m78

[gonzostar]

Hi

Good news bad news. Finally taking longer exposures with PHD2.  

This image is combination of 3 & 4 mins light frames (overall duration 46mins) also darks, flats and bias. Using a borrows 450d canon DSLR using my 8"SCT (6.3focal reducer) on a avx mount

As you can see on my image stars are "eggy" shape is that due to PA? 

Also how can i get rid of those horrible dots using gimp? Are they from guiding? I have never seen these before wnen i did unguided. Or is it i am using a borrowed camera, and using my sock of "darks and flats"

Thanks for looking any feedback appreciated

Dean

 

 

[steppenwolf]

The egg shaped stars look like a guiding issue to me.

Using darks from a different camera will result in all sorts of artefacts and you will be totally wasting your time as they will be meaningless, I'm afraid.

[gonzostar]

Ah i thought i could pull a fast one! being the same camera model as my usual one which is being repaired. 

Is there away of getting rid of those "speckle dots" using gimp?

 

Dean 

[steppenwolf]

The 'Y' shaped dots are indicative of a batch of hot pixels with a repeating pattern showing drift between images so I'd be tempted to take a fresh set of darks while you're asleep tonight then re-stack the images with the new dark frames. If you use SD Mask or Sigma Clip (Kappa Sigma Clip) for you stacking method you should be able to lose these artefacts easily as your image drift has given a quasi-dithered image run so the outlier pixels should be removed from the stack.

Apart from 'cloning', I can't think of any easy way to remove the artefacts using The Gimp.

[gonzostar]

Thanks for your advice make a lot of sense. Will give it ago tomorrow.   

[Carl M]

As Steve says, you can't really use darks from another camera because it's highly unlikely that the hot and cold pixels are in the same place on both cameras (which may well be the cause of those dots you are referring to.)

I would try and stack only the lights you have taken with the borrowed camera using kappa-sigma stacking method in DSS, or if you can, retake the darks and flats - may not be worth doing this until you get your guiding/PA sorted which is most likely the cause of the eggy stars. How are you guiding the SCT?

 

[gonzostar]

I am guiding with the orion autoguider and guide scope with the st-4 cable, And use qhy polemaster for Polar alignment

Still need a bit more practice with phd2. Only got that going a month ago. Need more clear skies!

[ecuador]

Eh, what do you mean "sock of flats"? Even though you can re-use darks with the same camera (not just same model of course), by matching the same temperature, you can certainly not re-use flats in any way. They have to be taken on the same session, with focus and rotation exactly the same as when taking lights.

No point in trying to correct this stack. Re-stack without the wrong darks & flats. If you want darks of course, you can get them on another night (taking care to match the temperature again).

[gonzostar]

yes i am going to redo the darks,bias flats tomorrow and attempt restack. Will let you know results later. Wont be perfect however i am still learning the processes

 

Dean

[Carl M]

I'm using a similar setup, rather with an ST80 guidescope on a C9.25. Was suffering from the same sort of trailing myself, and for me it was either polar alignment being off or from DEC axis being imbalanced.

Not familiar with the polemaster or your PA routine, but I used to do polar alignment first and then put my weights and telescope on the mount..soon found out that after I'd put the weights on, polaris had moved in the polarscope. Presumably from not enough tension from the latitude bolts. This may not apply to you but might be something to consider if you PA the same way I did. 

After sorting PA and the DEC axis balance my PHD2 graph settled down (especially DEC) and was able to do 5 minute exposures with my C9.25 without eggy stars.

[gonzostar]

My set up routine (i am currently updating this)

1) mount out point north and find Polaris with polarscope in mount

2)Add all equipment to be used, autoguider camera, ....

3) balance in Dec and RA. I get confused in DEC if you are aiming for a target south of the zenith, should there be a slight imbalance towards the finderscope end of telescope?? 

4) Initial polar  alignment using hanset, 2  star alignment and 4 calibration stars. Also adjusting the bolts to align the mount following the goto hand handset

5)I then use polemaster to do polar alignment. This i find far easier  and quicker then PHD2 polar alignment

6)Redo calibration stars as bolts may have to be adjusted again. Is it possible to use live view in camera for pointing accuracy?? 

7)Hopefully PA and PHD2 are happy  

If i am missing tricks please let me know

Cheers

Dean

 

 

[Carl M]

Blimey, a lot more long winded process than mine!

As I say, not too familiar with polemaster, surely once you've polar aligned with that you should be done? In step 6 you say you are adjusting the bolts again, are these the latitude/azimuth bolts you used to polar align with polemaster?

I would swap step 5 and 4 around. Polar align first using polemaster and then do your goto star alignment. Once you've aligned with polemaster you shouldn't need to touch polar alignment or the latitude/azimuth bolts again.

[gonzostar]

Cheers for that Did swap a few things around this evening. Polemaster poler alignment then did 2 star calibration with alignment stars. Did seem a little quicker. Especially with the polemaster For a change PHD2 didnt loose my guide star straight away either!  Managed to get new set of darks and bias subs. Then turned my attention to crab nebula. Busy evening

Dean 

[gonzostar]

This time image with the borrowed camera's own darks and bias subs. Hopefully slight improvement. Thanks for all tips

Dean

[Carl M]

You've got rid of the hot pixel trails which is a good, your image has a green cast to it which can be removed with HLVG filter in photoshop.

In fairness, I think you picked a pretty difficult object to image with the SCT. Crab nebula should be a much better target, how were the subs from that, stiill eggy stars?

[ollypenrice]

On 22/01/2017 at 00:48, ecuador said:

Eh, what do you mean "sock of flats"? Even though you can re-use darks with the same camera (not just same model of course), by matching the same temperature, you can certainly not re-use flats in any way. They have to be taken on the same session, with focus and rotation exactly the same as when taking lights.

No point in trying to correct this stack. Re-stack without the wrong darks & flats. If you want darks of course, you can get them on another night (taking care to match the temperature again).

I use my flats for months at a time. I don't take out the camera from the scope, though. I can rotate it because the camera rotator rotates everything which creates imperfections in the light path.

Olly

[gonzostar]

Well i am pleased that i am starting to be able to guide my set up for upto 4 minutes now. Only got the autoguider a month or so ago, and not many opportunities to use. Must say a long long way off some peoples standards though  

Clear skies 

Dean

 

[gonzostar]

52 minutes ago, Carl M said:

You've got rid of the hot pixel trails which is a good, your image has a green cast to it which can be removed with HLVG filter in photoshop.

In fairness, I think you picked a pretty difficult object to image with the SCT. Crab nebula should be a much better target, how were the subs from that, stiill eggy stars?

Thanks Carl for the effort. That is looking a lot better, Do you know if there is an equivalent to HLVG filter in gimp?

Cheers

[Carl M]

I'm not too sure on that one, with a little bit of research I found this guide which supposedly gives a similar result to HVLG but using GIMP's implemented features. Here is the link. It's an automatic translation from French but seems understandable and gives a few different ways of trying it. Might be worth giving it a go, good luck!

[gonzostar]

Cheers for that will try this evening on Crab nebula. Fingers crossed

[ecuador]

16 hours ago, ollypenrice said:

I use my flats for months at a time. I don't take out the camera from the scope, though. I can rotate it because the camera rotator rotates everything which creates imperfections in the light path.

Olly

Right. However, that applies only to very specific situations, for most (modest at least) setups even without removing the camera, just rotating, will affect at least vignetting (e.g. due to imperfect centering). So beginners should not be encouraged to recycle their flats

[gonzostar]

Spoil sport  

[gonzostar]

M1 taken a couple of days ago. 13* 3min light frames. Hopefully the stars are slightly less "eggy" Still not quite central. Working also on pointing accuracy after polar alignment. Can you use live view on back of DSLR to work on poing accuracy?

[Carl M]

By pointing accuracy do you mean for the goto alignment? You can use your camera if you can get the star in the middle of your LCD. I find it's much easier to use a high power eyepiece to center the star then add the camera later. On the final star of the alignment attach the camera and use the star also for focus. Then you've finished the alignment as well as sorting focus for your camera ready for when you want to slew to your target. 

Your stars still do seem to be eggy, how is your guiding graph looking and RMS values? Since SCT has pretty long focal length any guiding or polar alignment glitches will really show up. Try and spend a night trying to get your guiding and polar alignment down to a T, it will really help a lot on future outings.

[gonzostar]

Yes thats what i mean, goto alignment. Do you first PA or calibration stars first? I do a rough PA using the guide scope and tweek with polemaster  I just worry that if after balancing and take the camera off it will disturb the PA?

In term of RMS values what would you say are good values that you would be happy with? Also in terms of ratios between RA and DEC error.

Dean