66 replies to this topic

[Vinny1980]

Hello everyone,

 

in this period im star testing all my telescopes. In this 3d Id like to submit the output of Winroddier for my Dobson Orion 14xxg, that are in my opinion worth discussing.

Orion claims that the 14xxg is diffraction limited, so I understand it should have a minimum Strehl of 0,8 or lambda/4 P-V. I ran a cycle of 5 tests, in different nights, with different seeing and different stars. In all, I used a ASI174MM with Astronomik green filter.  The best result is Strehl 0,85 and lambda/3 P-V (not considering coma), that becomes 0,81 putting in all the aberrations; the worst, shows 0,71 Strehl and lambda/2.3, that becomes only 0,64 and lambda/1,6 adding coma. The results of the other runs are intermediate between these values, implying that the scope barely meets the "diffraction limited" requirements.

 

Other considerations:

- the scope was always exposed outside for several hours, before testing, to achieve thermal equilibrium;

- astigmatism is very clear also in visual for a trained eye. The axis of the aberration is not parallel to optical axis of the scope, forming with it an angle of at least 30 degrees;

- the main mirror has no pinches which can affect the test;

- i never can achieve a perfect collimation in both sides of focus, i mean if in extrafocal the secondary shadow is centered, it is off center in IF, and vice versa;

- the instrument is obviously suitable for deep sky but in lunary and planetary imaging details become confused, being overcomed in contrast by my C8HD that is much more obstructed;

- I never can eliminate from Roddier output a certain and not negligible amount of coma, despite I ve always tried to be as close as possibile to optical axis.

 

I  attach the 4 Roddier output for your reference (in this order: best without coma - best with; worst without coma - worst with).

 

What do you think about all this? Could the sample be considered of average quality, or I could open a claim to Orion?

Attached Thumbnails

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Edited by Vinny1980, 02 April 2018 - 08:13 AM.

[starman876]

What would it look like if the DPAC results were brought to 3 lines instead of  9?  Would look pretty bad if you ask me.

[Jeff B]

Please search this forum for the collimation issue you seem to have and take care of it first.  Also, the entire system is tested so the secondary mirror may have its own issues.

 

I'm glad you are testing though.  I find it great fun...but be prepared for some data you may not like.

 

Jeff

[Vinny1980]

Dears,

 

thanks for your replies, but please be patient: I need punctual answers. So please stress your replies, and feel free to link any resource that you think can be useful for me.

I dont wanna appear to be lazy, and before opening my topic, I tried to investigate the (wide) matter by myself and I ve some idea. I find this study very funny and instructive, by the way. But in my opinion is not wise wasting weeks upon weeks and many clear nights mounting,dismounting and testing, expecially now that spring has come: so before doing anything, I wanted to hear what you big experts think saving lot of time.

About differences between IF/EF figures, maybe the problem could be not perfect squareness of focuser with respect to secondary, but in pratice dunno how to do. I dont use cheshire nor lasers, I collimate only with stars. Before doing this, a rough alignment is achieved with help of center mark in primary mirror, as indicated in manual.

About astigmatism, I found this very useful page by Mike Lockwood:  http://www.loptics.c.../starshape.html but again: before moving the heavy 14" mirror, I prefer to read your opinion.

I got the scope few months ago and Im under warranty, so if something its wrong with it I can send something back.

 

I contacted Orion technical support several day ago and im waiting for a reply from them.  I confess I feel a little bit worried from what i read here about them, so finger crossed..

Edited by Vinny1980, 02 April 2018 - 02:54 PM.

[Adun]

I dont use cheshire nor lasers, I collimate only with stars

 

What are the steps you do to collimate the secondary with stars?

 

I'm imagining you defocusing a star and trying to center the shadow of the CO (like one does on an SCT) but doesn't sound effective with a newt secondary.

[dgoldb]

First of all, your results are pretty much what gso warrants, so I'm not sure what kind of claim you have against them. .80 strehl is a fine performer - you will almost certainly have other more significant issues in the system. You should really test your secondary, collimation, and thermal control, if you're concerned with image quality.

[Pinbout]

What eyepiece did you shoot thru?

 

how did you measure same distance from focus for both intra/extrafocal images.

 

the test does show a nice kink

 

the stig could be from the 2ndry, you have to rotate the primary and see if the stig rotates with the primary

Edited by Pinbout, 02 April 2018 - 05:38 PM.

[Pinbout]

Get a real $7  ronchiscreen.com 133lpi and do a intra/extrafocal image

 

heres my 12” I refiguring 

 

[starman876]

Get a real $7  ronchiscreen.com 133lpi and do a intra/extrafocal image

 

heres my 12” I refiguring 

 

great job.  Want to refigure a couple more?

[starman876]

Setting up a DPAC rig is not that hard and very rewarding as long as you have decent optics.   If the optics you have are not that good DPAC can be very depressing

[Pinbout]

great job.  Want to refigure a couple more?

Send them over 

[starman876]

Send them over 

you must be retired 

[Bean614]

".....your results are pretty much what gso warrants,"...????

 

  The Orion xx14g Dob has a Synta Mirror, not a GSO.

[Asbytec]

I have to squint to read it, but it looks like you're consistently getting Strehl of 0.86(?). I'd be happy with that for a mass produced mirror. The astigmatism in the first image seems to have disappeared the last, not sure what's going on with that. But, I agree with above comment. Look at collimation first for some improvement. Thermals, etc. 

[dgoldb]

".....your results are pretty much what gso warrants,"...????

 

  The Orion xx14g Dob has a Synta Mirror, not a GSO.

My mistake.  But its irrelevant - IIRC the two share functionally identical specifications.  

[TonyStar]

Assuming it was performed correctly (reported variability 0.85-0.71 is a bit too high), your Roddier test shows astigmatism and spherical aberration. As Danny pointed out, the astigmatism could be caused primarily by the secondary mirror. The simplest way for you to verify this is to rotate the primary mirror and run another test (without varying the camera orientation). Alternatively, you could have the secondary tested by an expert optician. 

I'm afraid Orion may argue that the scope almost meets their standards (Marechal criterion) and not replace it under warranty.

 

If the secondary is the culprit you could replace it with a better one with a minor investment, which should improve performances sensibly.  

 

For what concerns your collimation issues, do not use the secondary shadow as a reference, just collimate on a highly magnified (500X) star image and only defocus slightly.

Edited by TonyStar, 03 April 2018 - 12:03 AM.

[Vinny1980]

Hello guys,

thanks for your replies. Here s my observations:

-yes, i collimate mainly with secondary shadow but also looking at how the rings expands and contract at high magnifications, trying to find a compromise. I ll follow your suggestion and i ll use only the last method.

- i used no EP for testing and collimation, only camera.

- i think i got now what DPAC mean, we call it simply Ronchi test (logically, since Vasco Ronchi was italian!). I got a reticule that i never used, i ll give it a try. I really trust in Winroddier, anyway, and i think/trust results will be consistent.

- dont overlook at the best Roddier results, the other 4 are all under 0,8 strehl and no one meets the Rayleigh criterion of lambda/4 p-v. The average strehl of 5 tests is around 0,7.
Note for roddier users: i used a 100 pixel defocused stars, the optimum being into range 100-150. So I can give a try with 125 and see what happens.

- i ll try to rotate primary, and if astigmatism dont follow him, maybe i ve a no flat secondary. Will let u know.

- Orion still dont reply to my mail.

[Pinbout]

You may have to adj your focuser to get the co to be same-same on defocused stars

 

the kink is the most dominant error I see.

 

but you didn’t say how you measure defocus on both sides so you know the images are same distance from best focus.

Edited by Pinbout, 03 April 2018 - 05:49 AM.

[Vinny1980]

Danny, WR imposes using images with size in pix falling into range of 100-150 ( and its quite exigent about this). Defocusing is calculated by a tool which give you results both in mm and in waves, if i remember well im around 2 mm. We can assume my EF/IF stars have the same distance by focus since the stars have equal dimensions (+/- 1 px). Did someone else test his 14xxg optics ( also 16)? As i know Synta mirrors are quite reliable.

[peleuba]

Please search this forum for the collimation issue you seem to have and take care of it first.  Also, the entire system is tested so the secondary mirror may have its own issues.

 

I'm glad you are testing though.  I find it great fun...but be prepared for some data you may not like.

 

Jeff

 

Collimation is not too bad, and I am not sure it would materially effect Roddier results.  Coma can be removed -  but your point is well taken.  Optical alignment should be undertaken prior to testing.  I co-moderated the old Roddier Yahoo group and, unfortunately, this is about average from what I have seen in the larger imported dobs.

Edited by peleuba, 03 April 2018 - 08:33 AM.

[Pinbout]

 

We can assume my EF/IF stars have the same distance by focus since the stars have equal dimensions (+/- 1 px)

SA makes intra/extrafocal image disc sizes different when they are same distance from best focus. if overcorrected, intra - co shadow is larger but the extrafocal disc is larger, so the difference in sizes of the discs sizes are significant when judging SA 

 

and without that you can't read that much into SA except for disc brightness difference as well as ring brightnesseses.

Edited by Pinbout, 03 April 2018 - 08:34 AM.

[peleuba]

- dont overlook at the best Roddier results, the other 4 are all under 0,8 strehl and no one meets the Rayleigh criterion of lambda/4 p-v. The average strehl of 5 tests is around 0,7.
Note for roddier users: i used a 100 pixel defocused stars, the optimum being into range 100-150. So I can give a try with 125 and see what happens.

 

 

I have found the Roddier test to be very accurate when comparing results to known lenses/mirrors that have been interferometrically tested.

Edited by peleuba, 03 April 2018 - 08:43 AM.

[Vinny1980]

 I co-moderated the old Roddier Yahoo group and, unfortunately, this is about average from what I have seen in the larger imported dobs.

Yeah, this is what i wanted to know (even if really not to hear). If the WR result will be confirmed, being the scope on average, I guess its quite unuseful to claim anything to Orion, so I think I ll substitute the secondary or will refigure primary.

 

SA makes intra/extrafocal image disc sizes different when they are same distance from best focus. if overcorrected, intra - co shadow is larger but the extrafocal disc is larger, so the difference in sizes of the discs sizes are significant when judging SA 

 

and without that you can't read that much into SA except for disc brightness difference as well as ring brightnesseses.

As i know, WinRoddier only requires that the images have same pixel dimensions -  for him, the defocusing distance is unique for a given setup.  I guess they take in account physics of SA since this aberration is also calculated. Attached you can find a screen from WRcalc, that is the tool who helps you in calculations for WR inputs. It shows that defocus distance in my case is 2,77 mm.

 

I have found the Roddier test to be very accurate when comparing results to known lenses/mirrors that have been interferometrically tested.

You can found on internet several comparison between WR and interferometers output, which shows how reliable the software is. Im only a little confused about the variability of my own results.

Attached Thumbnails

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Edited by Vinny1980, 03 April 2018 - 09:42 AM.

[starman876]

Hello guys,

thanks for your replies. Here s my observations:

-yes, i collimate mainly with secondary shadow but also looking at how the rings expands and contract at high magnifications, trying to find a compromise. I ll follow your suggestion and i ll use only the last method.

- i used no EP for testing and collimation, only camera.

- i think i got now what DPAC mean, we call it simply Ronchi test (logically, since Vasco Ronchi was italian!). I got a reticule that i never used, i ll give it a try. I really trust in Winroddier, anyway, and i think/trust results will be consistent.

- dont overlook at the best Roddier results, the other 4 are all under 0,8 strehl and no one meets the Rayleigh criterion of lambda/4 p-v. The average strehl of 5 tests is around 0,7.
Note for roddier users: i used a 100 pixel defocused stars, the optimum being into range 100-150. So I can give a try with 125 and see what happens.

- i ll try to rotate primary, and if astigmatism dont follow him, maybe i ve a no flat secondary. Will let u know.

- Orion still dont reply to my mail.

A proper DPAC requires a flat a bit larger than the diameter of the optics being tested.  It is either the flat or an pan filled with oil.   That will set up the double pass required for the testing of the optics.  A star test with a ronchi screen is half the sensitivity of the DPAC test.  DPAC reveals the truth.  It takes good optics to be 1/4 wave and many will not test that good.

[TonyStar]

Im only a little confused about the variability of my own results.

Seeing might be responsible for the variability. How did you treat the intra and extrafocal images to mitigate seeing effects, did you perform image stacking? If so make sure you apply the same processing to either image.