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DPAC Test - Orion XT8 OTA

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#51 peleuba

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Posted 28 February 2021 - 12:43 PM

Has anyone see a chart that helps one determine what the DPAC test is revealing.  I have seen a few, but they are pretty simple.  

Here is one, I think from the Stellafane website.

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#52 starman876

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Posted 28 February 2021 - 04:30 PM

Here is one, I think from the Stellafane website.

I have not seen those charts before.  Thanks for posting



#53 Mitrovarr

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Posted 01 March 2021 - 10:35 AM

loosing one inch of aperture is not much at all

 

I don't know, I think it's pretty bad! Judging by the price hike between sizes with dobsonians, that's cutting at least $50 and more like $100 off the value of the scope. Also, you're still hauling around an 8" telescope. The scope now significantly underperforms its size and weight.

 

I would definitely send it back if the manufacturer will take it. If they take one lemon back they'll take another, so there's little danger of ending up with something worse (since you could just send that back, too). Plus, I think based on reputation, this is on the very end of the bell curve for GSO, you'd have to be really unlucky not to get an improved scope.


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#54 Jeff B

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Posted 01 March 2021 - 10:53 AM

I don't know, I think it's pretty bad! Judging by the price hike between sizes with dobsonians, that's cutting at least $50 and more like $100 off the value of the scope. Also, you're still hauling around an 8" telescope. The scope now significantly underperforms its size and weight.

 

I would definitely send it back if the manufacturer will take it. If they take one lemon back they'll take another, so there's little danger of ending up with something worse (since you could just send that back, too). Plus, I think based on reputation, this is on the very end of the bell curve for GSO, you'd have to be really unlucky not to get an improved scope.

I appreciate the input!



#55 peleuba

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Posted 01 March 2021 - 11:54 AM

I have not seen those charts before.  Thanks for posting

 

Johann, Very glad to do it.  It really gives you all that you need to know when starting out.  Though, I know its not your first rodeo with Autocollimation.    



#56 peleuba

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Posted 01 March 2021 - 12:16 PM

I would definitely send it back if the manufacturer will take it. If they take one lemon back they'll take another, so there's little danger of ending up with something worse (since you could just send that back, too). Plus, I think based on reputation, this is on the very end of the bell curve for GSO, you'd have to be really unlucky not to get an improved scope.

 

I think its remarkable (and not in a good way) that manufacturers find its cheaper to leave a leave a mirror unfinished and just replace it if it fails to deliver.  And then, here on CN, the company is praised for its "excellent" customer service.  Its not customer service, if the customer has to do the final quality control... 

 

Most of the commercial dobs from the Asian manufacturers, Synta, GSO etc. when critically evaluated have something wrong - anneal, errors of correction, edge issues etc.  It would be interesting to know at what point is the mirror rejected at the factory.


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#57 Jeff B

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Posted 01 March 2021 - 12:18 PM

Johann, Very glad to do it.  It really gives you all that you need to know when starting out.  Though, I know its not your first rodeo with Autocollimation.    

Paul, thanks for posting those charts!  waytogo.gif

 

My only additions would be to include astigmatism, a rotation or clocking of the bands as you sweep from one side of focus to the other, and, specific to refractor objectives, spherocromatism and color focus positions relative to green light....but, of course, I imagine the charts are "mirror-centric" too smile.gif .

 

Jeff


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#58 Jeff B

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Posted 01 March 2021 - 12:19 PM

I think its remarkable (and not in a good way) that manufacturers find its cheaper to leave a leave a mirror unfinished and just replace it if it fails to deliver.  And then, here on CN, the company is praised for its "excellent" customer service.  Its not customer service, if the customer has to do the final quality control...

 

Most of the commercial dobs from the Asian manufacturers, Synta, GSO etc. when critically evaluated have something wrong - anneal, errors of correction, edge issues etc.  It would be interesting to know at what point is the mirror rejected at the factory.

BOOM!



#59 Mitrovarr

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Posted 01 March 2021 - 12:26 PM

I think its remarkable (and not in a good way) that manufacturers find its cheaper to leave a leave a mirror unfinished and just replace it if it fails to deliver. And then, here on CN, the company is praised for its "excellent" customer service. Its not customer service, if the customer has to do the final quality control...

Most of the commercial dobs from the Asian manufacturers, Synta, GSO etc. when critically evaluated have something wrong - anneal, errors of correction, edge issues etc. It would be interesting to know at what point is the mirror rejected at the factory.


I mean, doing a better job would cost more. That's why a Zambuto mirror costs like 4-5x the cost of an entire GSO telescope of the same size.

#60 Jeff B

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Posted 01 March 2021 - 01:16 PM

But Paul still has an interesting point.  I'm curious too.  For example, the turned edge in this mirror covers the outer 10 to 12% of the diameter and is obvious, even in single pass Ronchi, and I suspect when they come off of the polisher, they at least look at them.  Or do they?  Regardless, it was still "Passed".  It, for me anyway, significantly degrades the performance of the scope.  I prompted the owner to let me have a look at it, not because he said he was dissatisfied, but because I was curious.  Only later did the owner express to me that he felt something was not right in the image.  Once in DPAC I immediately found out why.  It took me about 5 minutes to set it up, align it and have a look...and I was being slow.  

 

Well, there's the price point argument.  Ok, maybe, but my little Orion Star Seeker has a better figure than this scope has, in fact better, and is cheaper.  And then there is my Orion 80ED.  It and other samples I've tested , have some of the best optics I've seen, particularly the sample I kept.

 

But all that still leaves me with the question about returning this mirror.  The owner just got word that Orion has now twice refused the request to send out a new mirror, and we would then send back the lesser mirror.  If we send it in, I'm curious to see what we would get back....and when.  But since the owner is donating the scope to our club as a loner and for outreach, it also makes sense to just stop it to 7.25" and be done with it.

 

Still dithering.

 

Jeff


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#61 peleuba

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Posted 01 March 2021 - 01:26 PM

I mean, doing a better job would cost more. That's why a Zambuto mirror costs like 4-5x the cost of an entire GSO telescope of the same size.

 

Understood.  You are are correct, and I don't mean to be argumentative.  My reflectors all have CZ mirrors, so your point is not lost on me.

 

Since this is a double pass test, all errors are look twice as bad under test as they would appear in real life so-to-speak.  Even so, a 1" inch aperture mask seems excessive to me for an 8 inch scope.


Edited by peleuba, 01 March 2021 - 01:27 PM.

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#62 Jeff B

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Posted 01 March 2021 - 02:19 PM

Understood.  You are are correct, and I don't mean to be argumentative.  My reflectors all have CZ mirrors, so your point is not lost on me.

 

Since this is a double pass test, all errors are look twice as bad under test as they would appear in real life so-to-speak.  Even so, a 1" inch aperture mask seems excessive to me for an 8 inch scope.

Me too.


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#63 Pinbout

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Posted 01 March 2021 - 02:35 PM

... Even so, a 1" inch aperture mask seems excessive to me for an 8 inch scope.

but 1" is only .5" on any edge....

 

but in the 7.5" mask the dominating error are the zones.



#64 Jeff B

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Posted 01 March 2021 - 06:45 PM

but 1" is only .5" on any edge....

 

but in the 7.5" mask the dominating error are the zones.

Yeah Danny but that one inch on the diameter is enough to muck up the image, especially with the haze I see over the moon, which I have to assume is due to scatter and/or unfocused light from the bum edge.

 

But, yup, I'm left with the zones, which subjectively on the moon and star test anyway, are not destructive to me, at least to the 200x I was limited to, which around here is a high power.  

 

But I'd like your assessment of it stopped down.  I typically would say that, overall, the system "mimics" overcorrection, maybe about 1/6 to 1/8 wave.  What's your take?

 

Jeff



#65 CHASLX200

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Posted 01 March 2021 - 08:26 PM

I think its remarkable (and not in a good way) that manufacturers find its cheaper to leave a leave a mirror unfinished and just replace it if it fails to deliver.  And then, here on CN, the company is praised for its "excellent" customer service.  Its not customer service, if the customer has to do the final quality control... 

 

Most of the commercial dobs from the Asian manufacturers, Synta, GSO etc. when critically evaluated have something wrong - anneal, errors of correction, edge issues etc.  It would be interesting to know at what point is the mirror rejected at the factory.

These are not Zambuto optics Dorthy as i would say. They just don't make me happy when pushed to high powers.



#66 CHASLX200

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Posted 01 March 2021 - 08:27 PM

I mean, doing a better job would cost more. That's why a Zambuto mirror costs like 4-5x the cost of an entire GSO telescope of the same size.

And well worth it in my steady seeing.



#67 Mitrovarr

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Posted 01 March 2021 - 09:45 PM

And well worth it in my steady seeing.


I think more expensive equipment feels more worthwhile as you get more experienced.

Plus you gotta be able to buy it. If I waited until I could afford Zambuto to start I'd still be waiting.

#68 CHASLX200

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Posted 02 March 2021 - 06:41 AM

I think more expensive equipment feels more worthwhile as you get more experienced.

Plus you gotta be able to buy it. If I waited until I could afford Zambuto to start I'd still be waiting.

Well there are many used Starmasters that come up for sale for good prices with Zambuto optics. If i could find a used Zambuto 18" that had the exact FL as my Galaxy mirror i would jump.  Be nice if it was plug and play without changing focus and or cutting truss poles and having to get longer poles.
 


Edited by CHASLX200, 02 March 2021 - 06:42 AM.


#69 starman876

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Posted 02 March 2021 - 07:49 AM

Johann, Very glad to do it.  It really gives you all that you need to know when starting out.  Though, I know its not your first rodeo with Autocollimation.    

It is not the first,  and I have seen different charts.  I must have overlooked to one you posted.  I really appreciate it when members post information that is really helpful.  Thanks again.



#70 starman876

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Posted 02 March 2021 - 07:51 AM

And well worth it in my steady seeing.

Does not matter what the seeing is one can always benefit from beter opticsgrin.gif



#71 Pinbout

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Posted 02 March 2021 - 08:34 AM

 

post-21633-0-27434700-1612048757_thumb.j

on the far right... the center to whatever radius that is, is under.. from there out to the edge maybe ever so slightly over.... from .7 to edge is the most important part since its more than 1/2 the area contributing to the airy disc.


Edited by Pinbout, 02 March 2021 - 08:34 AM.


#72 Mitrovarr

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Posted 02 March 2021 - 10:34 AM

Well there are many used Starmasters that come up for sale for good prices with Zambuto optics. If i could find a used Zambuto 18" that had the exact FL as my Galaxy mirror i would jump.  Be nice if it was plug and play without changing focus and or cutting truss poles and having to get longer poles.
 

I mean, even at a good price, a used Starmaster is still going to be thousands of dollars. And believe me if they were common in my area, I'd have one by now.



#73 Jeff B

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Posted 02 March 2021 - 11:51 AM

on the far right... the center to whatever radius that is, is under.. from there out to the edge maybe ever so slightly over.... from .7 to edge is the most important part since its more than 1/2 the area contributing to the airy disc.

Yeah, I call it a "compound" figure.  I imagine if you were to try to mathematically describe the surface, sans edge, it would have higher order terms, but I'm just guessing which is why I say "overall mimics".

 

Thanks Danny.



#74 Jeff B

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Posted 02 March 2021 - 12:13 PM

I just dropped a note to Ed Jones inquiring about refiguring the primary:

 

https://opticaleds.c...r-refiguring-2/

 

I included my DPAC images.  Ed is only about 30 minutes away.

 

Jeff



#75 nirvanix

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Posted 02 March 2021 - 06:02 PM

So it's been established as a Synta mirror right? Why are people still talking about GSO mirrors?

 

By the by, I know many forum members that have excellent Synta and GSO mirrors, myself included. I  have a Discovery mirror and I had a Swayze mirror and my GSO is the equal of both of them.

 

I worked in an auto parts factory once and when we made a run of flawed parts the manager gave instructions to spread out those parts to different pallets for shipping. Such is life.


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