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Equipment Discussions >> Classic Telescopes

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rguasto
professor emeritus
*****

Reged: 11/18/10

Loc: Long Island, NY
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: strdst]
      #5690315 - 02/20/13 07:55 AM

That collimation cap is all you need. That F8 scope will have a large "sweet spot" somewhere around a centimeter. Don't even think about a laser in that focuser - it will seat differently every time you insert it and each time it will be way off. To adjust the tilt of the secondary you must loosen the center nut, then adjust the 3 screws. Start by loosening the screws so you don't strip the slotted heads
-Rob


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strdst
Post Laureate
*****

Reged: 03/23/08

Loc: Oregon Territory
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: rguasto]
      #5704676 - 02/28/13 01:55 AM Attachment (10 downloads)

Had some time to make adjustments of the secondary. The sky offered some sucker holes last night, the view of Jupiter was quick but not conclusive. Too much turbulence even for a 60mm refractor that I know can do so much better. Sometimes I just really hate the weather where I've chosen to live Waaaaughhhh!

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strdst
Post Laureate
*****

Reged: 03/23/08

Loc: Oregon Territory
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: strdst]
      #5725119 - 03/11/13 02:10 AM

I got a few opportunities to view Jupiter. The image seemed sharp, not very contrasty, and quite a bit of spurious light. I shined a maglight through the backside and it looked like M-13 through a really big scope. Do I need to recoat? Will it tame the spurious light?

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: strdst]
      #5725466 - 03/11/13 09:51 AM

Great scope! The perfect seductress for a now former refractor guy. Love the round spider. Neat ventilation plug.

The cap over the mirror end of the tube may or may not effect air flow appreciably, but it does reinforce the edges of the fifty-year-old tube. Maybe desirable to leave it in place.

I recognized the view from ActionHac's yard even before reading his name. Guess I've been on the forum for a while!


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bremms
Carpal Tunnel
*****

Reged: 08/31/12

Loc: SC
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: Joe Cepleur]
      #5725568 - 03/11/13 10:50 AM

It should have very good contrast at F8 with a small diagonal.
If its the original coating. it's going to not give great images. I have a few really nice mirrors in my storage area. I'm planning on some nice reflector biulds in the next few months.


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actionhac
Post Laureate
*****

Reged: 08/09/08

Loc: Seattle
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: bremms]
      #5725699 - 03/11/13 12:05 PM Attachment (9 downloads)

I just did the same with my RV-6 mirror.
I haven't viewed with this mirror yet.
I don't know much about this subject, but looking at my picture below I'd say the rather spotty unevenness of the aluminum coating is probably erosion.
I would imagine when the aluminum was originally deposited it was much more uniform and probably the aluminum molecules were nice and evenly distributed.
Now with this erosion and the hills and valleys in the coating we will see a loss of contrast and some scattering of the light.


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Bill Griffith
sage


Reged: 08/12/09

Loc: Ca.
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: actionhac]
      #5725788 - 03/11/13 12:49 PM

Robert,

Does your mirror need recoating? Sure looks like it.
Loss of contrast probably. Not so sure if scattering is a function though. I don't think the non coated surface sends photons on a different path. My understanding is that the figure has not changed.
Reflectness is actually higher with thin coatings.

Many mirrors with higher reflectness will show a flashlight image through the backside. A visual glance tells our brain something is wrong with what we see. Yours is pretty thin though!

Bare aluminum coating is on average about 1000 to 1500 angstroms which I believe amounts to .1 micron. 25 microns to .001 of an inch.



Coatings do not effect a figure of a mirror or lense if done close to correct.

Bill


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bremms
Carpal Tunnel
*****

Reged: 08/31/12

Loc: SC
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: Bill Griffith]
      #5725816 - 03/11/13 01:04 PM

I did some experiments with coatings in an independent lab study in college. Reflectivity was pretty constant above 1/4 wavelength thick ~1500 angstroms. Below 1000 it did start to fall off and below about 700 was the knee of the curve. We went up to about 4000 angstroms and there was no easily measurable change from about 1200-4000 angstroms.

Yes that is an eroded coating. A few pinholes is normal but that coating is not good and will cause scattering.and you should be able to see a bulb filament through the coating.


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bremms
Carpal Tunnel
*****

Reged: 08/31/12

Loc: SC
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: bremms]
      #5725837 - 03/11/13 01:14 PM

I just found my data from the thin film experiments. The coatings were flat in response down to about 700 angstroms then gently tailed off the "knee" was more like 400 angstroms.
Better to check something you did nearly 30 years ago than rely on memory. Thicker coatings are going to be more uneven due to the vaporization techniques and are to be avoided.
Bob hit the nail on the head with this one.


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Bill Griffith
sage


Reged: 08/12/09

Loc: Ca.
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: bremms]
      #5725995 - 03/11/13 02:37 PM

Mea Culpa

My apologies Robert I didn't mean to imply you were wrong. Just asking the professionals out there lurking

Thanks Marc for getting the rusty gears turning in the right direction.

I had completely forgot about introducing the index of refraction! That would scatter visable wavelengths.

Although I was under the impression that a 1/4 wave step would need to be 10 times the amount of a 1500 A deposition.

The reflectness not reflectivity measurements I have acquired are from a Carey 5000 and hyperspectral cameras.

Bill

Edited by Bill Griffith (03/11/13 02:45 PM)


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bremms
Carpal Tunnel
*****

Reged: 08/31/12

Loc: SC
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: Bill Griffith]
      #5726166 - 03/11/13 03:47 PM

1/4 wave is about 1500 angstroms. 5500 angstroms is about the center of the visual spectrum. Which is 550 nanometers. Mixing the two can be confusing. Angstrom is 10-10 and Nanometer is 10-9. I always try to use nM.

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strdst
Post Laureate
*****

Reged: 03/23/08

Loc: Oregon Territory
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: bremms]
      #5729302 - 03/12/13 11:08 PM

Ok Robert is a reflector kinda guy although his inner refractor kinda guy burns bright. Robert is conflicted, which makes him totally awesome here. Robert knows when a mirror ain't up to potential and knows a great achromat when he sees through one. Me... I know when I'm sockless looking through a refractor but reflectors puzzle me.
So far this beautiful Criterion 6" comes in second to my standard grab and run, a HOC.Monolux 4380 60x910 refractor Goodwill scope. Certainly the Criterion is brighter, but both scopes are comparable for image sharpness at similar magnifications. If the haze of light around bright objects (Jupiter) indicates a recoat of the mirror or mirrors is necessary, where should I send it? The scope was born in 1957 and has never been touched. At that time Criterion was promising 1/20 wave optics IIRC.
I certainly don't want to recoat if it is unnecessary. Don't even want to remove the mirror if it is unnecessary. I'm a cheap refractor kinda guy. These fussy mirrors scare me!

So should I consider a recoat in order to minimize the stray light or keep working on collimation instead?


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strdst
Post Laureate
*****

Reged: 03/23/08

Loc: Oregon Territory
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: strdst]
      #5729379 - 03/12/13 11:40 PM

BTW,

Trying to post on a phone totally sucks. Which hopefully explains a few things I missed or messed up in the previous post. Sorry.


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actionhac
Post Laureate
*****

Reged: 08/09/08

Loc: Seattle
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: strdst]
      #5729467 - 03/13/13 12:43 AM Attachment (4 downloads)

I'm not so sure about the flashlight coating test, I think it may be overly critical shinning a flashlight at any lens or mirror its going to look horrible.
I put my sheer no-nonsense mirror back in the tube and it really looks good through the eyepiece. So I did the flashlight coating test on a brand new mirror and it looks like *BLEEP* too!
How clean is your mirror Keith, a really dirty mirror can scatter light. Nice thing about your Dynascope is you can take out the screws holding the cell in the tube and if you mark the cell and tube and put it back with the marks in the same place it shouldn't affect collimation.
I'd forgot just how good the Criterion's are. If your mirror isn't worse than mine your scope should perform beautifully. I know it will need to be working very well though to please a refractor guy!
OK here's another shot of mine:


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strdst
Post Laureate
*****

Reged: 03/23/08

Loc: Oregon Territory
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: actionhac]
      #5729521 - 03/13/13 01:30 AM

I think mine looks about like your last post... 55yrs into it ,never a recoat. This scope for its size is pretty heavy. To replicate a view through a fantastic 60 m isn't enough to keep it around. I've heard a few recently bad recoater stories. If I send it away to.who?

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actionhac
Post Laureate
*****

Reged: 08/09/08

Loc: Seattle
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: strdst]
      #5729543 - 03/13/13 01:40 AM

I don't know who to recommend Keith. I thought I would send one here:
http://www.nova-optical.com/
I have a Nova mirror in my dob and the coatings have held up very well for ten years old exposed in a open truss that lives out in my shop year round next to the lawn mower and the rototiller.


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dgreyson
professor emeritus
*****

Reged: 11/06/12

Loc: South Carolina
Re: 36.4mm to 1.25" diagonal to 6" Dynascope new [Re: actionhac]
      #5731048 - 03/13/13 08:21 PM

I had my 10 inch recoated by nova and was very pleased with their service several years ago. Havent delt with them recently but was planning on sending my 12 inch later this summer.

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