6 replies to this topic

[Jeroe]

This weekend I had first light of my refurbished Newtonian Telescope. It is 60 years old and was used in observatories as a visual scope for a long time.
I took it home, cleaned everything, Flocked the Tube, made a mirrormask, new Focuser, new Tuberings etc. to make a astrograph out of it.

 

As a first test, I aimed it a several bright targets to see how it performs. I expected nothing but was pleasently suprised to see that it performs pretty good.
Now I realized in my tests that even though it seemed sharp there was still a slight blur to the photos especially the stars.

I have this video as reference on how sharp a newtonian can be : https://www.youtube....h?v=f5bLMDQN2NA

 

I feel like my stars look a bit bloated and not quite round and in Focus even though my 6 spiked defraction spikes indicate that I was on point with my focus.

What could be the cause of this? maybe the backfocus? Coma corrector?  or are the mirrors indeed to old and not quite up to the task?
Or am I just expecting to much out of my system?

 

 

[happylimpet]

Could certainly be the coma corrector. Which one are you using? Some introduce spherical aberation.

 

Could also be just that you imaged on a night of poor seeing.

[TOMDEY]

A telescope should not degrade with time. The resolution depends on three things --- the aperture, the seeing, and the wavefront quality (including proper focus). If your stars are not as condensed as your aperture could deliver --- then one of more of those contributors are deficient. That should be fairly easy to isolate --- and then decide if anything can be done to fix it economically enough to justify the cost.     Tom

[Jeroe]

Could certainly be the coma corrector. Which one are you using? Some introduce spherical aberation.

 

Could also be just that you imaged on a night of poor seeing.

I have this Coma Corrector: https://skywatcher.c...ma-corrector-2/

I thought it was a 8" f5 Scope with 1000mm, it turned out to be a f4.5 scope with 900mm. Could that be the problem? I mean the difference is not huge between f5 and 4.5 right?

[CowTipton]

Seeing (will bloat all stars)

Colllimation (will bloat and make stars non-round)

Tracking (will make stars elongated)

 

etc.

 

I would assume it's one or all of those before thinking it's the mirrors.

 

Shorter subs can help overcome some of that but it'll mean more files to stack for your computer.

[happylimpet]

I have this Coma Corrector: https://skywatcher.c...ma-corrector-2/

I thought it was a 8" f5 Scope with 1000mm, it turned out to be a f4.5 scope with 900mm. Could that be the problem? I mean the difference is not huge between f5 and 4.5 right?

I dont know about that one. I had the 0.9x skywatcher coma corrector which certainly added noticeable spherical aberation, as (as i recall) do some models of the Baader MPCC.

[Dan_I]

I have this Coma Corrector: https://skywatcher.c...ma-corrector-2/

I thought it was a 8" f5 Scope with 1000mm, it turned out to be a f4.5 scope with 900mm. Could that be the problem? I mean the difference is not huge between f5 and 4.5 right?

It could very well be. This is a Ross-type corrector with two lenses that does add spherical aberration as others posters suggested.

 

 

Maybe you could try shooting a test image without the coma corrector. Of course the edges will be horrible, but the center of the image will tell you what your mirror is capable of.

 

There are few coma correctors  that do not add spherical aberration below f/5: the Televue Paracorr and the GPU corrector, also sold by Skywatcher under the name F4 Quattro coma corrector. It seems to be the case also for the Explore scientific HR, but I never tested it.

Edited by Dan_I, 09 October 2024 - 03:43 PM.