Want to use achromat for planetary observing but CA is bugging you?

[vlaiv]

There won't be much wisdom here - except the fact that I simplified the process / math for you.

You need to make aperture mask - but what size aperture mask is right for your scope?

Process is rather simple - take focal length, multiply it with number between 5 and 8, where 5 is CA free view, and 8 CA noticeable but not really ruining the view (choose 5 if you are sensitive to CA) and take square root of resulting value.

Have ST120 and want to observe planets and the moon CA free? No problem - we start with 600mm FL and multiply that with 5 to get 3000. Take square root and result is 54.77mm - use 55mm aperture mask for CA free views.

If that is too small for you and you want max aperture - go with number 8, in this case it will be 69.3mm or round that to 70mm.

In the end to reiterate - aperture mask diameter = square root ( focal_length * 5 )

(replace 5 with 8 to get "upper limit" of aperture size mask with some CA).

 

[Rusted]

Just as additional information: The elderly usually experience yellowing of the eye lens.
Which acts as a natural filter for false colour. So they may more easily enjoy the Sidgwick correction factor.
Rather than needing the far more stringent Conrady standards.

Personally, I never found any improvement from stopping down an achromat.
Whereas I always enjoyed the benefits of a Fringe Killer filter with my 6" f/8.
Though not a common No8 yellow filter.

My iStar 180/12/16 R35 achromat does not improve with stopping down nor using the "FK" filter.
This is probably due to its unique colour correction failing to respond to "normal" filtration.

 

[Tiny Clanger]

1 hour ago, vlaiv said:

There won't be much wisdom here - except the fact that I simplified the process / math for you.

You need to make aperture mask - but what size aperture mask is right for your scope?

Process is rather simple - take focal length, multiply it with number between 5 and 8, where 5 is CA free view, and 8 CA noticeable but not really ruining the view (choose 5 if you are sensitive to CA) and take square root of resulting value.

Have ST120 and want to observe planets and the moon CA free? No problem - we start with 600mm FL and multiply that with 5 to get 3000. Take square root and result is 54.77mm - use 55mm aperture mask for CA free views.

If that is too small for you and you want max aperture - go with number 8, in this case it will be 69.3mm or round that to 70mm.

In the end to reiterate - aperture mask diameter = square root ( focal_length * 5 )

(replace 5 with 8 to get "upper limit" of aperture size mask with some CA).

 

 Thanks for that, I tried the maths for the little Orion st80 I bought second hand recently , and got a (rounded to the nearest cm) number of 45cm , which just happens to be the exact diameter of the smaller cap-within-a-cap hole in the lens cover .

Almost as if they knew 

Heather

[vlaiv]

Yep, all the math involved is just above table that @Rusted posted condensed in easy to use formula for calculating aperture mask diameter.

Numbers 5 and 8 come from 25.4 / 5 = ~5 and 25.4 / 3 = ~8, where 25.4 is number of millimeters in an inch and 5 and 3 represent Conrady and Sidgwick standards.

This can also be used for astrophotography purposes, but in that case - one should consider using fringe killer type of filter as well as sensors are more sensitive than our eyes.

Here is example of different aperture masks with and without filter:

Columns are just different levels of stretch so same star image only stretched less or more.

Each pair of rows represents certain aperture mask and same aperture mask + wratten #8 filter. First row is no aperture mask, third row is 80mm aperture mask, 66mm aperture mask and last one is 50mm aperture mask (odd rows are just repeat + filter).

This was done with ST102. Even Conrady standard in this case showed some blue fringing - but this is because of sensor sensitivity - visually you would be hard pressed to see any of it. On the other hand 66mm aperture with filter has almost no CA even photographically.