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Made a decision.... down the slippery slope

Naemeth

By Naemeth
in Discussions - Eyepieces

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Naemeth

Naemeth

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Coma is a result of the telescope not the EP, and is worse the faster the scope is.

When you get a bigger scope, you will want the wider FOV...double whammy :D

I knew that, but I thought it was worse if you had a really wide FOV and low mag? Or is it connected more to exit pupil than anything else?

I guess what I'm asking is, in an F/5 scope, is the coma worse in a 32mm TV Plössl or a 31mm T5 Nagler?

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Dunkster

Dunkster

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I knew that, but I thought it was worse if you had a really wide FOV and low mag? Or is it connected more to exit pupil than anything else?

I guess what I'm asking is, in an F/5 scope, is the coma worse in a 32mm TV Plössl or a 31mm T5 Nagler?

Coma increases the further you go away from the centre, so (ignoring any differences due to the slight different magnifications) they will be the same out as far as the edge of the plossl, but the Nagler edge goes further out so the stars in this extra space will show more coma still.

This is why a lot of the Dob Mob buy coma correctors to go with their Naglers, etc.

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Naemeth

Naemeth

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Coma increases the further you go away from the centre, so (ignoring any differences due to the slight different magnifications) they will be the same out as far as the edge of the plossl, but the Nagler edge goes further out so the stars in this extra space will show more coma still.

This is why a lot of the Dob Mob buy coma correctors to go with their Naglers, etc.

I think I really need to go to a star party / observatory night (might be able to do this Friday :)) and find someone with the Plössl and Nagler and a Dob to see how bad it is for myself. I can't think the coma in an F/5 Dob would be anywhere near as bad as F/4 though.

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Moonshane

Moonshane

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Shane,

I have managed to cause enough damage to some kit without people helping. I had a good long look at the floor of the observatory yesterday and thought , concrete it will be easier, I know I am going to regret it.

Alan

it's a great solution I think but personally, I'd probably buy a few camping mats to act as cushions just in case.

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John

John

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I think I really need to go to a star party / observatory night (might be able to do this Friday :)) and find someone with the Plössl and Nagler and a Dob to see how bad it is for myself. I can't think the coma in an F/5 Dob would be anywhere near as bad as F/4 though.

The coma visible with my 12" F/5.3 dob is noticably less than it was with my 10" F/4.8. In fact it's only noticeable at all in the outermost part (the last 5%) of the field of view of my ES 20mm 100 degree eyepiece.

It's a pity that Skywatcher don't do F/5+ 10" and 12" newtonians - they are easier to collimate and can have proportionately smaller secondary obstructions too.

Making an accurate F/6 mirror is easier than an F/4.7 as an added incentive.

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Naemeth

Naemeth

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The coma visible with my 12" F/5.3 dob is noticably less than it was with my 10" F/4.8. In fact it's only noticeable at all in the outermost part (the last 5%) of the field of view of my ES 20mm 100 degree eyepiece.

It's a pity that Skywatcher don't do F/5+ 10" and 12" newtonians - they are easier to collimate and can have proportionately smaller secondary obstructions too.

Thanks John, how noticeable is the coma in a 30mm-ish Plössl at F/5.3?

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Stargazer_00

Stargazer_00

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The coma visible with my 12" F/5.3 dob is noticably less than it was with my 10" F/4.8. In fact it's only noticeable at all in the outermost part (the last 5%) of the field of view of my ES 20mm 100 degree eyepiece.

It's a pity that Skywatcher don't do F/5+ 10" and 12" newtonians - they are easier to collimate and can have proportionately smaller secondary obstructions too.

Making an accurate F/6 mirror is easier than an F/4.7 as an added incentive.

it's a pity no one does them except OO :/

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RikM

RikM

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maybe I'm mad but I use the same eyepieces at star parties as I do at home as I don't have any others. OK I don't use my eyepieces at school for the reasons above but I like everyone to experience what I am seeing rather than " Oh just wait a minute while I put a cheapo eyepiece in for you to look through". then again it did irritate me once when someone tested where the eyepiece was by touching the eye lens of my 13mm Ethos. Thankfully it was cold and he had a dry finger tip (or missed).

I use the TV Plossls for public observing events rather than my XW's but at proper star parties I use both. It gives people the chance to compare.

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