Purple stars?

[Jjmorris90]

so i managed to take my telescope out tonight as the skies were reasonably clear. I have to say, I'm fairly disappointed. I connected my Canon 550d and snapped a few shots. I am using a heq5 pro mount. most stars were ok, albeit out of focus. I struggled to get focus on the brighter stars as when i turned the focus wheel the stars turned a bright purple? even my shot of Jupiter had a purple glare around it? 

 

it is a bresser ar152L

 

any advice? 

 

thanks

[Jjmorris90]

Also, should i use a barlow lens to capture the planets? here is my attempt at jupiter. you may notice it has a purple hue,..

[vlaiv]

9 hours ago, Jjmorris90 said:

most stars were ok, albeit out of focus. I struggled to get focus on the brighter stars as when i turned the focus wheel the stars turned a bright purple? even my shot of Jupiter had a purple glare around it? 

This is quite normal and to be expected. You are using achromatic refractor telescope. It can only bring to focus two wavelengths of color - others will be out of focus. This usually results in violet "halo" around objects. This is why such scope is not recommended for astrophotography (camera is much more sensitive, and while you can see it visually in high magnification - it usually is not an issue in low to medium powers, but it will always be issue in photos).

[Jjmorris90]

59 minutes ago, vlaiv said:

This is quite normal and to be expected. You are using achromatic refractor telescope. It can only bring to focus two wavelengths of color - others will be out of focus. This usually results in violet "halo" around objects. This is why such scope is not recommended for astrophotography (camera is much more sensitive, and while you can see it visually in high magnification - it usually is not an issue in low to medium powers, but it will always be issue in photos).

Are there any filters that can reduce it?

[vlaiv]

5 hours ago, Jjmorris90 said:

Are there any filters that can reduce it?

Sure, there are several types - but none of them works 100%

Cheapest is Wratten 8# yellow filter.

It gives distinct yellow cast - nothing you can do about it for visual, but you can apply color correction to an image (like white balance thing) to get better image.

There are Baader semi apo, fringe killer and contrast booster.

Each of them reduces violet bloat to some extent - both visually and for imaging.

You can find reviews for each of them online - and even some comparisons - like this one:

Then there is aperture mask. Aperture mask works very well for planetary observation. It works well for DSO and astrophotography - but it cuts too much light, so unless you know what you are doing - stick to planetary with aperture mask use.

Aperture mask is simply a mask with smaller circular aperture in the center. It is restricting light to only central part of lens and blocks outer regions that bend the light the most and cause the most issues with chromatic aberration.

You have 150mm/1200mm FL scope

Here is good guide for chromatic aberration:

You want light green for visual and dark green for astrophotography to have minimal levels of CA.

Your scope has CA index of 1.33 - which is rather poor. If you make aperture mask of 100mm - that will give you 4" F/12 scope (100mm and 1200mm FL so 100/1200 = F/12)

That is already at CA index of 3 - or in the light green. With 80 aperture mask - you'll have ~3" scope and F/15 - which gives you CA index of 5 - that is deep green.

You can even combine things - filter + aperture mask.

I made following image using very fast F/5 102mm achromatic scope (loads of color), wratten #8 filter and aperture mask:

Some stars show very slight sign of violet edge - but overall, not many people would think it was captured with F/5 achromatic scope.

[Zermelo]

Another couple of threads that might interest you:

https://stargazerslounge.com/topic/386412-wratten-filters-for-ca-correction-in-achromats/

https://www.cloudynights.com/topic/518512-best-minus-violet-filter-baader-fringe-killer-or-baader-semi-apo/

[astrolulu]

Hi, you will also receive a residual CA in the case of an APO, and even an SCT telescope, so it is worth learning the techniques limiting this phenomenon also at the stage of processing and post-processing. At the stacking stage, it is worth using the RGB-align option offered, for example, by AutoStakkert! CA remediation tools can also be found in bitmap editors such as Photoshop, which allow you to relatively effectively remove CA residues from a photo. One of the best I know is the optics correction filter in Camera Raw in Photoshop, which allows you to remove artificial tint from aberration without much crippling the image from the color information. The negative effects of its use can be mitigated by combining it with a color noise reduction filter.