Pentax 60mm F5
City: 7" MN78: MK4#2, 10" F6.3: MK4#1, 16" F5 ParaCorr
Country: 5" F9.5, 8" VISAC: GP2
Car: 6" F5 MPCC: SP, 5" 127mm F7.5
TV 55mm, Paragon 40mm, UO Pretoria 28mm
B&L 32 Pl, Clave's 25, 8, 6, 2x
Hyperions 5, 8, 13, 17, 24, 31
Nagler1 9mm, Meade 14mm 4000 UWA
Antares 1.6x, 0.7x, 0.5x
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