FRA500 Star Shapes

[PottyMonster]

Hi All, I've not long taken delivery of my new FRA500 (my first refractor) and I love it, however....looking at some of the first frames I've captured over a few nights and I'm noticing something "odd". In the areas where there are really bright stars the halos around them (which seem to be much worse with a Optolong L-eXtreme) have a weird shape to them. If this was my reflector I'd think this was the result of being out of collimation and some interferance with the secondary mirror vanes when looking at out of focus stars. 

I think it's fairly obvious from the pictures attached, but to cliary I'm refering to the split down the middle of the halos. I'm not surprised there are halos around the bright stars but I would have just expected them to be fully circular. I'm seeing this in all single frames (it's not a stacking issue), and all/no filters. Images taken with my 200PDS with the same camera don't have this effect, so I'm pretty sure it's down to the telescope's optics.

I'm not sure if there's a reasonable explanation for this or if there's something wrong?...it doesn't look right to me.

 

[GoldTop57]

Hi I had a similar issue with an Askar 130PHQ but not as extreme. Not sure if you will be able to see it in this attachment but the large star to left of M51 in this image exhibits a similar pattern - note the triangular stars too. I thought it could have been pinched optics? I wasn't happy with the scope and asked Askar for a replacement and they duly obliged; the second scope has been fine (their after sales service was very good). Are you able to return it?  The L-extreme is known for it's halos.

Edited June 15 by GoldTop57

[Stuart1971]

That is a lighthouse beam effect, and is common on the Takahashi FSQ85, it can be caused by a couple of things, on the tak, there is a runner seal O ring around the front lens, and where the two ends meet there is a tiny gap, and this is the effect it causes in the light path, so look for something like that around the lens cell…👍🏻

[Elp]

Sometimes with scopes with greater than two elements you need to allow them to acclimatise properly (this is with any scope) or use dew bands where the glass elements are located for them to come up to temperature (this sometimes works). I've also read with some scopes to check the tightness of the tube rings and if they're tight back them off slightly.

[Stuart1971]

This is the same effect from my Tak, and it’s nothing to do with temp, dew bands or tightness of rings, I spent 18 months trying to figure it out and in the end I found out on a forum that it was common when imaging with these scopes, some people were not bothered, but I did not like at all…

Ignore the bad star shapes this was before I got the flattener, but even after this lighthouse beam effect was still there on all fairly bright stars…

 

[GoldTop57]

Presumably it varies from scope to scope as my replacement 130PHQ scope did not exhibit this effect. Would be interesting to find out if other FRA500 owners have the same issue.

[Elp]

I've read of their latest triplets doing the same.

[inFINNity Deck]

Hi PottyMonster (is that your real name?),

the effect you see could be an uneven spot along the edge of the aperture, although I would expect it to be symmetrical, which it is not in your case. The same for pinched optics, these should be symmetrical as well.

The Tak FSQ85 indeed has these dark spikes, which are always oriented perpendicular to the centre of the image. Most likely cause is an unfortunate lens spacing in combination with the type of coating of the lenses and that they are the result of physical amplification and extinction of the light.

More information on imaging artefacts in a white paper on my web-site: https://www.dehilster.info/astronomy/imaging_artefacts.php

Nicolàs

[PottyMonster]

On 15/06/2024 at 09:57, inFINNity Deck said:

Hi PottyMonster (is that your real name?),

the effect you see could be an uneven spot along the edge of the aperture, although I would expect it to be symmetrical, which it is not in your case. The same for pinched optics, these should be symmetrical as well.

The Tak FSQ85 indeed has these dark spikes, which are always oriented perpendicular to the centre of the image. Most likely cause is an unfortunate lens spacing in combination with the type of coating of the lenses and that they are the result of physical amplification and extinction of the light.

More information on imaging artefacts in a white paper on my web-site: https://www.dehilster.info/astronomy/imaging_artefacts.php

Nicolàs

Thanks for the reply. That's really interesting.

It's odd because it seems to be a consistant artifact across the frame, all of the artifacts "point" in the same direction , but not "point symetrical" as you describle for pinched optics.

https://telescopius.com/pictures/view/190770/deep_sky/north-america-nebula/NGC/7000/diffuse-nebula/north-america-first-askar-d2-filter-test/by-pottymonster

https://telescopius.com/pictures/view/190613/deep_sky/north-america-nebula/NGC/7000/diffuse-nebula/first-light-with-fra500/by-pottymonster

Some links to the full size images.

What I didn't mention in my origional post is that this telescope has had issues with pinched optics in the past but was "repaired", so my original concern was that's what it would be. I'm still not sure!

Dan, aka PottyMonster

[inFINNity Deck]

Hi Dan,

indeed it is quite curious why these artefacts are not point symmetrical. Did you check the front of the lens-cell to see of the are any small uneven spots along its circumference?

Do you also see the artefacts in each and every sub?

Nicolàs

[Xilman]

4 minutes ago, inFINNity Deck said:

indeed it is quite curious why these artefacts are not point symmetrical. Did you check the front of the lens-cell to see of the are any small uneven spots along its circumference?

Do you also see the artefacts in each and every sub?

An objective which is tilted with respect to the optical axis of the telescope will also give this sort of effect.

[inFINNity Deck]

28 minutes ago, Xilman said:

An objective which is tilted with respect to the optical axis of the telescope will also give this sort of effect.

Ok, that is interesting. The tilt can be checked by placing a Cheshire eyepiece in the focuser and brightly illuminate it while the lens-cap is on. If the lens-cell is tilted you will not see concentric reflections of the Cheshire's reflector.

Nicolàs