Narrowband with an achromat (SW StarTravel102)

[alexbb]

I'm back from holiday and my 130P-DS imaging scope is being under surgery for a secondary mirror replacement and cutting a bit the focuser.

Since last night I had a few hours of clear sky, I put the camera on my SkyWatcher StarTravel 102 achromat. 102/500. It has a focuser upgrade and I used the OVL field flattener that I purchased for using it with my small 72ED. I placed the sensor ~55mm away from the flattener's thread and the image seems pretty flat.

I always wondered if this fast achromat could be used for narrowband imaging and I did not find too many examples on this internet. I hope this will help more people trying to find out if it's worthy or not.

I let the image speak for itself. 6x60s with the ASI1600 and an Optolong 7nm Ha filter.

Clear skies!

Alex

[Paul67]

Looks good to my eyes, I would be interested to see this image or similar in a bi-colour.

[alexbb]

Hmmm, to my eyes, there's a lot of under/overcorrection. You can see in the smaller stars a kind of core, but also a halo around it. The bigger ones are just bloated. I don't know all the theory behind, but you can also see some more advanced analysis around this at Tommy's interferometrie blog: http://interferometrie.blogspot.com/2017/06/3-short-achromats-bresser-ar102xs.html

I don't think I will ever attempt to shoot again through this scope except some white light solar work as this scope is the only one I have a solar filter for.

[happy-kat]

Is there a chance it isn't focused?

Using my really CA evident 200mm lens only the bright stars are soft/bloated to various degrees, the other stars are fairly small and tight once processed in a stack, whilst I am not using NB I would think the result might be similar.

[alexbb]

There could be some focus shift after I locked the focuser, but I didn't get it in the trials before the final shots. And I don't believe there could be any significant temperature shift during 10-15 minutes. I used a bahtinov mask for focusing. I can't say that I'm 100% sure, but I really doubt that I missed the focus.

[SteveNickolls]

Thanks for starting this thread, this is something that I may also get to try, perhaps next season if I go the way of getting a Polemaster device and begin the joys of guiding. Do hope the idea works out for you.

Cheers,
Steve

[alexbb]

Hi, Steve!

Maybe it could work with a slower achro, like the one tested by Tommy, the 100/660, but my 102/500 is not working good enough for me. I don't even know if it has an adjustable cell so it could be tuned for a specific wavelength, but I won't bother anyway. I'll just stick with the 130PDS.

[SteveNickolls]

Hi Alex,

Sorry your attempt has put you off. I have some elements for giving this a try-the Startravel 102mm achro, modified Canon 700D with a Astronomik clip in 12nm filter. I am aware I need to be able to get my CG-5 mount to track for long enough (it's doing sterling work with camera and lenses at the moment) and improve the polar alignment. I am under no illusions on what will be required to make the step from 300mm lens to 500mm telescope. However as good nights are few and far between I first want to explore using the camera and lens combination before embarking on another step.

Good luck with your future imaging with the 130PDS.

Best Regards,
Steve

[alexbb]

Hi, Steve!

I'm not even sorry it's not working. I wondered almost since I got the camera how it could work with a nb filter, this scope and a flattener. I did not believe it would work good enough so I didn't buy a flattener for it. However, now I bought a smaller 72ed and the universal OVL flattener (a while ago, a dedicated flattener for the 72 was not available) and now I had the chance to test them together.

The answer was a resounding no so now I have a concern less

I have the feeling you are not guiding now. Does your mount have a guide port? If I were you, that's the route I would go for. Perhaps I would put a guide camera on the ST102 and secure the imaging camera and lens on top of the guiding scope. You should almost certain get enough guiding stars at 500mm, but you can also put a 0.5x reducer in front of the guiding camera. I did this in the past with an ASI120 so I'm sure it works.

[alexbb]

This is a 12x60s stack, shot with the 130PDS. Same, only some DDP applied.

Seeing was poor and guiding went pretty bad. Still, compare the stars...