Feedback Please

[DanLXIX]

This is a 62 second shot of Capella taken tonight. I've really only taken it to test out how well my PA accuracy is coming along and the clouds have rolled in now stopping me from doing more.

All I've done to the pic is de-hazed it and done a little noise reduction in Lightroom. I seem to have tadpole stars, so my question is is this PA errors, or something else? Am I simply pushing the limits of exposure time?

Any thoughts and comments on this that might help me improve things would be most welcome. I'm hoping to get on to more interesting subjects soon.

[vlaiv]

What mount did you use?

If it is EQ5 PRO from signature and you did not guide then it is probably RA periodic error (short period component). It might be coupled with some sort of wind or external shake.

There is small spike on stars. It is however aligned with star elongation. This can be by chance or it can be sign that you have roughness / backlash in one axis, probably RA.

To diagnose further it would be good to check what is direction of RA and DEC in image. Plate solving can help there. If it is in distinct direction of one axis (for example RA) - it is probably related to periodic error / roughness of tracking.

You can try to tune / regrease your mount to help a bit, but guiding is better way of dealing with this.

[DanLXIX]

Thanks for the feedback. Yes it's the EQ5 Pro mount in my signature. I've not looked at plate solving so that's a bit of reading up for me, and I'll explore the Synscan app because that has PEC on it.

[vlaiv]

48 minutes ago, DanLXIX said:

Thanks for the feedback. Yes it's the EQ5 Pro mount in my signature. I've not looked at plate solving so that's a bit or reading up for me, and I'll explore the Synscan app becasue that has PEC on it.

I think that easiest way to plate solve is just to use astrometry.net - upload your image and it will solve it for you.

http://nova.astrometry.net/

[DanLXIX]

32 minutes ago, vlaiv said:

I think that easiest way to plate solve is just to use astrometry.net - upload your image and it will solve it for you.

http://nova.astrometry.net/

Thanks - unfortunately it failed. I looks like it hasn't picked up Capella.

http://nova.astrometry.net/user_images/2555539#original

[vlaiv]

8 minutes ago, DanLXIX said:

Thanks - unfortunately it failed. I looks like it hasn't picked up Capella.

http://nova.astrometry.net/user_images/2555539#original

Sometimes it does that, but there is always "manual" solving

Little tails seem to be aligned with RA axis - which supports my assumption that it is periodic error or roughness in RA axis (maybe backlash). Here is screen shot of the region from Stellarium with Equatorial grid superimposed on it:

and your image with same stars marked out as above:

If you look at Stellarium you will notice that RA axis is pretty much aligned with three stars above Capella. In your image, elongation of stars and streaks pretty much match direction of three stars.

Like you said, first step to do would be to look at PEC feature of Synscan app - that will help somewhat. Next thing to do would be to look at maximum exposure before stars start to trail - this might not happen if you apply PEC, but there is always some residual error after correction that is not harmonic to base worm period that might cause such irregularity and trailing. Depending on exposure length that might be noticeable or not, so experiment with max exposure.

After that, next step would be auto guiding if you are not happy with results of above methods.

[DanLXIX]

Brilliant, thank you. I'm going to have to learn this stuff properly.

[RolandKol]

Hi Dan,

I will not agree with Vlaiv on this one.

Usually he advises me on all kind of stuff as he is much more advanced in all technical and mathematical fields

And I really grateful for it.

but...

I afraid, PEC will not help Much and you will be made to stay below 60s exposure time without Guiding... 
I was in your shoes around 12 months ago, my mount NEQ6 with smaller 130PDS scope on the top... 
I tried to save money doing all kind of tricks to increase exposure time without guiding... PEC was one of them...

And unfortunately, without any proper result... Of course, you may succeed, but... 

problems I faced:

1) Lots of rain in UK... no proper time for experiments;

2) SW mount quality limitations;

3) lack of personal knowledge and experience with Polar Alignment (plus my Polaris was invisible... House in the north...);

4) No knowledge of the software at all...

if you sum up 1)+2)+3)+4)

You will get LOTS of Wasted nights!  which are so rare in our island...

My 2 pence will look like this:

Start guiding... 

And if it's not yet possible:

1)  Balance your rig as perfect as possible (keep in mind, Nikon shifts the scope's gravity center once on,

the best position for the cam is under the scope, at around 6 o'clock if you do not afraid to loose it, or at around 12 o'clock as alternative, you will probably need to turn the scope in the rings);

2) Polar align as Perfect as Possible;

3) Go for higher ISO and stay below 60s.

4) Be prepared to delete around 30-40% of the images due to the star trailing...

Good Luck!  and clear nights for you!

[DanLXIX]

All good points, and you've made me realise I balanced the scope prior to putting the camera on, so that won't have helped. I want to go guided, but I view this as learning the ropes so I want to see what I can do with the kit I've got before jumping ahead into a guided rig.  I'm also not sure if the EQ5 with the 150PDS on might be a little overloaded with a guide scope as well and if so that's an expensive step up to a new mount .

[RolandKol]

10 minutes ago, DanLXIX said:

All good points, and you've made me realise I balanced the scope prior to putting the camera on, so that won't have helped. I want to go guided, but I view this as learning the ropes so I want to see what I can do with the kit I've got before jumping ahead into a guided rig.  I'm also not sure if the EQ5 with the 150PDS on might be a little overloaded with a guide scope as well and if so that's an expensive step up to a new mount .

 this is what actually happened to me also 

keep all cables on, while balancing  they do have effect! plus you will see if they catch any adjustment bolts and etc.
P.S.
Balancing slightly East or West Heavy also may help (a bit...), it will slightly reduce the backslash... But I doubt you will notice much difference, in addition, you will need to change position of weights once passed the meridian.

[RolandKol]

While hunting down my "lunch sandwiches" I been thinking about your "Overloading" issue...
And in my opinion, you should not afraid of it in this particular case.

Your Focal length is not extreme, plus, not sure if you bought one, if not, you can use Skywatcher's Coma Corrector, which is also 0.9 reducer.

If you place  optimally small guiding scope and and cam on top + one more cable, you will probably add only up to 800g.

Even if you will end up a bit overloaded, you will manage to do around 3mins and even longer subs if not windy, plus you will keep around 90% of data...

And later, on a better mount, you will be able to use the same guiding rig anyway...

[RolandKol]

Plus, 

nice examples 

https://www.astrobin.com/141251

https://www.astrobin.com/224877

Two different people: up to 10min exposures with 150PDS on HEQ5, with quite nice round stars 

[DanLXIX]

I believe you are helping me spend money here ?

[RolandKol]

1 minute ago, DanLXIX said:

I believe you are helping me spend money here ?

Nope!
I want to save the most precious thing for you!
Time!

[RolandKol]

And sorry for spaming...
I have suggested Skywatcher's Coma Corrector for you and felt I need to let you know.

Be aware... it may cause some troubles for you, like here:
https://www.astrobin.com/full/36156/0/

Or like in the image below..
If you look at the bottom of the star disk, you will notice it is flat...

This is caused by a Focuser's tube... it protrudes quite deep into the scope and casts shadow on the primary mirror.

I have "Circumcised" mine by 1cm, but as you see... it was not enough for my ASI1600MM camera. 
Your Nikon may have focus in a completely different position and you may avoid this problem, I do not know.

Just wanted to let you know...