What are these streaks across my nebula images? (secondary diffraction spikes?)

[imakebeer]

Hi All,

SW150PDS + Canon 450D + Coma Corrector+ L-Pro

Can anyone help explain what these streaks are across my images? Two processed starless images are shown below, NGC281 Pacman Nebula and IC443 Jellyfish Nebula.

Diffraction spikes are expected of course around bright stars, but this is something else altogether. I note in the image of IC443 that the streak is parallel to the diffraction spike around the very bright star (Propus?), which seems VERY suspicious!

It's a bit of a bummer because while Pacman has some serious issues I'm quite pleased with the Jellyfish for a first stab and minimal processing.

I've done some Googling here and wonder if it might be secondary diffraction spikes cuased (possibly) by out bright star outside the FOV of the camera but within the scope's overall FOV. Could this be it? If so what can I do to prevent or reduce it?

A few things that are really odd:

• It's like I've been at this for years but I'm surprised this issue has only appeared recently.
• These streaks are present in the original light frames but not flats.
• They're too persistent to be satellites or aircraft etc.
• On NGC281 it only appeared after the meridian flip.
• I've imaged NGC281 three times since the start of october and had no issues first time round and was really hoping to combine the data, but I got these streaks on the second and third attempts

TIA for any help and suggestions 🔭👍

NGC281 Pacman Nebula

 

IC443 Jellyfish Nebula

[bosun21]

When you say too persistent to be a satellite what do you mean? Is it on every frame?. Have you checked the frames individually?

[imakebeer]

Yep, it's on every non-cloudy frame and yes I've checked them manually. I could literally see them appearing in the ASIAIR app as each new light frame loaded during the imaging session and they are in the identical position from one frame to the next. and they appear in multiple imaging sessions.

A hundred and million percent, it's not a satellite, aircraft or similar.

[ONIKKINEN]

Looks like rogue diffraction spikes from out of field stars. Can be a pain to prevent, but if you have photoshop you can fight the symptom by lassoing the artifact and doing some content aware fill action which works surprisingly well. Could hide the issue completely if you can isolate just the problem area.

For prevention you need to make sure there is nothing shiny anywhere inside the tube or imaging train. This means flocking the tube interior and covering any shiny screwheads, Flocking the sides of the spider vanes (often have shiny black paint), and possibly the interior of your focuser although your coma corrector probably fills most of the drawtube. These can also happen on the inside of one or more adapter in the imaging train. Huge pain in the backside to be honest... If youre unlucky the problem is in the coma corrector itself and you are out of luck.

As for why it only happens on one side of the meridian is probably just a tube orientation thing. Seen it before, not too surprising.

Edited November 26, 2024 by ONIKKINEN

[imakebeer]

6 minutes ago, ONIKKINEN said:

For prevention you need to make sure there is nothing shiny anywhere inside the tube or imaging train. This means flocking the tube interior and covering any shiby screwheads, Flocking the sides of the spider vanes (often have shiny black paint), and possibly the interior of your focuser although your coma corrector probably fills most of the drawtube. These can also happen on the inside of one or more adapter in the imaging train. Huge pain in the backside to be honest...

Kiitos - Thanks for the info, much appreciated, looks like I was on the right track as far as the cause goes 👍

Issues with the 150PDS and similar scopes are well documented - OK out of the box but lots of bare metal parts that could do with painting, shortening and modifying - a lot of room for improvement if you're so inclined. Looks like I have some jobs ahead of me (either that or get the credit card out!).....

Real bummer, the Jellyfish is looking OK in GIMP with only a modest amount of data (apart from the wildly out of control star which I reckon I can tame a bit!)

Edited November 26, 2024 by imakebeer

[ONIKKINEN]

Some googling suggests that there are plugins for gimp that imitate the content aware fill in Photoshop. You could try that, might be able to salvage the data in the end. The Photoshop version works really well so long as the rogue spike gets deposited in the starless layer. If it goes in the star layer then its more complicated as there are bound to be stars in the path that will get altered with content aware fill.

Edited November 26, 2024 by ONIKKINEN

[alacant]

1 hour ago, imakebeer said:

streaks

Hi

Is this an out of the box PDS? 

[imakebeer]

25 minutes ago, alacant said:

Hi

Is this an out of the box PDS? 

Yes - apart from some tape around the focuser and a woolly hat over the primary mirror end (both to reduce light leaks)

And to be fair you did already tell me a couple of months ago about various recommended mods (here), many of which are well documented.

But hey, look on the bright side - at least my flats are working consistently these days 😊🤣

Edited November 26, 2024 by imakebeer

[alacant]

12 hours ago, imakebeer said:

recommended mods

I remember now. It's the only way we've found of avoiding issues such as this on budget Newtonian Reflectors. Either that or get a TS UNC or ONTC.

Even then, stuff caused by Propus and Alnitak type objects may still need the big hammer treatment.

Cheers and CS

**EDIT. Ah, I see you're using a filter. If it's on the telescope side of the cc, I'd put good money on it being the culprit.

Edited November 27, 2024 by alacant

[imakebeer]

18 hours ago, alacant said:

Either that or get a TS UNC or ONTC

You're gonna have to help me there - TS UNC? ONTC?

18 hours ago, alacant said:

Ah, I see you're using a filter. If it's on the telescope side of the cc, I'd put good money on it being the culprit.

Camera, coma corrector, filter, telescope - why is this an issue? Unfortunately the 2" filter I have will only screw into the coma corrector so right now I have no other option AFAIK 😕

[ONIKKINEN]

I've used a filter like you do now as well as in between the CC and camera and not sure i could tell if there is a difference.

If the filter edge is not properly blackened then that could lead to excessive reflection issues of some kind but this is mostly an issue with some budget filters out there. I would assume the L-pro is competently designed so probably not an issue. My Baader and Antlia filters all work well at the end of the CC at least.

[alacant]

12 minutes ago, imakebeer said:

no other option

Many filters are reflective so no filter is an option. Worth a try? I find a lot of astrophotography is trial and error.

14 minutes ago, imakebeer said:

TS UNC

https://www.teleskop-express.de/de/teleskope-4/newton-teleskope-315/unc-ontc-high-end-newtons-aus-deutschland-312

[imakebeer]

3 hours ago, alacant said:

Many filters are reflective so no filter is an option. Worth a try? I find a lot of astrophotography is trial and error.

https://www.teleskop-express.de/de/teleskope-4/newton-teleskope-315/unc-ontc-high-end-newtons-aus-deutschland-312

I'm sure when you and others were helping me with the Crescent Nebula there was a suggestion that a NB filter was needed - even though my camera is modified maybe it was to focus just on the H-alpha and/or exclude light pollution??? 🤔 

That said, at the moment I've got the L-Pro fitted rather then the L-eNhance so maybe I'll give it a try.

As for those German Newtonians -thanks for the link. My first thought is "Ouch! The price!" But on the other hand it's probably no more than the parts + labour costs to bring a PDS or similar up to a similar standard. 

I had looked just out of curiosity on FLO for a top end Newtonian such as this but found nothing so interesting to know they do exist.

[imakebeer]

Update: 

It's been a long wait to get some clear skies to start imaging again but I think I solved the problem...

I simply rotated the OTA in its rings by roughly 45°.  Link below to my latest images - they still need some work but at least it seems to have resolved the problem with these ugly secondary discretion spikes.

 

 

[michael8554]

"I simply rotated the OTA in its rings by roughly 45°."

That sounds like a one-time fix for that target only.

Michael

[imakebeer]

41 minutes ago, michael8554 said:

"I simply rotated the OTA in its rings by roughly 45°."

That sounds like a one-time fix for that target only.

Michael

Maybe. But I had the problem (so far) on two targets and my simple fix has cured them both. It may reoccur and hence require further work but it's sorted for now at zero cost so I'm happy for the time being.

[900SL]

20 hours ago, imakebeer said:

Maybe. But I had the problem (so far) on two targets and my simple fix has cured them both. It may reoccur and hence require further work but it's sorted for now at zero cost so I'm happy for the time being.

That sounds like something in the light path reflecting in one position. Do you have any exposed screws, etc anywhere in the tube or focuser?

[imakebeer]

2 hours ago, 900SL said:

That sounds like something in the light path reflecting in one position. Do you have any exposed screws, etc anywhere in the tube or focuser?

It's a stock SW150PDS which is well known for having myriad issues, so take your pick! 🤣 (lots of info here, for example)

I probably said this above a few weeks ago (and if not it was discussed in another thread a few months back) but my suspicion is it was a reflection or secondary diffraction spike off the spider vanes caused by a bright star in or just outside the FOV.

One of the many jobs on my list to improve my scope is to get busy with some matt black paint on the spider vanes, back & sides of the secondary mirror and the focuser tube to help alleviate this.

(actually I've just ordered an aperture mask for the primary mirror this morning, and I'm giving serious thought to setting about the focuser tube with a hacksaw)

[Martyn87]

Rather than painting your spider, there are CNC ones available on AliExpress which are much nicer in every way:

I'll drop you a PM with some info.