Short tube refractor optical woes

[matt_man21]

Hi folks,

I've had a old Skywatcher 104mm F5 refractor for some time now and it's a reasonable visual scope but since taking the dive in to DSLR astrophotography i have been blighted by two or three nasty little optical aberations... The most degrading to my images is color fringing from a rather 'chromatic' achromat. The others that look like coma and astigmatism i can just about live with...

Center of the FOV

Edge of the FOV

Does anyone out there have any creative ideas as to how i might go about correcting these issues or would the inevitable investment on solving these problems be better spent on a low end Apo? (Remember i'm a broke student surviving on pasta, oxo cubes and beer!)

[ollypenrice]

Yup, that is why fast photographic refractors of comparable aperture cost over £4000.

However, you might find that one of the fringle killing filters would make a big difference. They come in various flavours. There's the Minus Violet, the Fringe Killer and the UV/IR blocker. This latter might not cut off the blue close enough to the visible spectrum for your scope, however. There may be other filters available, too. Check out the Astronomik site and Baader Planetarium.

Quite a good image was posted recently from one of the ST range of scopes so you should certainly get better results than those above.

There's a photoshop bodge for non-round stars, best saved as an action because it only handles one star at a time. Magic wand to select a star. Expand the selection by four. Feather by three. Filter, Blur, Radial Blur (choose spin/best), deselect.

Olly

[Merlin]

I'd recommend BAADER's Semi-Apo Filter. I use it with my CELESTRON 100m/f.5 'scope.

[Ags]

With my ST80 i have tried defocusing slightly - this causes the halo to almost completely dissappear, but of course the stars are slightly less sharp.

Depending on the target you can use a luminance layer composed of some of the wavelengths to suppress the haloes, although I find the blue halo leaves a black scar on the green channel - no idea why.

It if is just one star, why not paint it out manually?

[Gina]

Or if you're a good artist, paint the lot ;)

[Ags]

Yes, sketching fully compensates for CA!

[Moonshane]

does shooting in monochrome remove the CA?

[Ags]

Shooting in green removes the CA, but the blue/red haloes somehow appear as a dark shadow in the green (noise 'snow' seems suppressed where the halo would have been).

[ronin]

There are very few high end triplets at f/5 using exotic glasses that manage to control CA adaquately. Look at the TMB scopes and these are very good but not f/5, they are longer. Why they make an inexpensive achro at f/5 I have no idea since CA will dominate.

Cut out the beer and get an ED if you want to do astrophotography. Even an ED will have some CA unless you get something like an f/7.5 scope. Again at low f numbers it is difficult to control CA.

[Moonshane]

or a small newt and coma corrector.

[matt_man21]

Cheers for taking the time to respond folks!

I've been looking at fringe killers but am a little dubious as to how effective they might be (any links)

I've been shooting through the QHY dichoric RGB filters lately as they seem to mitigate the flood of light pollution from Brighton 4 miles down the road and have noticed that the red and green exposures look sharp and fringe free but the blue exposures are horrible. Any star brighter than mag 8 suffers from the dreaded blue splotch of doom!

@ Moonshane : When shooting in Monochrome the CA is still there, i'm using a DSLR.

As for Using a small newt I only have one motorized mount, the one that comes with the LX-10. Although it can support and guide a small refractor on top, it's at the limits of what the mounts motors can deal with, I can see there being trouble trying to attach even a small newt to it... A reasonable mount is on my upgrade path but it's a long way off for the time being...

[SiriusB]

I no nothing about astro imaging, found this on my internet surfing adventures a while back though, maybe its of some use??

How to Fix Chromatic Aberration in Photoshop

[alacant]

Hi

Was the blue coma (?) ever diagnosed? At the edges of the field in my refractor, the coma extends to the centre of the frame, not the edge which seems to be more usual. I think it's called internal coma. It gets worse the further out from the centre you go. Maybe a field flattener woud get the halos round again? Lots of unanswerd thoughts...

[rl]

You might find that something like a deep sky/ CLS type of filter makes a lot of difference and gets rid of some of your light pollution too. Some of them pass just the teal colours around the H-beta/ oxygen lines and cut out the extreme blue. You want one that passes a big wide band in the green/blue and the red H-alpha line as well. A UHC filter is probably a bit harsh. The cost for a 2" version is £50 - £150...

Your mount would probably handle a 130pds Newtonian which are available for about £100 secondhand. You will probably have to spend another £80-100 on a coma corrector so the total budget soon gets to the point where you could buy a 70mm ED doublet at F/6... but then you'll want the reducer/ field flattener....

If such a thing as cheap good astrophotography exists I've yet to meet it! there is always some extra "essential" piece of glass or metal to buy...

RL

[alacant]

Hi

Thanks, but a newtonian would be even worse. It adds those false spikes to every star in the photo.

I found a fix. The lenses in the focussing tube (Bresser-ES call it 'Petzval')  can be moved to correct the coma (or chromatic?). Moving them toward the main lens. But be careful. Even a few mm and you lose it. Even over a 760mm focus length. Now round (spike free!) stars.

[Bobby1970]

Theres a tool in Lightroom for removing colour fringing. I used this in my pics where CA is quite severe. Along with playing with various levels and saturation settings i find i can achieve a satisfactory result (for myself anyway).

Others have also mentioned the various filters and i myself have used the baader semi apo filter in my ST102 (F5 achromat).

 

 

[Mr Spock]

These short tube achromats are a bit 'colourful'. I've never found filters to be of use.

I got rid of my st80 in favour of an 80 apo - the difference between the two is staggering.

[happy-kat]

Have a look at member space cowboy they have used the ST102 and have grabbed some very pleasing images to look at m31 was one of them and even using a altaz mount.

Edit. Pleasing for the equipment used.

[Mr Fiery Jack]

On 2 February 2016 at 16:19, Mr Spock said:

These short tube achromats are a bit 'colourful'. I've never found filters to be of use.

I got rid of my st80 in favour of an 80 apo - the difference between the two is staggering.

I found returning to an achro, even a really good one like a Vixen after using an ED120 a real shock. I am purely a visual astronomer but I do know achros and AP are never a good combo. Visually I found my old ED100 to be the most perfect optical view I ever had, not a hint of CA to my eyes

 

 

[Moonshane]

I agree about ed scopes. Being used to ca free newts I was never happy with achromats /  refractors until I tried an ed. 

[timwetherell]

Those relatively inexpensive short focus achromats are very cool wide angle visual scopes but not perfect in more demanding roles. I had a 100mm f5 which was reasonable and a 120mm f5 which was unreasonable. I think they vary a lot from one scope to the next too. I'm not an imager but I would imagine that if you used a minus violet filter things would improve dramatically

[Dave Lloyd]

On 03/12/2011 at 11:53, ronin said:

 

Cut out the beer and get an ED if you want to do astrophotography. Even an ED will have some CA unless you get something like an f/7.5 scope. Again at low f numbers it is difficult to control CA.

Cut out the pasta and oxo cubes, keep the beer. Then you can have "beer optics" and everything will look better.

[spaceboy]

I guess you could try a #8 wrattan filter ?? as this would cost only a few pounds.... if you can find one that is. It is suggested as the poor mans fringe killer in visual use but it may tidy up images that you only require a small amount of editing??

[Bobby1970]

On 2 February 2016 at 16:19, Mr Spock said:

I got rid of my st80 in favour of an 80 apo - the difference between the two is staggering.

Unfortunately, so is the price for some people at 3x approximately. ;-)