Svbony SV48 500mm f5.5 results

[Mick W]

G'day all. I received my OTA (above) from China this week and managed to get a couple of hours imaging. The scope is supposed to be an achromatic doublet but the results don't bare that out or am I doing something wrong? As you can see from the Pleiades there seems to be massive chromatic aboration around the main stars. In the image of Capella there is a purple halo but that also shows up when I use my reflectors. I just had my Canon 650D moded (IR filter cut) and I'm wandering if that is a factor? If we get a clear night tonight I'm going to test with my Sony A7rii to compare.  I'm a bit of a newbie to AP but a pro photographer of over 25 years and theses images are not giving me the satisfaction I expected. Any pointers/advise would be most welcome.

Equipment used: Svbony SV48. HEQ5 pro. Canon 650D.  OVL Light Pollution Filter 2 Inch

[vlaiv]

I'm somewhat confused with what you've written.

10 minutes ago, Mick W said:

The scope is supposed to be an achromatic doublet but the results don't bare that out or am I doing something wrong? As you can see from the Pleiades there seems to be massive chromatic aboration around the main stars.

If scope is achromatic doublet - then purple halo is very much to be expected. In fact in bottom image, level of purple halo is much less than I would expect from F/5.5 achromatic doublet. You need at least slow well corrected ED doublet to avoid chromatic aberration if not proper APO triplet.

13 minutes ago, Mick W said:

In the image of Capella there is a purple halo but that also shows up when I use my reflectors.

This one is confusing because I suppose first image is of Capella, right? It has diffraction spikes and no chromatic aberration visible at all - that image was in all likelihood taken with reflector and not refractor. Thing that is in image that could be interpreted as some sort of halo around star is just unfocused reflected light. Setup used to make that image has some optical element that is not properly coated and that results in reflection halo (Maybe everything is properly coated - Capella is such a strong star and produces so much light that there is bound to be reflection artifact visible in long exposure).

 

[Mick W]

Thanks Novi. Wow! That is depressing. I've been using 2 Skywatcher reflectors for the past year (130 and 200P) and I thought I'd give a refractor a go for more wide field imaging. I didn't want to break the bank just yet and see if I liked the results then step up to the big league. I thought the specs on the Svbony wouldn't produce the kind of aberration shown here. That's practically unusable and is going to take some serious photoshopping. I think the image of Capella might of been mixed in from a previous shoot and it is with the 220P reflector. I can see a purple circle behind the star that looks like a reflection of the scopes mirror. Is that right and if so how on earth do I get rid of it? I know people keep saying what an expensive and complicated hobby this is but man it's just relentless. Still, it's in the blood now and I'm in for the long haul.

 

[vlaiv]

Unfortunately, good imaging refractors tend to be a bit more expensive so there is no cutting corners there.

However, not all is lost here. Above scope produces less chromatic aberration than one would expect from fast achromatic doublet of that aperture. You can further reduce it by few tricks.

You can try using Wratten #8 filter in front of camera. Not sure if there is clip in version, but you can get 2" version and screw that in your DSLR adapter (if it is 2" one).

Another trick is to use aperture stop. This can be simple cardboard with circular cutout at the center placed over telescope aperture (just make sure it is centered and cut out is clean without rough edges). This will reduce amount of light entering your scope so telescope will be slower - you will need to expose for longer to get nice image (same as stopping down the lens).

Here is comparison of stars in similar 100mm F/5 scope when using first, second and combined approaches:

Columns contain same image of star but stretched to different level (strong, medium and light stretch). Rows contain following: clear aperture, clear aperture + wratten #8, 80mm aperture mask, 80mm+#8, 66mm aperture, 66mm + #8, ....

I think that for that scope I found that 66mm aperture mask and #8 produces image with virtually no chromatic blur / halo.

Following image was taken with such combination:

As you can see there is no purple halo around stars that can be seen (but note spikes / rays around bright stars at the bottom - that is because aperture mask was not cut smoothly).

For reflections - that depends on filters that you use, and sometimes it is unavoidable. If they are caused by filters or similar - you might try to replace used filter to see if that helps. Sometimes only thing that you can do is fix that in post processing.

Here is example of filter reflections in my RC scope. This was due to UHC filter used:

here is doughnut of light around that bright star. I switched to narrowband filters instead of UHC and did not have such issues any more (although NB brought their own issues - such is AP, constant struggle )

 

[Mick W]

Outstanding reply and thank you very much. That has given me a lot to think about and I now see it's not the end of the world. Bravo and love you work Novi. 👍

Edited November 19, 2019 by Mick W

[windjammer]

Hi - it is definitely not the end of the world.  I use a SW Startravel 150mm F5 doublet - with a mono cam and filters: RGB and narrowband.  It works well, just that an L filter is a mush of out-of-focus light.  No reason why you should not use RGB filters with a  colour cam.  Just a bit more work processing !  I found that the focus positions (Bahtinov mask) of R and B light is very close, and G is quite different - which I was not expecting.  B is difficult, it needed and an L3 astronomik filter stacked on the quite flabby astronomik B to get acceptable blue exposures, and even then stars tend to bloat badly when images are stretched - but the photoshop carboni star tools fix this.  On the other hand narrowband Ha, SII, OIII, Hb are all sharp.

Here is an Ha of Flame and Horsehead (bright star by the Flame shows collimation a bit off)

https://www.flickr.com/photos/31131978@N00/49098502228/in/photostream/lightbox/