First Light with 8" RC has ghosts

Hi Ian!

I am responding to this topic, as I am also considering to buy an RC sometime in the future and want to follow. But let me also ask a question please: I see you use a DSLR or mirrorrles camera on your scope. Do you also use a filter there? Could that be an internal reflection between the field corrector and a filter?

One thing I did notice was there were some test shots when I was trying to frame the shot that would change the angle, shape and size of the blue apparition. I should probably post one of those for more clues.

WIth what little I know about the optics, I would assume, that there was an internal reflection between the back end of the flattener and some component in the camera, like wiring around the sensor, or a sensor filter array/protective window. You were shooting a very bright star, which would have producet sufficient light to register a reflection in the image. So, I do not think that the ghosts are caused by bad collimation. But, I am no expert and I could be completely wrong. However, if this assumption is correct, then a light pollution filter will solve the problem quite easily, as it is supposed to have an antireflective coating, to prevent exactly this sort of problems.

What model were you looking at. The place I purchased this from was really pushing the Apertura 6". But I really wanted a little more reach. One thing that I am really impressed with is it was a little breezy when I shot this and the tube and mount combo did a heck of a job compensating, despite my bungling the guide operation.

Now I have two wide angle scopes with 250mm and 530mm focal legnth. Eventually, I want to add one more, with long focal length to work on small planetary nebulas and galaxies. I certainly do not want to use Schmidt Cassegrains. I think these are good for planets, but not so good for DSO. Therefore, I am considering two options: RC or Vixen's modified cassegrain. I think in either case I will go for 8" model, as these appear to have slightly faster focal ratios than 6" and the price premium is not all that big. My mount should be able to support the 8" weight easily, very much like yours. Another consideration is the usability for visual observation, which I still do occasionally for family and friends. Some people say, that RCs have less contrats, due to wider secondary mirror and, therefore, are less suitable for visual astronomy.

It should be stray light, may be a problem with the internal baffle. Look to this issue here:
https://www.youtube.com/watch?v=nHuLrYe-oik

Ian,
Bright stars, both within and just outside, the field are the acid test for any Cassegrain type system (RC, CDK, SCT, Cass, etc).  Imaging a bright star will quickly reveal problems with stray light due to poor optical finish, improper baffling, or vignetting from a bright edge.  Optical alignment generally has no effect on the kind of stray light that you have in your system.

The rectangular stray on the left side of your image looks like it might be coming from the edge of a rectangular window or filter that is near the sensor.  Tell me more about what equipment you are using.  What camera, what filters, and how big are they?

The stray on the right looks like it might be coming from a lens but it could be related to the stray on the left.

This may be a case where it would be valuable to pull the camera, point the scope at a flat panel, and look backwards through the system.  You’ve got something in the field that shouldn’t be there.  You can see what I did in similar circumstances here:  https://www.astrobin.com/xfedon/F/.

John