Imaging with the 130pds

[Uranium235]

The double diffraction spikes have been caused by having one of your spider vanes slightly twisted. Flexure of the tube on a 130pds is quite minimal due to its size.

Check your spider vanes carefully, and tweak as required. Its easy to accidentally twist them when collimating but should take no more than a few min to fix.

[Adam J]

23 hours ago, Uranium235 said:

The double diffraction spikes have been caused by having one of your spider vanes slightly twisted. Flexure of the tube on a 130pds is quite minimal due to its size.

Check your spider vanes carefully, and tweak as required. Its easy to accidentally twist them when collimating but should take no more than a few min to fix.

How much twist are we talking about here? I cant see any at all...so unless we are talking fractions is a mil differenct from each end then I am not sure. Its a DSLR so the focuser is almost fully retracted into the tube and is within the light path. Ill pist a picture of the vanes later tonigt.

[Uranium235]

It doesnt take much of a twist to cause it, you have to check very, very carefully. I know this because I had exactly the same issue which was only cleared up by ensuring all four vanes were perfectly straight.

[richyrich_one]

Just a thought, I have seen this once before when my scope started tracking below the obs wall.

My point being that an obstruction of some kind could cause it.

Are you using a dew shield that's restricting the scope at all?

I don't remember the focuser intrusion having this effect but that would be a partial obstruction just the same.

Edited November 8, 2016 by richyrich_one

[sagramore]

Hey all, two quick images from last night - approx. 1 hour on each. They aren't very good really, need more data, but they could be worse!  Quite pleased with how well the M1 image turned out, given the short focal length of the 130-PDS and the fact I'm using a DSLR. I overcooked M45 a bit but it's not enough data really. Also, just purchased Gradient Xterminator, and I cannot believe it took me so long to fork out for it. It's brilliant when you don't have Pixinsight!

Thanks for looking.

 

Messier 45 (Pleiades) - Flickr Link

2016-11-07, near Swindon, England

Gear:
* Skywatcher 130-PDS with 0.9x coma corrector (585 mm, f/4.5)
* Skywatcher NEQ6-Pro Synscan
* Canon EOS 550D (unmodified) and Skywatcher 2" LP filter
* ZWO ASI120-MC guide camera
* Skywatcher Startravel 80 guide scope

Acquisition & Processing:
- AstrophotographyTools (APT) and PHD2 guiding with dithering
- 15 x 300s = total 75 minutes @ ISO 800
- 14 flats, library darks (12-14 C), library bias
- Stacked in DeepSkyStacker and post-processed in Photoshop CC 2015 with Gradient Xterminator

 

Messier 1 (Crab Nebula) - Flickr Link
2016-11-07, near Swindon, England

Gear:
Skywatcher 130-PDS with 0.9x coma corrector (585 mm, f/4.5)
Skywatcher NEQ6-Pro Synscan
Canon EOS 550D (unmodified) and Skywatcher 2" LP filter
ZWO ASI120-MC guide camera
Skywatcher Startravel 80 guide scope

Acquisition & Processing:
- AstrophotographyTools (APT) and PHD2 guiding with dithering
- 11 x 300s = total 55 minutes @ ISO 800
- 14 flats, library darks (12-14 C), library bias
- Stacked in DeepSkyStacker and post-processed in Photoshop CC 2015 with Gradient Xterminator

Edited November 8, 2016 by sagramore

[richyrich_one]

Nice!

As you are dithering you might want to try without darks...some say they can ADD noise.

Oh, and Pixinsight isn't all it's cracked up to be, it's better!

 

[Adam J]

12 minutes ago, richyrich_one said:

Nice!

As you are dithering you might want to try without darks...some say they can ADD noise.

Oh, and Pixinsight isn't all it's cracked up to be, it's better!

 

I would agree with this. I just dither to remove dust and hot pixels from the image and even out the color noise takes about 16 frames to be effective. I do use a chunk of Bias frames normally about 24, but as I cooled my DSLR to -10c in my M45 at 300s bias is actually the bigger noise component not thermal. 

[Adam J]

2 hours ago, Uranium235 said:

It doesnt take much of a twist to cause it, you have to check very, very carefully. I know this because I had exactly the same issue which was only cleared up by ensuring all four vanes were perfectly straight.

Ok here is a good question, assuming it is a twist that I have missed, what is a good way to test for it? I am basically eyeing it up against a steal ruler and cant see anything. Is there a better method?

To help me out, given that it is the vertical spike in the image that is doubled and that the camera was mounted with the long axis of the sensor parallel with the OTA, am I correct in thinking that with the focuser pointing straight down as I look at it will be the horizontal pair of vanes that I need to be looking at? 

Edited November 8, 2016 by Adam J

[sagramore]

32 minutes ago, richyrich_one said:

Nice!

As you are dithering you might want to try without darks...some say they can ADD noise.

Oh, and Pixinsight isn't all it's cracked up to be, it's better!

 

 

14 minutes ago, Adam J said:

I would agree with this. I just dither to remove dust and hot pixels from the image and even out the color noise takes about 16 frames to be effective. I do use a chunk of Bias frames normally about 24, but as I cooled my DSLR to -10c in my M45 at 300s bias is actually the bigger noise component not thermal. 

Thanks for this advice! I dither on literally every frame so I will try a stack without darks and see how it goes..... 

[Susaron]

2 hours ago, Adam J said:

Ok here is a good question, assuming it is a twist that I have missed, what is a good way to test for it? I am basically eyeing it up against a steal ruler and cant see anything. Is there a better method?

To help me out, given that it is the vertical spike in the image that is doubled and that the camera was mounted with the long axis of the sensor parallel with the OTA, am I correct in thinking that with the focuser pointing straight down as I look at it will be the horizontal pair of vanes that I need to be looking at? 

Maybe is not the highest accuracy instrument but I did use a small bubble level with all the imaging train attached to the focuser.

It is curious but if you take a look to Sagramore´s pictures of M45 they have the same effect on the vertical spikes.

[Adam J]

5 hours ago, Uranium235 said:

It doesnt take much of a twist to cause it, you have to check very, very carefully. I know this because I had exactly the same issue which was only cleared up by ensuring all four vanes were perfectly straight.

I have a little play but it looks ok to me here are some pictures for you guys to look over. I added a straight line to show that they are straight. Let me know what you guys think. The thing that confuses me is that the unwanted spikes are much dimmer. The focus tube is in the same position i locked it off in for focus with the M45 picture. It looks to me like it may be clipping the very edge of the optical path.

Am actually looking at it now wondering if the secondary is slightly rotated anti-clockwise....

 

Edited November 8, 2016 by Adam J

[richyrich_one]

Surely those pictures only prove that they aren't bent. They could still be twisted slightly.

 

[Adam J]

38 minutes ago, Susaron said:

Maybe is not the highest accuracy instrument but I did use a small bubble level with all the imaging train attached to the focuser.

It is curious but if you take a look to Sagramore´s pictures of M45 they have the same effect on the vertical spikes.

Yes but not quite as pronounced....i see it on several DLSR pictures in this thread so it makes me suspect that the cause may be the ingress of the focus tube due to the DSLR back focus I cant find an example on a CCD image.

[Adam J]

5 minutes ago, richyrich_one said:

Surely those pictures only prove that they aren't bent. They could still be twisted slightly.

 

Thats why I tried to place the center of the secondary mount in the center of the primary so that in theory the vane would appear to vary in width if twisted. Or at least that was my theory. What i need is a short flat surface to lay on top of the vane so that I can see if it is flat against its entire length.

Edited November 8, 2016 by Adam J

[richyrich_one]

21 minutes ago, Adam J said:

Yes but not quite as pronounced....i see it on several DLSR pictures in this thread so it makes me suspect that the cause may be the ingress of the focus tube due to the DSLR back focus I cant find an example on a CCD image.

Try moving the primary up to eliminate the focus tube ingress and see if that cures it.

[Adam J]

 

6 minutes ago, richyrich_one said:

Try moving the primary up to eliminate the focus tube ingress and see if that cures it.

I am assuming that i do that by winding in the adjustment screws....if i go too far will the primary fall off? I dont want that to happen lol

[happy-kat]

The vane at three oclock is catching the light on a small part of the length nearest the secondary, might that suggest the vane is slightly twisted.

[richyrich_one]

7 minutes ago, Adam J said:

 

I am assuming that i do that by winding in the adjustment screws....if i go too far will the primary fall off? I dont want that to happen lol

 

No you add some spacers like I did here:

 

 

[Adam J]

7 minutes ago, richyrich_one said:

 

No you add some spacers like I did here:

 

 

Mine was almost entirely towards the rear end of the scope on the current screws even without changing the current screws I just moved if forward about 6mm. Might that explain something?

What was it that prompted you to do it?

Edited November 8, 2016 by Adam J

[Adam J]

21 minutes ago, happy-kat said:

The vane at three oclock is catching the light on a small part of the length nearest the secondary, might that suggest the vane is slightly twisted.

There might be something very slight there but to be honest if it is that then its not a twist its a defect / dint in the vane...not sure if skywatcher would do anything about that after 9 months of ownership...

[Alien 13]

To put all this into perspective even a strand of spider silk will give diffraction spikes...

Alan

[Stub Mandrel]

1 hour ago, Alien 13 said:

To put all this into perspective even a strand of spider silk will give diffraction spikes...

Alan

Thinking about it you must be right. Diffraction is caused by edges not thickness. An inch-diameter steel bar across an aperture has the same amount of edge as a spider silk the same length!

[StamosP]

When i took my 130pds to pieces i discovered that the vanes were not screwed properly to the cylindrical central "thing", two of the vanes hadn't their plane parallel to the axis of this "thing" (i cannot describe it better...).

So i unscrewed them and with careful measurements and the help of a digital caliper i managed to align them perfectly.

I cannot tell if this is the case here but i have the impression that the 3 o'clock vane is slightly bent-twisted (as happy-kat mentioned.)

Furthermore i think that the secondary has to be slightly rotated clockwise...

[carastro]

Finally got first light with my SW130PDS

Not the greatest result as I live in the suburbs of London and I had problems with guiding over 500secs.  It wasn't until after I packed up I realised that Dec was not properly balanced, so wasted a lot of time trying to find out why it was trailing and trying out shorter subs.

Since I wanted a result to se how the SW130PDS performed and we get so few clear nights I decided to cheat a little and made up two Ha images one with 15min subs with trailing and 1 with 500sec subs without trailing.  Erased the stars on the 15 minute image using Straton and overlaid the 500sec image over the top to replace the stars.  As a consequence of all the messing around trying to find out why I was trailing and trying different lengths I did not get as much data as I would have liked.  I think I really need a dark location for the Oiii, and Sii. 

Anyway - enough rabbit - taken on 5/11/2016 - yes bonfire night -  I hoped that narrowband would be OK with fireworks.

First light for my SW130PDS, Atik460EX, NEQ6, PZT
Ha 5 x 15mins + 2 x 500secs
Oiii 3 x 150secs binned
Sii 4 x 150secs binned

Looks better if you don't zoom in (ha ha) but just glad to get a first light with this new scope.

Carole 

 

 

Edited November 12, 2016 by carastro

[Susaron]

I´d like to share with you a photo taken of the field around M29 a couple of weeks ago, thanks to the sw Cartes du Ciel, I have added some of objects around M29, even there is a red supergigant ( BD+37 3903, http://simbad.u-strasbg.fr/simbad/sim-id?Ident=BD%2b37 3903 ). 

Best Regards.