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Looking for images with examples of COMA


thehand

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I have been reading the different articles regarding the need for a COMA corrector for fast Newtonians but other than a profile of what the COMA effect looks like I have not found any photos which display the affect in a starfield.  I am debating whether to purchase an F/4.5 or F/6 and at what point I will decide I need the COMA corrector.  According to some of the articles I have read the COMA corrector will increase the focal length of my scope by 15% which will be about F/5.2 for the F/4.5 scope or about half way to the F/6.  Therefore, without a reference of what COMA looks like in an F/4.5 with and without a COMA corrector as well as how an uncorrected F/6 would appear I am unable to determine my best course of action.  Any help would be appreciated.  Thanks.

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Coma in a image looks like tear drops they all point towards the centre of the image from the very edge getting to be less of a problem as you look towards the centre, if your just viewing there's not much to worry about, i no nothing about the CC changing FL........

Found this it has some Coma...

capella.jpg

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Thanks!  I can still see a small amount of distortion around the edges if I zoom in on the photo but it's a big improvement.

Ok, I think that answers my question on that front.  Anyone have an idea about the apparent difference in COMA in an F/4.5 vs. an F/6?  Thanks.

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Coma corrector present some issues that depends from the quality of the accessory.

My CC - GSO cause:

1.  not perfect correction beause it needs a precise distance from sensor. My set is at the limit of this distance.

2. it cause a barlow effect (increase FL) with 14 % ( the MPCC from Baader is better for this issue)

3. reflection of bright stars over the filters. I have more spikes and if the star is very very strong I have a ghost star  near the real star.

And I suppose it cause distortion of field at periphery. I am not sure about.

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no, is the distance that the accessory needs to work. My set has: coma corrector + OAG + filter weel + Canon.

photoset-measure.jpg

The MPCC from Baader has small permition for distance. You must pay attention about this. The good accessory can have bad performance because this.

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if you want know waht is to loos performance because of distance, you can see my post Eta Carina - wide field - mosaic. I haven't the comet appearance, but I have oval stars at periphery. And you can see the reflex: the bright stars have more spikes than four.

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How does Baader's CC compare against Televue's Paracore? Does anyone have experience with both?

The Paracorr increases the focal length slightly while the MPCC doesn't. The MPCC is great for the price but is very sensitive to sensor distancing, what I mean by that is the optical distance from the corrector to the sensor must be spot on...and it will change if you add a filter. It will be more noticeable on a larger sensor. The Paracorr is more forgiving.

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I had issues with my first Baader MPCC III not being machined correctly (it didn't sit orthogonally in the focuser)- so something to watch out for if you get one. Today I also aquired a Skywatcher Aplanatic coma corrector so I will be doing a head-to-head comparison in the near future to see which is best. Getting the coorector to CCD distance correct is essential for the best best results with any kind of field flattener or corrector. So a range of T2 or M48 spacer rings is useful.

The back focus distance can be an issue if you are planning to use both a filter wheel and OAG rig. In this case the Baader RCC Newton Coma Corrector might be the best choice as it has 94mm back focus distance. I will probably end up getting one of these.

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