Imaging with the 130pds

[Jim Smith]

Off Centre Diffraction Spikes.

Some of the brighter stars seem to have off centre diffraction spikes. Any ideas?

130pds - drawtube shortened - mirror clips removed - ZWO ASI 533 MCP - Baader MPCC

60x30s - gain100 - APP - Affinity Photo - Astroflat Pro - Topaz deNoise AI

[Rallemikken]

Seen this on my own newt sometimes. Comes and goes. Only a minor annoyance. Is the direction consistent? Can it be moonlight or other stray light entering over and into the top of the tube? Maybe a long dew shield? If your scope isn't flocked, I'd start with that.

[Spaced Out]

On 05/04/2022 at 21:27, Bibabutzemann said:

I always thought the focuser is really stable. I would have thought flex in the tube would be a bigger concern then the focuser. 

How did you measure the weight induced tilt?

 

I had streak like stars on one edge of the frame, they moved around when I moved the mount position, it seemed to correspond with gravity. Once I’d worked this out I put a hotech laser in, put my full camera gear in a carrier bag and hung it (very carefully !) off the laser. This tilted the laser dot and as I gently moved the mount around the laser dot moved about. I tightened everything up on the focuser the best I could but couldn’t fix it. Used a lighter DSLR only and there were no issues, just when I used heavy mono gear, so I got the impression 1.3KG is too much for the stock focuser to take.

Fitted a TS V-power focuser instead and problem solved.

[barbulo]

2 hours ago, Jim Smith said:

Off Centre Diffraction Spikes.

Some of the brighter stars seem to have off centre diffraction spikes. Any ideas?

130pds - drawtube shortened - mirror clips removed - ZWO ASI 533 MCP - Baader MPCC

60x30s - gain100 - APP - Affinity Photo - Astroflat Pro - Topaz deNoise AI

Might be wrong but I'd say it is a bit out of focus: the four long diffraction spikes should be thinner.

[alacant]

4 hours ago, Jim Smith said:

drawtube shortened

... and blackened? As too, edge of secondary mirror, spider vanes and nuts which hold the focuser. Matt black.

Do the spikes change size or position? Remove the rubber 'o' rings from the focuser. Mirror sealed to its cell?

Look down the tube for anything shiny. finger prints etc.

But TBH, I wouldn't worry. It's a lovely image.

Cheers and HTH.

Edited April 9, 2022 by alacant

[Ratlet]

2 hours ago, Jim Smith said:

Off Centre Diffraction Spikes.

Some of the brighter stars seem to have off centre diffraction spikes. Any ideas?

130pds - drawtube shortened - mirror clips removed - ZWO ASI 533 MCP - Baader MPCC

60x30s - gain100 - APP - Affinity Photo - Astroflat Pro - Topaz deNoise AI

Incredible picture.

[rotatux]

4 hours ago, Jim Smith said:

Off Centre Diffraction Spikes

I can see double spikes on some, but none clearly offset from centre ; I read this can be caused by twisted spider vanes.

[bottletopburly]

5 hours ago, Jim Smith said:

Off Centre Diffraction Spikes.

Some of the brighter stars seem to have off centre diffraction spikes. Any ideas?

130pds - drawtube shortened - mirror clips removed - ZWO ASI 533 MCP - Baader MPCC

60x30s - gain100 - APP - Affinity Photo - Astroflat Pro - Topaz deNoise AI

Nice image , bit of walking noise which Dithering would sort , 👍

[Jim Smith]

6 hours ago, Rallemikken said:

Seen this on my own newt sometimes. Comes and goes. Only a minor annoyance. Is the direction consistent? Can it be moonlight or other stray light entering over and into the top of the tube? Maybe a long dew shield? If your scope isn't flocked, I'd start with that.

There was no moon and no dew shield. Yes, maybe it's some sort of internal reflection in the tube. I don't have much experience with the 130pds yet so I will see how future sessions go to see if there is a pattern to it.

[Jim Smith]

4 hours ago, barbulo said:

Might be wrong but I'd say it is a bit out of focus: the four long diffraction spikes should be thinner.

I used a Bahtinov mask but focus may have slipped afterwards.

[Jim Smith]

1 hour ago, rotatux said:

I can see double spikes on some, but none clearly offset from centre ; I read this can be caused by twisted spider vanes.

I will check the spider vanes.

[Jim Smith]

1 hour ago, bottletopburly said:

bit of walking noise which Dithering would sort

I have yet to try dithering. I'll put it on the list!

[Bibabutzemann]

Here my attempt on M101.

The stars are bloated. I dont know if this is due to bad seeing.

But i didnt want to apply star shrinking, because it would most likely also shrink small background galaxies. So i decided to just let it be.

 

130 PDS, EOS 1200Da cooled, EQ3 Pro

16hours @ISO1600, Bortle 5

[alacant]

Hi everyone.

Software marathon. Same image. 3 apps.

Can't decide between noise, fat stars or just plain colourless! This is ~3h so there's hope that another 3 may bring something a little less lifeless. I don't think the no-name AliExpress cc helped much.

Thanks for looking.

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[pipnina]

How have you guys flocked your 130-pds? I have the flocking material (and lint roller) recommended from FLO and plan to take the primary mirror cell out this weekend to get good access to the inside of the tube. I am not sure if I should stop with the inside of the tube, or do other bits as well like the inside of the focuser tube (IIRC it's silver?) and the edge of the secondary (not blacked by default and faces the camera sensor...)

I have some bad reflection-like effects in the middle of all my images and have to work DBE in pixinsight HARD to get rid of it.

Doesn't seem to affect subs though, or maybe, at least not much (ABE applied to this single sub below, maybe some effects but maybe it's just ABE not correcting the vignette or background gradient properly?)

I'm using a Baader MPCC coma corrector, ASI EFW, chroma LRGB filters, RisingCam 571 with AR window. I also have a big lens hood over the front of the scope, but even bore I had that there were still issues with this sort of pinching effect in the center of the frame. I take my flats by smoothing out a white cloth over the aperture and kept it taught with a thick elastic band. Then exposed it to the sky opposite where the sun was at half peak (max ADU in my flat subs was 30k'ish out of 65k). (no lens hood at this stage since I found it made the vignetting much stronger and figured it would upset the correction)

Anyone else seen this effect?

[alacant]

10 hours ago, pipnina said:

no lens hood at this stage

Hi

If you image with it in place, then you should take the flat frames with it in place. That will probably solve the issue. Otherwise...

The biggest improvements for us was a black shower cap over the 1º mirror end, uprated springs, matt black secondary spider+focuser barrel and removing the 1º mirror clips. 

[barbulo]

6 minutes ago, alacant said:

If you image with it in place, then you should take the flat frames with it in place.

Interesting. I normally use a home made dew shield, though I remove it before taking the flat frames because the light panel mask has the exact size of the OTA. I thought it wouldn’t affect the light path, but now I’ll try and take the flats with it on (or the lights without the dew shield 😜) to see if it improves their effectiveness.

Thanks @alacant

By the way, I’ve just put the mirror clips away and glued the primary to its mirror cell. Now I have to tackle the mysterious tilt. 

[alacant]

8 minutes ago, barbulo said:

I remove it before taking the flat frames

With our 130s, so do we (it is however NOT an option for many of our other reflectors), but it looks as though the OP may have flat frame issues, so best to go the official line whilst trouble shooting, the clue being that '...made the vignetting much stronger...'. If you have a short dew shield, you're more likely get away with it.

8 minutes ago, barbulo said:

the mysterious tilt

Do you still have the rubber 'O' rings in the focuser?

Saludos

Edited April 13, 2022 by alacant

[barbulo]

12 minutes ago, alacant said:

Do you still have the rubber 'O' rings in the focuser?

The 4 tiny ones in the 4 screws that hold the focuser knobs? Yes I do. All 4. 

Gracias!

Edited April 13, 2022 by barbulo

[alacant]

25 minutes ago, barbulo said:

All 4

There are three other culprits in the push-pull system holding the focuser to the base;)

Suerte

[pipnina]

6 hours ago, alacant said:

Hi

If you image with it in place, then you should take the flat frames with it in place. That will probably solve the issue. Otherwise...

The biggest improvements for us was a black shower cap over the 1º mirror end, uprated springs, matt black secondary spider+focuser barrel and removing the 1º mirror clips. 

Number one mirror end? Is this a mod or something you put in place for flats only?

I suppose removing the primary clips and glueing it could work, what glue should I use?

Is there anything you'd recommend for making the focuser barrel black? I can spray paint it or similar? And the sides of the secondary I was thinking about using a little strip of that stick-on velour I'm going to baffle the inside of the tube with?

[alacant]

22 minutes ago, pipnina said:

Number one mirror

Primary mirror.

22 minutes ago, pipnina said:

glue

Neutral silicone sealant. A generous blob to coincide with the SW cork.

23 minutes ago, pipnina said:

focuser barrel black

Gouache.

HTH

[Bibabutzemann]

16 hours ago, barbulo said:

Interesting. I normally use a home made dew shield, though I remove it before taking the flat frames because the light panel mask has the exact size of the OTA. I thought it wouldn’t affect the light path, but now I’ll try and take the flats with it on (or the lights without the dew shield 😜) to see if it improves their effectiveness.

Thanks @alacant

By the way, I’ve just put the mirror clips away and glued the primary to its mirror cell. Now I have to tackle the mysterious tilt. 

 

You can simply calculate as following:

Inner diameter of the dewshield=outer diameter of scope=160mm. Mirror diameter is 130mm. So the distance between mirror and dew shield is 15mm. This distance determines, how long of a tube+dew shield is allowed (given the sensor size).

The light path in the image is the most outer light that isnt obstructed by the dewshield:

                                                                                   

We have

tan alpha = (15mm/(540mm+x)) , alpha = FOV/2

-> x=15mm/tan(FOV/2) - 540mm

For APS-C Camera the FOV is 2.36° , so the maximum dewshield lenght is x=189mm

Note that its not the total dew shield length, only measured from the end of the tube.

 

So as long as you are under 19cm, you are right to assume that your Dew shield doesnt affect the light path. Doing flats with dew shield can also cause additional problem, when its a flexible dew shield.

 

17 hours ago, alacant said:

Hi

If you image with it in place, then you should take the flat frames with it in place. That will probably solve the issue. Otherwise...

The biggest improvements for us was a black shower cap over the 1º mirror end, uprated springs, matt black secondary spider+focuser barrel and removing the 1º mirror clips. 

 

In which way did those mods improve your images or how did you conclude that it helps?

 

On 13/04/2022 at 02:04, pipnina said:

How have you guys flocked your 130-pds? I have the flocking material (and lint roller) recommended from FLO and plan to take the primary mirror cell out this weekend to get good access to the inside of the tube. I am not sure if I should stop with the inside of the tube, or do other bits as well like the inside of the focuser tube (IIRC it's silver?) and the edge of the secondary (not blacked by default and faces the camera sensor...)

 

Anyone else seen this effect?

If the cloth you are using for flats it too thin, then internal reflections could effect your flats. Daylight sky is already too bright for a normal shirt in my experience. 

Maybe you could give us a unstretched single calibrated frame.  Then its easier to see what is causing your gradients.

I would use a flocked dew shield as first measurement to block out stray light . Its easy, riskfree and protects the 2nd mirror from dew.

Edited April 14, 2022 by Bibabutzemann

[alacant]

5 hours ago, Bibabutzemann said:

how did you conclude that it helps

Stars are without the sw spikes and the telescope holds collimation at all angles, not just the one at which you collimated. Fewer gradients.

Edited April 14, 2022 by alacant

[pipnina]

12 hours ago, Bibabutzemann said:

If the cloth you are using for flats it too thin, then internal reflections could effect your flats. Daylight sky is already too bright for a normal shirt in my experience.

Maybe you could give us a unstretched single calibrated frame.  Then its easier to see what is causing your gradients.

I would use a flocked dew shield as first measurement to block out stray light . Its easy, riskfree and protects the 2nd mirror from dew.

Cloth I'm using is sort of flannel thickness- almost like a thin bath towel but much thicker than a shirt

Here's a calibrated image, although made of only 3 of the 40 odd flats, I had to remake them. Plus a raw flat and a raw light. star_Light_001.fitsFlat_001.fitsstar_Light_001_c.tif

I have the astro essentials flexible dew shield, which seems pretty black on the inside already, so I don't know if I need to flock it or if it's already suitably flocked?