Hi ! Question for MarkS - Focal Reducer Problem

[hgg]

Hello friends of Astrophotography & Astronomy!

While searching the internet I found another person that had the same problem with me.
Coma when using a focal reducer...

I have the Meade f/6.3 Focal Reducer which I've started using lately for astrophotography.
Recently I took a photo of the Globular Cluster M5 with the following setup :

Celestron C9.25" --> Meade f6.3 --> Baader 20mm SCT Adapter --> SBIG AO-8 unit --> SBIG ST-2000XCM ccd.
The whole setup distance from the reducer to the CCD chip is approximately 77mm

While everything was perfect, guiding @ 24hz, collimation with an artificial star and sharp focus,
star coma is apparent especially in the upper part of the image.  Please have a look :



Even the flat image has an off center vignetting :


I am posting because I've read that the member MarkS had the same problem.
Hello MarkS, did you finally found a solution? 

From my internet research, nobody seems to know exactly the focal distance of the reducers !!!
How strange.  I measured the focal distance of my Meade focal reducer by holding it
near a wall and changing its distance until the image of the room lamp comes to focus on the wall.
I found that the distance is 210mm.

In the post you say that given a 285mm focal length you need an 105mm distance
from FR to CCD.  How do you calculate that distance?

Just by analogy my 210mm focal reducer needs a 77mm spacing from FR to CCD.
If this is correct, then I am using the correct spacing, just by accident!

Why then I have the star coma?  Either the focal reducer spacing is wrong or I might
have a faulty reducer.  I am confused...  

I hope MarkS or any other member can help.
Best Regards,
George.

[MarkS]

George,

That's a very nice image.

Answering your points one by one:
1)  My Celestron C11 coma/astigmatism problem was on a much larger DSLR chip - 24x16mm.  Your chip is 12x9mm so you will have less of a problem.
2)  Assuming the reducer reduces from F10 to F6.3 there is a lens formula that gives a 105mm CCD distance for a 285mm reducer.
3)  So you're correct that 77mm is the right distance for a 210mm reducer.
4)  However check your focal length by using something near inifinity (sun or moon or distant streetlight) rather than a lightbulb.  There is also the problem of where to measure it from - somewhere in the centro of the lens assembly, I guess.  The distance is not actually critical for this reducer on the SCT - if you get it slightly wrong it only affects image size, not the image quality.
5)  Your image has greater coma in the top corners than in the bottom corners - this indicates collimation is probably off in an up/down direction.  If your vignetting is off-center in the same up/down direction then these two facts are probably related (I can't actually see your vignetting image).   I always find that if my vignetting is central then my aberrations are also minimised and are symmetrical about the centre of the CCD.

However, in the end, the Celestron SCTs without the new "Edge" optics will always exibit a mixture of coma and astigmatism and will have a curved image plane.  The Celestron F6.3 reducer (and  I think the Meade also) are not flatteners and even if they were, they would not get rid of all aberrations without being prohibitively expensive.

Mark

[hgg]

Hello Mark!

Thank you for your reply. 
I am setting up the telescope right now for a second try with a greater distance.
Some people suggest the 105mm...  I don't know, I will give it a try.
I will also measure the focal reducer distance as you suggested, with a moon projection.

The center of the flat and the center of the coma, I think are the same.
Here is the flat again : http://www.postimage.org/image.php?v=gxm9TP0

The strange think about the problem being a collimation one is that I collimated the
telescope with an artificial star out of focus and in focus.  When the star was in focus,
I could clearly see the airy disk symmetrical around the center!  The best indicator for
a perfect collimation.

Both Celestron and Meade reducers are advertised as flatteners/correctors as well.
Meade : http://www.shopatron.com/products/productdetail/f/6.3+Focal+Reducer/Field+Flattener/part_number=07545/1323.0.1.1.42062.45518.0.0.0?
Celestron : http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&nav=0&kbarticleid=2243

So they are supposed to make the image even better...

What happened with your problem?  Was it minimized with a change in spacing?

Regards,
George.

p.s. Rick, thank you very much.

[Mike]

Guiding at 24Hz?! How do you managed that?

[hgg]

Hi Mike,

I use the SBIG AO-8 adaptive optics unit and if I find a bright guide star,
that is almost the maximum frequency of corrections per sec.  It can go a little
higher.  I usually guide from 4Hz to 12Hz.  The AO-8 is an amazing tool!

Regards,
George.

[MarkS]

George,

For flatteners on other scopes, the spacing to the CCD is critical and it is published by the manufacturer.  If you deviate from this spacing you get distorted stars.
On the other hand, the spacing for the Meade and Celestron SCT flatteners is not published by the respective manufacturers and in practice it really seems to make no difference to the star shapes - it just magnifies or shrinks the image.  I've done a lot of testing to come to that conclusion.  Hence I don't believe these reducers are field flatteners at all.  Other people have come to the same conclusion.

Like you, I also often had the problem that I could carefully collimate the scope and then still find non-symmetrical star distortions caused by mis-collimation.  I never had any real success with  artifical star collimation - probably because the focuser and hence the mirror is in a completely different place for a nearby artificial star and then you have to move it to focus on the real thing.  I now adopt the policy of collimating the scope immediately before an imaging session using a real star close to my target - this means the scope does not need to change its orientation after collimation.   I use a "concentric rings on a de-focused star" approach.  I also sometimes use CCD Inspector to confirm the accuracy of that collimation.  The defocused star also warns me if there are internal tube currents which mean that the scope still needs to cool down further before imaging begins.

Hope that helps.

Mark

[Carole]

flatteners on other scopes, the spacing to the CCD is critical and it is published by the manufacturer.  If you deviate from this spacing you get distorted stars.

Hmmm, that's interesting information Mark. 

Would you just get distorted stars around the edges or does it affect the ones in the middle as well.  There is a reason for asking.

Carole

[MarkS]

In general, the further from the centre, the more obvoius the distortion.

[hgg]

Mark,

Your collimation approach is correct.  I do almost the same with the artificial star.   I place it
approximately at the elevation that I am imaging.  I use a custom made 50μm fiber optic.
In my case it works perfectly!

I am currently imaging M3 cluster, under a full moon... and I think that I have news...
I am trying the 105mm optimal suggested distance.  When I centered M5 I could not find
the usual guide star that I use.  It was way outside of the field of view which means that
the new setup is greater than f/6.3.    So now I am trying M3 and the first pictures show
minimal coma, if any!  It looks like that this is not an f/6.3 reducer as advertised...

I will upload the results tomorrow.

Thank you!
George.

[Carole]

Hmmmm, Mark, do you remember the strange shaped stars I was getting around the edges at Kelling Heath?  Well they were also around the edge in my successful image of M13 but not in the centre.  

Just before Kelling I had been supplied with an extension ring for my flattener as some-one had suggested the distance wasn't right (on UKAI), and at Kelling I was getting these really strange shaped stars which I've never had before.  

I was wondering if this could be the reason.  

Useful thread thanks George.  Lovely M5 by the way.

Carole

[MarkS]

Just before Kelling I had been supplied with an extension ring for my flattener as some-one had suggested the distance wasn't right (on UKAI), and at Kelling I was getting these really strange shaped stars which I've never had before.  

An extension ring for your flattener?  Which flattener have you got?  As far as I'm aware, the Williams Optics flatteners are designed with DSLRs in mind.  They take a Canon adapter ring and then the DSLR attaches directly to give the precise distance (56mm).  If you add anything else then, yes, you will get odd shaped stars.

Be careful of advice you get on forums!  (Yes I know that comment is self referential!)

Mark

[Mike]

Ahhh... Adaptive Optics. You jammy ****.

[MarkS]

George,

I've found an old thread of mine: http://forum.orpington-astronomy.org.uk/index.php?topic=4243.0

At that time, I measured the focal length of my Celestron reducer to be 230mm and thus needs a spacing of around 85mm to give F6.3
When I used it with a spacing of 105mm I was getting approx F5.2
To be honest though, it didn't make any difference to the star shapes.

You can find some of my C11 images on my website: http://www.markshelley.co.uk/Astronomy/   
Look carefully and you'll find they all have some degree of coma/astigmatism but I am using a DSLR sized chip.
Ignore the 2010 images - they've all been taken with a recently acquired Tak Epsilon 180ED - now that really does have a big flat field (at least it will have when I've ironed out my teething troubles)!

Mark

[hgg]

Hello,

Mark, this is the thread I found while searching the net for the specific problem....  
By the way, nice pictures you have in your site!  The new Takahashi is great!

I measured again the focal length of my reducer with a projected full moon and its 205mm
So I suppose that the distance to the CCD should be 75mm.

I've tried yesterday the 105mm distance on the M5.  I think the coma is slightly better. ??
You cannot judge by looking at the same stars, because the 105mm gave me a wider field
of view.  Is there any simple way to find the actual f from the new data and compare it to
the previous image of the 77mm distance?

I had CCDInspector, but the trial ended.      Can anybody measure the curvature and
aberrations of the old and new image to see which one is better?  That would help.

So, here is the new image of M5 with a 105mm spacing from the FR to CCD.
M5 was 22 degrees away from the full moon...  and the image information was much less
than the previous one.  Dark skies are valuable!  

The image had extreme gradients and some strange black spots.  You can see the spots
if you stretch the midtones.  I've never seen anything like that.  
Anyway here it is:

Celestron C9.25" on the Takahashi EM200 Temma2, Meade f6.3 FR @ 105mm spacing.
SBIG ST-2000XCM with AO-8 guiding @ 26Hz
32 x 120sec, 32 flats w/darks, 8 darks.



Regards,
George,
hgg2010.

[RobertM]

George,

The off centre illumination of your flat (assuming the flat was in fact evenly illuminated) indicates to me that the optical alignment doesn't match the mechanical centre of your image.  If your star looked perfectly collimated then something else might be awry.

If you have a laser collimator and some means of centering then I'd check that the secondary is perfectly collimated (when it is by artificial star) - the beam should bounce back perfectly.  If it doesn't then it could mean that the secondary or corrector aren't centered, I'd doubt this though as the stars look nice and tight.

I'll look at some of the images from my old C9.25 but from memory the illumination was dead centre if the collimation was spot on.

I'll check the image out tonight with CCDI if someone else hasn't already...

Hope that helps
Robert