Canon 100mm Macro as Astro lens

[koputai]

Firstly I'm pretty new to astro photography, so advice from those
with any experience is great.

Tonight I whacked the 100mm macro on the 7D and put them on
the EQ6Pro. I thought this lens should be a very good astro lens
due to it's reputation as a terrestrial lens.

I was suprised by the result below. This lens is coma city!

Does anyone think there is a reason for this? It's as good a focus
as I could get using Live View zoomed in x10.

The image is a 100% crop from just above centre, but it's the same
all over. 2 mins @ f/4, ISO1600.

Edit - The second image is the same area, same camera and exposure
but using the EF 70-200 f/4L IS set to 100mm. A much better image
than the macro. 2 mins @ f/4 (wide open on this lens), ISO 1600.

Cheers,
Jason.

93337

93341

[luigi]

Very strange:
I took this with a 100mm F2.8 macro last year:
http://www.luisargerich.com/night/h31d3b235#h31d3b235
At F 5.6 the stars were good border to border.

Maybe you have a bad copy
or maybe is not really comma, I've seen that happen when the lens gets fogged due to condensation.

[rcheshire]

I have done tests shots with the same lens and expect the same result as Luigi - now deleted.

[acropolite]

It's worth noting that Louis' shots were most likely taken with a 5DII, not the 7D.

The 7D has a much higher pixel density (the 5DII has the same pixel density as the old 20D at 1.6 crop size) and is going to be far more demanding on lens performance. That said, I've only seen that sort of abberation on my 15mm fisheye and then only at the edges, centre is clean, doesn't look right to me.

[bojan]

It seems Canon has quality problems from time to time. If your lens is new, return it and ask for replacement - it won't perform as terrestrial lens either.
It is also possible the lens was hit and damaged (internal elements moved out of position ? ).
I noticed similar issues on some of my lenses but not to that extent.

[koputai]

Thanks guys.

The lens had an LPS filter on the front, and was fitted with a dew
strap, as was the 70-200mm for the second shot. There was no dew
on the front.

The image looked the same when looking through the view finder, so
I don't think it comes down to pixel size. I reckon I may have a bad
copy of the lens. Strange though, as my terrestrial macro shots look
fine. Unfortunately I bought this lens second hand, which I normally
don't do. That'll learn me!

I might give it another try stopped down to f/5.6, but I'm not holding
much hope.

Cheers,
Jason.

[bojan]

Well, not really... Star test is the most revealing of all tests you can do.. and many lenses (including the most expensive ones) will not pass it as satisfactory for astrophotography.
However, some much cheaper lenses will perform quite well actually... so here you go.
Also, macro lenses are designed for focussing on close objects.. while star fields are far away. Personally I don't have experience with Macro lenses, but I wouldn't be surprised if some designs would not perform well on stars... However I would expect symmetrical coma around star images, so something is definitely not right with your copy.
F/5.6 will certainly help.
And try to get rid of filter.. also try with external diaphragm (in front of the lens.. in general this works with longer lenses only)

[koputai]

Thanks Bojan, I will try f/5.6, and also an external aperture.

What I suspect really points to a misaligned piece of glass is the
fact that over the whole frame, the abberation points in the same
direction. If it just came down to the design, surely the abberation
would be symetrical around the centreline.

Cheers,
Jason.

[bojan]

Have a look at test results on my Canon 100mm FD (converted to fit EOS).
http://www.iceinspace.com.au/forum/showthread.php?t=58629&highlight=canon+100mm
http://www.iceinspace.com.au/forum/showthread.php?t=62377&highlight=canon+100mm

[luigi]

@koputai good point about pixel density but the 7D also crop the borders and my 5dII sees all the lens image, I guess those things are at least neutralized.

I'd try without any filter and no dew to make sure it's the lens and nothing external.

[koputai]

There's hope yet!

I decided to check the dew straps for correct operation. The Firelite 2" I was using on the lenses gave fluctuating current draw when I put the Fluke on it. Resistance check didn't tally with my other straps, and was variable. I pulled the strap apart and found the fitting that crimps on to the resistor film was not fully crimped. There was also just some electrical tape used as insulation, and this was not even between the two contacts, so not only was there intermittance, there was the likelihood of short circuiting as well, as the contacts are a fraction of a mm apart.

So, I crimped the contact and put insulation between the them, and now it's working like a charm.

Unfortunately it looks like this has caused a fault in the Kendrick controller, and even though it is meant to be able to supply 5A per channel, the whole controller will only supply 720mA, which gets shared across whichever channels are active. I can't say I'm overly impressed with the Kendrick quality control.

I'll give the 100mm another try with a fully functional dew strap.

Cheers,
Jason.

[bojan]

Just a thought.. thermal imbalance due to a local heating may cause lens glass distortions..

Give it a try without it.

[koputai]

Worth a try, thanks.

Cheers,
Jason.

[leon]

Looks like the classic Lens slipping out of focus as it is imaging to me, but i may be wrong :shrug:

Leon :thumbsup:

PS: David Malin actually did an image purposely just for the effect, looked pretty cool actually

[hotspur]

I would have thought a 100 mm macro could have been used for astro work,but may be not.Bojon's comment on cheap lens working while other more expensive ones do not work as well,could be well founded-I had a cheap 17-85 lens which I sold when I got much more expensive ones.I often wish I had not have sold that lens as it gave me some good results for astro work

I have enclosed a image with the 17-85 on my old 300D so it is noisy,with some amp glow

with regards to Kendrick-well where to start,I paid an a lot of money for there set up,and as you mention the quality control in there factory has room for improvement,my system failed on every level,lucky I have a friend into electronics he pulled the contoller apart and it was a mess,he cleaned it all and got it working,3 oput of 4 drew straps have gone to the bin,for $500 I'd have expected the very best.But what a waste of money.

I have the EFS 60 mm Canon Macro lens I hope to give this a go soon.The below image was at 70 mm,so hope I could get similar results,Geoff Johnston now makes dew heaters and gets very good straps imported from U.S,far cheaper all up than kendrick,and way better.

[koputai]

I think I've found the problem.

The rear element and it's holder in the back of the lens are wobbly.
I set up an artifical star and shot down the hallway. What was strange
was that the abberation stayed in the vertical plane no matter which
way up I held the camera. This made me think the was glass moving
due to gravity. On inspection yep, the rear element is flopping about.
It doesn't unscrew or screw up though, so I'm not sure what is loose.
I think I'll break out the little screwdrivers and pull it apart.

Thanks for all your ideas guys, much appreciated.

Cheers,
Jason.

[bojan]

Excellent !!
Good thinking mate, that's it. :thumbsup:
Sometimes the last element (in it's cell) is centred by grab-screws.
Maybe retaining ring (inner side of cell) is loose .

[koputai]

Woohoo!! All fixed!!

It was the entire focus assembly that was loose. Once I got the
lens apart I found that as normal there were three screws with
shrouds around that slide in helical tracks, which slides the focus
mechanism back and forth. Well, two of the three were totally out,
so the assembly was hanging on one screw. When I took the circuit
boards etc out which were holding it up, it just about fell apart.

I refitted the two that were out, and tightened the third. This was
all a looooooong way inside the lens!

All back together now, and the artificial star looks good. Also the
autofocus performance is WAY better than it was before!

Thanks for your assistance.

The weather's not looking good for testing it out tonight though......

Cheers,
Jason.

[bojan]

No worries..
You've just made ~$300-400 (minimum cost of repair like that).
It may prove to be a good second hand purchase after all :thumbsup: