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Secondary quality

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#1 Mr Magoo

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Posted 20 August 2011 - 09:59 AM

Looking at several suppliers for secondaries has me thinking about how good is good enough? One supplier offers a secondary of BK7 Glass, 1/10 Wave, Al & SiO2 Coating, 92% Reflectivity in a size that would work for me for around $60.00. Another supplier offers secondaries with a wave rating ranging from 1/15 to 1/30 wave. The price range for a size that would work for me ranges from $170.00 to $260.00.

First of all for visual use I have been told that 1/10-1/15 is plenty good enough and anything over that is really only desirable for imaging. Opinions? Obviously the lower end range of 1/10 or 1/15 wave is a huge price difference between the 2. Incidentally the the secondary I'm using now is from the first vendor and was rated at 1/10. Seems to be performing good to me, but I'm no testing expert and have never had it tested. That bring me to my next question.

I have tried star testing my primary and can see the results. Is there a way to star test the secondary? Do problems with a secondary show up at the eyepiece during a star test?

#2 Old Will

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Posted 20 August 2011 - 10:25 AM

Just my opinion, which won't buy much, a secondary being so much closer to the focal plane than the primary affects the overall quality of view by a proportional lesser amount. Also since 1/10 wave is surely enough visually, going to extremes in this case would be putting "pearls on a pig" other bragging rights....

#3 Alvan Clark

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Posted 20 August 2011 - 11:50 AM

I agree you probably won't see a difference. The bigger issue is the overall quality and smoothness of the mirror and whether those advertised as 1/10 wave actually are.

#4 DAVIDG

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Posted 20 August 2011 - 12:22 PM

Raytracing will show that you need at least 1/10 wave in flatness or it will introduce astigmatism into the image. So if you keep seeing astigmatism when you optics look well aligned then you need to start to question the quality of the flat.
The issue is that, what a supplier states the flatness is and what it truely is can be very different. I purchased a couple of "1/10" wave small diagonals and they all were junk, maybe 2 waves at the best. That is why I test my diagonals using a known quality flat. The flat and a CFL bulb is all you need. You can purchase 2" diameter flats from Ebay and Surplus Shed for not much money and use them to test other flats. If you purchase three of them you can test one against the other and determine which one is best and return the other two. Another method is to use the diagonal in question in front of a small refractor to aim starlight into the refractor. You then examine a star at high power. The steeper the angle of the flat the more sensitive the test and if you see astigmatism you know the flat is not very good.
Here is a picture of good diagonal being tested.
- Dave

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  • 4758748-DIAGONAL.JPG


#5 Mr Magoo

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Posted 20 August 2011 - 03:05 PM

That's good info to have. I kind of figured that the smoothness would be pretty important on a flat. I'm still trying to figure the best size. Using the formula in the K&B book gives me a size of 2.87". This is for a 13.1" f4.5. secondary to focal plane distance of 10", and using the fully illuminated field diameter of .78". Lockwood uses .5" in his suggested size tables which gives me a size of 2.64" The available size of 2.7" seems like it would be a good compromise. The 3.1" that came with the original scope causes vignetting at the focuser, but I'm still not convinced that is is that bad of an issue.

#6 Pinbout

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Posted 20 August 2011 - 03:48 PM

David,

Do you have a diagram of how you set up the test?

#7 DAVIDG

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Posted 20 August 2011 - 04:54 PM

David,

Do you have a diagram of how you set up the test?


You just place the master flat on top of the flat your testing and shine monochrome light on it. Both surfaces need to very clean. If your have a coated flat to be tested then the Master needs a partial coating to have the reflectivities be close so th finge contrast is high enough to easily see.
For critical testing like trying to determine between a 1/20 and 1/30 wave surface you need a source of collimated light so you'll need to build a Fizeau interferometer but for commerical diagonal testing, in my experience they are usually what the wave rating was suppose to be or miles off so the simpler contact test is usually all you need.

- Dave

#8 John Carruthers

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Posted 21 August 2011 - 02:15 AM

I say get the best optical surfaces you can afford, you won't regret it. I used a 1/20 quartz flat with my best mirror and though most nights you'd never know, on those rare magical nights (less than 5/year) when it can be used to its full potential it's worth it.

#9 Mark Harry

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Posted 21 August 2011 - 07:26 AM

The quality of the flat can get pretty stringent with large fast scopes I would think. (?)
Dave's pic of the flat looks good across most of the aperture, with a slight edge defect. If I was making the test, I'd like to see about a quarter of the bands shown in the pic, and make them "sweep" across the surface to highlight any defects better. :tonofbricks:
A pic of my first 1" MA elliptical; highlighting the edge issue. Quite minor, but the fewer bands makes it easier to see and quantify.
Best,
Mark

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  • 4760088-success 1inch.jpg


#10 Mark Harry

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Posted 21 August 2011 - 07:39 AM

I have to differ with Dave on a couple minor points:
1. It's possible, and quite easily, to test a coated flat with an uncoated reference. Same method applies, you just have to pay attention to what your eyes are capable of relaying for information. Experience is a big help.
2. If wanting to confirm accuracies that equal or exceed 1/20th wave surface error, reference flats need to have accuracies considerably better than these tolerances, and this applies to direct interference testing, or IFs. I personally do not like to use IFs when looking at miniscule errors like this; for there's far more that can go wrong. Reference accuracy I would like to see somewhere as being confirmed in the 1/50-1/100th range. With references in the 1/20th range, you're only going to reliably ascertain about 1/10th surface error as a best case.
(A good smooth 1/10th wave with very little edge hook is likely as good as needed with most moderate focal length Newts anyhow; as noted previously above somewhere)
Mark

#11 Ed Jones

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Posted 21 August 2011 - 09:16 AM

Some vendors will give an interferogram with the diagonal they sell. I would recommend to only buy from those who did.

#12 Mark Harry

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Posted 21 August 2011 - 05:23 PM

I bought one once with an IF from a known maker, and it was nfg. Depends on the rep of the maker, and the testimonials backing them up!
fwiw,
M.

#13 Mr Magoo

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Posted 21 August 2011 - 05:29 PM

Wait a minute. So what your saying is that the Bic ballpoint pen I bought off of eBay that was used by the signers of the Declaration of Independence may be a fake even though I have a C.O.A? Darn it!

#14 DAVIDG

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Posted 21 August 2011 - 10:11 PM

I have to differ with Dave on a couple minor points:
1. It's possible, and quite easily, to test a coated flat with an uncoated reference. Same method applies, you just have to pay attention to what your eyes are capable of relaying for information. Experience is a big help.
2. If wanting to confirm accuracies that equal or exceed 1/20th wave surface error, reference flats need to have accuracies considerably better than these tolerances, and this applies to direct interference testing, or IFs. I personally do not like to use IFs when looking at miniscule errors like this; for there's far more that can go wrong. Reference accuracy I would like to see somewhere as being confirmed in the 1/50-1/100th range. With references in the 1/20th range, you're only going to reliably ascertain about 1/10th surface error as a best case.
(A good smooth 1/10th wave with very little edge hook is likely as good as needed with most moderate focal length Newts anyhow; as noted previously above somewhere)
Mark


Mark,
I agree with you. I think the point I'm trying to make is that it isn't hard to test a diagonal and get some idea if it is good or junk and worth the effort to be sure your getting what you paid for. In my experience if a manufactor states a certain wave rating, and I test it, it is usually what they say or very far from it. I haven't seen too many 1/10 wave flats test out at 1/8 wave but I have seen a number of 1/10 wave flats test out at 1 to 2 waves !
I also agree that the picture I posted would need to show few fringes to get a better understanding of the exact wave rating. The reason it was photographic the way it was, was because my cheap digital camera has a hard time when there are few fringes. Like I said I have tested a number of "1/10" wave flats with that many fringes showing and even then they were waving around all over the place. So with that many fringes showing and they are straight, when you reduce the number your not going to be looks at a surface that is junk.

- Dave

#15 sixela

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Posted 22 August 2011 - 02:35 AM

Looking at several suppliers for secondaries has me thinking about how good is good enough? One supplier offers a secondary of BK7 Glass, 1/10 Wave, Al & SiO2 Coating, 92% Reflectivity in a size that would work for me for around $60.00. Another supplier offers secondaries with a wave rating ranging from 1/15 to 1/30 wave.


If the first vendor gets its secondaries from GSO and the second from Antares Optics (New York), chances are that the second one is a lot better and you run a (smallish) risk with the GSO that it's actually inducing a lot of astigmatism (i.e. doesn't comply with that stated goal).

If it's a "1/10th wave" ProtoStar vs. an Antares Optics, then it's a different story, but given the price, I think it's very likely the first source is GSO.

#16 Mark Harry

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Posted 22 August 2011 - 06:28 AM

Your observations actually back up what I've seen also- either the errors coincide quite closely, or they're way off!!!
It's always a good idea to be able to verify what one buys, or know someone who can. I think the secondary scene has gotten better, but 10-15 years ago, really bad rejects were more common. Think so?

Regards,
Mark

#17 Mark Harry

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Posted 22 August 2011 - 06:32 AM

Better luck next time, Ken! :lol:

#18 Mr Magoo

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Posted 22 August 2011 - 10:25 PM

People have primary mirrors tested, but I'm guessing hardly anyone has the secondary tested.

#19 dpwoos

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Posted 22 August 2011 - 11:59 PM

I buy mine from Protostar, and am confident that the mirror will be as advertised. It is a real pleasure dealing with Bryan Greer - his stuff is top notch and I don't worry about getting a lemon secondary or anything else. Of course, I am not claiming that Protostar is the only reliable secondary supplier.

#20 davidpitre

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Posted 23 August 2011 - 03:40 PM

Ostahowski has an impeccable reputation for his secondaries as well.

#21 Mark Harry

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Posted 23 August 2011 - 04:03 PM

I've never seen a bad one from these last 2 vendors.
Mark

#22 Ed Jones

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Posted 23 August 2011 - 05:06 PM

Here is a place to have your secondary tested. :lol:

#23 Mark Harry

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Posted 24 August 2011 - 06:20 AM

Nice little online store, Ed! (I'll have to try my hand at one of those "solar projectors" someday.)
M.

#24 Pinbout

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Posted 24 August 2011 - 08:19 AM

M. Harry,

I was playing around with a flat and wanted to try to test my diagonal but I couldn't see anything. Could I partially remove silver on one flat [with Ferric Cloride] to test a silvered flat.

Here I was testing the back of a pcx. just wanted to see if I could see anything. as I applied more pressure I could see wider bands. is that correct way your suppose to vary the number of bands.

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#25 sixela

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Posted 24 August 2011 - 11:28 AM

If you apply pressure, obviously you're measuring the surface after pressing on it, not the original unstrained one. Unless, of course, you press on it to get rid of the air film in between and then release again.

To increase the numbers of band you use the wedge between the two objects (i.e. you raise one end slightly). If the two have no wedge and they're both flat and perfectly parallel, you should have "infinitely" wide bands.

If you have bands and they're both flat, they're not parallel and there's a wedge, but pressure is not what you need to get rid of the wedge. Obviously if there's a speck of dust or dirt between the surfaces you'll get a wedge, and pressing or moving the surfaces with respect to each other is, in that case, Not a Good Idea.



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