Comet shaped stars facing center of image

[Josh40996]

Hi

I have been using a skywatcher 114mm reflector for the past 2 nights but I am running into problems with comet shaped stars that seem to all be facing the center of the image, whilst the central stars are (almost) pinpoint. I was just wondering if anyone had any idea whats causing this? I have tried collimating, but that didn't change anything.

I've attached a single 30s frame

Thanks

[orion25]

Don't worry. It's normal, lol. Technically, it's an off-axis aberration called "coma" and it's typical of reflector type scopes because of their optics. The optical path in a telescope is curved (because of the curved mirror and/or lenses used to bring light to a focus), revealing distortion more and more as you move from the center of view. Light is not being transmitted evenly across the focal plane because of this curvature, resulting in comet-shaped images the further you move away from the center "sweet spot". "Fast scopes", say f/5 and below show more of the optical field, so you'll notice coma much more in reflectors. Slower scopes with narrower fields of view like refractors, SCTs and MCTs do not show as much coma because the effect is masked (you're seeing more of the center field of view, not so much the periphery). There are coma correctors that can help alleviate this effect.

Reggie

[alacant]

Hi. Nice idea using the 114 for ap. You'll need a coma corrector. Unless you change your focuser to a 2" model, you'll need to get one made. This place does them to order. Others may know off the shelf models; a quick search didn't reveal any though. HTH

[Stub Mandrel]

I suspect its a double whammy made worse by the 114 not having a parabolic mirror.

FLO sell the 114P for £10 more.

A custom coma corrector might be a bit expensive, but you have lots of options:

Cheapest - crop out the middle of your photos.

Next cheapest - get an x2 barlow, smaller field of view and less light gathering but you can use much more of the image.

Consider swapping to a 130P-DS which also has coma (but less so), but has a 2" drawtube and a number of coma correctors available off the shelf for reasonable prices. You could then sell the 114 to someone only interested in visual.

[carastro]

Can't add to all that good advice, but this is what i would do:

Quote

Consider swapping to a 130P-DS which also has coma (but less so), but has a 2" drawtube and a number of coma correctors available off the shelf for reasonable prices. You could then sell the 114 to someone only interested in visual.

 

[Josh40996]

Thank you for the help, I usually use a 130mm reflector but decided to give this scope a try just because it is much lighter and a bit faster, and it has just been lieing around. Back into storage it goes I guess.

[bobro]

Hi Josh,

Coma correctors for 1.25" eyepieces aren't generally available, but can be put together from parts. See : https://stargazerslounge.com/topic/288608-diy-125-coma-corrector/?tab=comments#comment-3163837

If your 114mm scope is the long focal length type with optical corrector (fl typically 1000mm) then a cc can't be added. If the focal length is 500mm then it may be possible - would need to check.

It may be better to add a cc to your 130mm scope if it is fast (650mm focal length) and coma is a problem. If it's the 900mm fl type then coma should not be a problem.

The DIY cc works quite well. Below is an image of Andromeda taken with my 650/130 reflector using the DIY cc.

HTH