Roddier test of two scopes: TS (GSO) RC8" and TS 80mm F/6 APO

[vlaiv]

I'm by no means expert in either conducting nor interpreting Roddier test, but I did two of them in order to check optical quality of my imaging scopes.

First is TS (GSO) RC 8". Test was performed last summer, camera used was ZWO ASI 185mc, under real sky. Seeing was ok, but not great. I've heard that quality of test does depend on seeing, but in general obtained results are either that good or better - not worse.

This makes me very happy since Strehl for this scope comes out to be at least 0.94.

Test was conducted as per instruction manual for Win Roddier, and with use of that software (again, as I've said, I'm no expert so I followed the manual to the letter).

Here is result of the test (screenshot):

For the second test, conditions were not that great, a bit worse seeing than in case of the first test, this one was conducted in second half of October, 2016. This time I did deviate a bit from manual. I shot defocused star again with ASI185mc but used R, G and B channels of stacked frames to make three separate tests - one for each channel. OSC camera is probably not best choice for such work - I guess mono cam with filters would be better way to do it (there is some overlap in bayer response curves - over all three channels). Also I suspect that B result is most heavily impacted by not so good seeing, still I'm happy with results as well. Even in conditions of not so good seeing and with OSC camera, I got "diffraction limited" performance in Blue - Strehl 0.8. Green gave 0.94, and the Red was best with 0.98.

My main concern with this scope, given it has traveled quite a bit in the Post - it was sent to the wrong country, and I had to wait 3 weeks for it to return to Germany and then be shipped again to correct destination - was that of possible damage in transport. I was really pleased to see that collimation was spot on, both visually and in tests.

Here are results in Blue, Green and Red (again screenshots):

I actually wanted to do proper reviews for each scope, but unfortunately I did not have good weather / time to do extensive work with them - only couple of recording sessions with each - in less then satisfactory conditions.

From these tests and those few imaging sessions all I can say is that I'm really pleased with quality of the scopes.

All comments are welcome, and I hope someone will find this information useful.

[prusling]

Hi, I saw this old topic referenced in a current thread and wanted to ask how you conduct a Roddier test. I also have a TS 80/480 - of 2011 vintage - and I seem to get a spread of colours within stars - with red more prominent in one segment. I'd like to determine whether it's the scope or another part of the optical train.

[vlaiv]

10 minutes ago, prusling said:

Hi, I saw this old topic referenced in a current thread and wanted to ask how you conduct a Roddier test. I also have a TS 80/480 - of 2011 vintage - and I seem to get a spread of colours within stars - with red more prominent in one segment. I'd like to determine whether it's the scope or another part of the optical train.

Here is a brief overview of how it is done, but there is much nicer explanation in document accompanying the software that is used:

It would be best if you have access to NB filters - that way you can do exact measurement in certain wavelengths. I have OIII, Ha and also Baader solar continuum filter that is around 530nm or so, but have not done measurement with those. You can also do it with regular RGB filters but results will not be as precise - you will get a sort of "integrated" strehl ratio over respective bands. Same happens with OSC cameras.

You will need WinRoddier 3.0 software (or any more recent version, last time I did this, it was with version 3.0). You will also need some sort of application for fast capture - like planetary imaging. Sharpcap will serve this purpose. Last you will need planetary stacking software - AS!3 for example.

Measurement is done by recording defocused star pattern in particular wavelength (or filter). You should select fairly bright star - like mag 2-3, and high up in the sky to minimize seeing effects. Night of good seeing will give you more precision in your results (similar to planetary imaging). If you have OSC - you record all three bands at the same time, then split channels later and examine each R, G and B recordings. You proceed by doing short videos of about 1 to 2 minutes. You need one of in focus pattern and one of out focus pattern. There is method to calculate what diameter of defocused star pattern needs to be in pixels - depending on focal length of scope and size of pixels on camera, it is included as separate small piece of software and it is described in how to document / manual.

When you finish your movies and calibrate them - you stack them in AS!3. It is important that you don't change intensity of image in any way, so avoid auto scaling of intensity and such. Also - no sharpening must be applied to image. Once you have your stacked images prepared you load them in WinRoddier, set wavelength (for NB filters - pick proper wavelength, and for RGB use wavelength in center of the band for each color).

Software does the rest and prepares wavefront images, Zernike polynomials / coefficients and calculates strehl / star profile - look at images that I attached it is screen capture from WinRoddier software.

This is process in the nutshell, for more detailed instructions - read the "manual" (or rather document describing workflow).

All needed files (both program executables and manuals) can be obtained via Roddier yahoo group. You need to join the group (it might need permission from group maintainer/admin if I remember correctly - it took a day for me if I remember correctly).

https://groups.yahoo.com/neo/groups/roddier/info

Once you have joined the group look under Files / WinRoddier Ver. 3.0 (Latest)--Apps and User Manuals folder for pretty much everything you need.

If you want to check out manuals and "how to" before joining the group - it looks like these are available on different urls on the net, so here are some to get you started:

http://www.compubuild.com/astro/download/Roddier_Ver.3.0_Quick-Start_Guide.pdf

http://www.compubuild.com/astro/download/New_WinRoddier_User_Manual.pdf

 

[prusling]

Fantastic info, many thanks. I'll digest it and have a go!

[oymd]

On 28/01/2017 at 14:09, vlaiv said:

I'm by no means expert in either conducting nor interpreting Roddier test, but I did two of them in order to check optical quality of my imaging scopes.

First is TS (GSO) RC 8". Test was performed last summer, camera used was ZWO ASI 185mc, under real sky. Seeing was ok, but not great. I've heard that quality of test does depend on seeing, but in general obtained results are either that good or better - not worse.

This makes me very happy since Strehl for this scope comes out to be at least 0.94.

Test was conducted as per instruction manual for Win Roddier, and with use of that software (again, as I've said, I'm no expert so I followed the manual to the letter).

Here is result of the test (screenshot):

http://serve.trimacka.net/astro/Forum/2016-07-30/post_01/roddier_v1.png

For the second test, conditions were not that great, a bit worse seeing than in case of the first test, this one was conducted in second half of October, 2016. This time I did deviate a bit from manual. I shot defocused star again with ASI185mc but used R, G and B channels of stacked frames to make three separate tests - one for each channel. OSC camera is probably not best choice for such work - I guess mono cam with filters would be better way to do it (there is some overlap in bayer response curves - over all three channels). Also I suspect that B result is most heavily impacted by not so good seeing, still I'm happy with results as well. Even in conditions of not so good seeing and with OSC camera, I got "diffraction limited" performance in Blue - Strehl 0.8. Green gave 0.94, and the Red was best with 0.98.

My main concern with this scope, given it has traveled quite a bit in the Post - it was sent to the wrong country, and I had to wait 3 weeks for it to return to Germany and then be shipped again to correct destination - was that of possible damage in transport. I was really pleased to see that collimation was spot on, both visually and in tests.

Here are results in Blue, Green and Red (again screenshots):

http://serve.trimacka.net/astro/Forum/2016-10-31/post_01/rodder_blue.png

http://serve.trimacka.net/astro/Forum/2016-10-31/post_01/roddier_green.png

http://serve.trimacka.net/astro/Forum/2016-10-31/post_01/roddier_red.png

I actually wanted to do proper reviews for each scope, but unfortunately I did not have good weather / time to do extensive work with them - only couple of recording sessions with each - in less then satisfactory conditions.

From these tests and those few imaging sessions all I can say is that I'm really pleased with quality of the scopes.

All comments are welcome, and I hope someone will find this information useful.

@vlaiv

Hi Vlaiv

I've come across Roddier 3.0 on Cloudy Nights, and after a google search, your post came up!!

Being the genius we know you are, can you please break this test down into simple steps for us mere mortals?

I'm interested in giving it a go?

The camera I have is an OSC, the 294MC Pro. I have a mono 290MM Mini guide cam. Can I use that if Mono is a better option?

Also, I do not have NB filters. I have a set of CHEAP RGB filters. Would they work?

Thanks in advance!!

[vlaiv]

12 hours ago, oymd said:

@vlaiv

Hi Vlaiv

I've come across Roddier 3.0 on Cloudy Nights, and after a google search, your post came up!!

Being the genius we know you are, can you please break this test down into simple steps for us mere mortals?

I'm interested in giving it a go?

The camera I have is an OSC, the 294MC Pro. I have a mono 290MM Mini guide cam. Can I use that if Mono is a better option?

Also, I do not have NB filters. I have a set of CHEAP RGB filters. Would they work?

Thanks in advance!!

First I need to point out that I ran some tests on WinRoddier software with synthetic data and it turned out that test is not reliable (or my test methodology is flawed).

You can read about results of these tests here:

If you still want to try this test - then I can give you a brief overview of that is involved and also point you to some PDFs that contain detailed instructions on how to perform the test.

1. You start by using small utility program that helps you calculate defocused star pattern size. You enter focal length of telescope, pixel size, level of defocus (which should be about 20-30 wave) and it calculates how many pixels your defocused star should have

2. Next step is to find suitable software that will measure size of features on screen - like screen ruler or something like that. You'll need that to get wanted level of defocus

3. Start some capture application like SharpCap and record about 1-2 minute of SER video (similar to planetary imaging) or sequence of fits files. You'll need to make two recordings - one for in focus image and other for out focus image. Defocused star image should be calculated size in pixels (you use ruler app to measure it and adjust defocus).

4. Use planetary stacking software like AutoStakkert to stack those recordings and produce single stack of each - in focus and out focus. In this step it is important that you don't do anything fancy with the data  - don't auto stretch it, don't equalize histogram, don't sharpen - don't do anything except regular average stacking with alignment

5. Start WinRoddier software and import in / out focus images and enter telescope parameters and hit "calculate"

That is what is generally involved - rather simple. There are things that you should be careful about like - choosing the night of relatively good seeing (can be full moon - it won't be a problem), scope needs to be properly cooled down and collimated, star should be placed in center of the FOV - on optical axis, it should also be high in the sky to minimize atmospheric influence and so on ...

If you wish, I can share those PDFs with you - but you can also find them in files section of WinRoddier group. It was previously hosted on Yahoo groups, but now seems to be moved to groups.io or something like that (let me see if I can find that for you).

https://groups.io/g/roddier

(you'll need to apply for access and yes, all material is there in files section - just browse thru 3 pages and find one that has: Roddier_Ver.3.0_Quick-Start_Guide.pdf and New_WinRoddier-3_User_Manual.pdf )

 

[oymd]

@vlaiv

amazing Vlaiv. Thanks a lot for your help!