M64 Black Eye Galaxy

[Carole]

2nd image of the smaller stars near the centre of each image both with DSLR:

the left crop is BEFORE I got a coma corrector
the right crop is WITH a coma corector (Black Eye image).  I have enlarged the bottom star in the box on the left, note the comet type shaping and this is in the middle of the image.

It seems to me I got better results BEFORE the Coma corrector.



Carole

[RobertM]

It does look corrector related.  Are you sure everything is seated correctly ?

[Carole]

I'll take another look at it all Robert, but as I said before everything is right up against the focusser (no space).  Can't fit in a centring adapter as I won't get focus.

Thanks for looking.

Carole

[MarkS]

I've done an analysis using a program that analyses star elongations in nine parts of the image - corners, middle of eges and centre.  It works by taking an average of stars in each part of the image.

http://www.markshelley.co.uk/webdisk/CaroleBlackEyeAnalysis.jpg

There aren't really sufficent stars in the image and a 16 bit raw stack would work much better but the indication is a general left/right star elongation across the whole image (guiding issue maybe) combined with stars in the corners going "diagonal" (almost certainly a coma corrector with the wrong spacing to sensor).

The weird double vertical diffraction spike on that star you pointed out is definitely caused by the same issue that is causing the star elongations.

Have you re-checked what the spacing should be?

Mark

[Carole]

Thanks Mark, I could let you have the complete Tiff files if you like.

I was told by some-one I know with a similar scope that the spacer to coma distance should be 55mm, but that is exactly the distance of sensor to front of T piece of the DSLR, so I have nothing left to play with.  

On looking this up to answer your question I have realised I have made a boo boo, as I forgot to factor in the actual spacing to lens in the CC itself which is a further 17mm, so with the T to M48 adapter it is actually 18mm too large - duh!!!  (Stupid thing is I factored it in for the other cameras, how did I miss it for the DSLR?)

However I have also been googling trying to confirm the CC spacing in case for some reason my friend's kit varies in some way, though his is an F5 Skywatcher Newtonian.  But I have not been successful in finding any info, so have E Mailed Astronomia where I bought the scope, and am currently waiting for a reply.

So if 55mm is the correct spacing, it looks like I won't be able to use a coma corrector with the Newtonian and the DSLR.  

Bummer.

Carole

[Carole]

http://www.markshelley.co.uk/webdisk/CaroleBlackEyeAnalysis.jpg

That's a clever bit of software Mark.  Not that I would be able to interpret it.

Carole

[MarkS]

Carole,

Which CC is it - I'm trying to interpret what you are saying.  55mm is a standard focal spacing for such a corrector and it is measured from the rear flange of the adapter (not the lens glass) to the CCD.  This is perfect for screwing on a Canon T-mount adaptor (10mm thickness) and then the DSLR itself (45mm) - total is 55mm.

Or does the CC have a M48 thread on the back instead of the usual T-thread? 
Like the Skywatcher one:
http://www.firstlightoptics.com/coma-correctors/skywatcher-coma-corrector.html
If so buy one of these:
http://www.firstlightoptics.com/adaptors/skywatcher-dslr-m48-ring-adapter.html

Or is the problem completely different?

Mark

[Carole]

Hi Mark

it is measured from the rear flange of the adapter (not the lens glass) to the CCD

that makes a lot of difference, as I thought it was to the rear of the glass lens.

In that case I should only be 1mm OUT which is what I thought in the first place.

That is indeed the coma corrector that I have Mark and I bought a T ring to M48 adapter to make the CC fit my current T ring but this adds 1mm to the train.  There is possibly another 1mm error because I can't screw the thread all the way and I don't want to force it.

I could get the adapter that you suggest but this will only remove 1mm and I want to make sure it's going to make that much difference before I shell yet more money out. currently I feel there is too much error that 1mm is going to make that much difference.

thanks for your continuing help Mark.

Carole

[MarkS]

So you seem to be saying that the spacing could be up to 2mm too large.  Unfortunately I don't know how critical this distance is.  However I do know that on a refractor the equivalent corrector has very critical spacing. Maybe the instructions give a clue, if there are any, which often there aren't.

I have bought a number of correctors, flatteners etc. over the years and they typically come with no instructions.  The manufacturers' collective attitude seems to be "you've bought this expensive product, now guess how to use it using trial and error and wasted nights of imaging".  It's one of the frustating things about this hobby.

Mark

[Carole]

So you seem to be saying that the spacing could be up to 2mm too large.  

that's correct Mark.  

I do have a set of instructions with the CC but all it does is tell you to use the T ring that you recommended above, then the coma corrector and attach to the DSLR so we have to assume that is the correct spacing. Says nothing about spacing otherwise.  Interestingly it doesn't mention CCD cameras except to say "You don't need to use complicated CCD cameras" (Great!)

So assuming that's all I need according to the instructions, it should be right.  But it wasn't.  
So maybe that 1 - 2mm is critical.  
As you say very frustrating that the info isn't out there.

I'll wait to hear what Astronomiser have to say.

Carole

[RobertM]

Carole I would think 2mm would make it out of spec.  I've seen 55mm +/-0.5 mm mentioned and would think you should aim for that.  As Mark mentioned there is a dedicated SW Canon m48 adaptor for this.

Regrettably, absolutely nothing is as expected in this hobby !

Robert

[mickw]

I believe Ian King sells the thin adapter

[Mike]

Try Bern at Modern Astronomy - he will be cheaper than Ian King.

[Carole]

Yes looks like I will have to get the TRing/M48 that will take 1mm out of the equation, and then see what i can do about getting the coma corrector on a bit further.  Bit of WD40 maybe.

Carole

[mickw]

If the CC is not screwing in fully, it might not be M48.
You should probably check with the suppliers of all the parts to find out if the threads match.

I can't remember what the different threads are (possibly M48 and 2") but they are very similar and seem to fit but don't properly

There is also the possibility that the anodising is slightly blocking the threads - repeated screwing and unscrewing might ease them and they will gradually clear the threads - do not overtighten in case they jam together.