About the Spiral Search Direction

[takuya_t]

Hello. I recently captured Jupiter using a focal length of 6000mm. I set the focal length in SharpCap to 6000mm. After performing polar alignment and star alignment, I used GoTo to point at Jupiter. I then tried to use the spiral search to finally bring Jupiter into the field of view. However, even though I kept holding the spiral search button, Jupiter, which should have been very close, never appeared in the field of view.

Finding this strange, I switched the screen to the guide scope and observed the movement of the spiral search in a wide field of view.

I noticed that although the movement started from the center of the image, the image center was not the actual center of the spiral search. The image center (starting point) was at the right edge of the spiral, and the spiral expanded toward the left side of the screen. The right side of the screen was never searched. This means that if the target was even slightly to the right of the field of view, it would never be found.

Do you have any advice regarding this issue? Is there any setting I might have configured incorrectly?

The equipment I am using is as follows:

Mount: Celestron AVX (ASCOM connection)
Primary Telescope: SkyWatcher BKP Mak127　　×3 Barlow , Ext.tube
Guide Scope: SVBONY SV165

Best Regard,

[admin]

Hi,

I must admit that the spiral search function in SharpCap is really quite an old piece of functionality and hasn't had the improvements or updates that it maybe deserves...

It tends to give a more even spiral when used at higher speeds rather than lower ones, but that may make the spiral too spread out to catch all the nearby sky. The problems really arise from how the mount responds to movement commands of the RA/Dec axes - does the mount add the movement rate to the tracking rate when moving or pause tracking while moving in RA? Does the same sort of pause happen when moving in Dec? How long does it take to resume tracking after the movement has been stopped, etc? If you have a larger movement rate the effect of these unknowns is reduced (at least compared to the total movement of the mount).

I think I will put the spiral search on the list of things to try to improve for the future, as you are quite right that it is not always useful in the current form.

cheers,

Robin

[SteveInNZ]

Well, while you have the notebook out ....

Would it be possible to specify the coarseness of the spiral ?
If you're setup for planetary imaging and the search moves a degree for each spiral, it's unlikely to find the target.
Maybe use the focal length set in the plate solve section if it doesn't already.

Also, some means of back-tracking along the spiral. If you've been through several rotations and you see Saturn whisk past, it can be quite hard to get back to it.

Steve.

[takuya_t]

Hello Robin,

Thank you very much for your prompt response. I also greatly appreciate that you are considering this issue for future improvements.
I have an additional question and one more,would like to hear your opinions.

First, my question: I assume that the mount’s step size during the spiral search is calculated based on the focal length that is manually entered in the plate-solving settings (in my case, 6000mm) and the camera's pixel size that SharpCap automatically reads when the camera is connected. Is this assumption correct? If so, then the commands issued by SharpCap are independent of the mount’s movement speed, and therefore, a too-high movement speed should not create gaps in the spiral search. In your answer, you mentioned that “Too-high movement speed may make the spiral too spread out to catch all the nearby sky,” does this mean that even if the commands from SharpCap remain the same, the mount—as a mechanical device—might not move exactly as commanded if its speed is too high? (i.e., the mount might overshoot the commanded movement)

Secondly, as a provisional measure until this function is improved, I would like to get your opinion on the following approach. If the root cause of the problem lies in the uncertainty of the mount's behavior when it receives spiral search commands while in auto-tracking mode, would it be effective to disable auto-tracking before executing the spiral search, and then re-enable auto-tracking once the target has been acquired, thereby eliminating the uncertainty of the mount's response to the commands?

Thank you in advance for your advice.
Best Regards,
Takuya

[admin]

Hi,

unfortunately the current spiral search is far less sophisticated than you imagine The code to do it was written over a decade ago when mount control was first added to SharpCap and hasn't really been updated since.

The algorithm moves

1second up
1second right
2s down
2s left
3s up
3s right
4s down
4s left
... and so on ...

If you trace this out on squared paper with 1s = 1 square then you will see it does give a spiral working out from the initial point. The tightness of the spiral on the sky is controlled purely by the selection of the movement rate, but as noted above, there is also a problem as tracking is dropped during some of the movements.

The suggestions that have been made here - calculate the amount of movement required from the field of view, a way to go back, etc, are all things that I have considered adding, but I still have concerns about how to present the additional options needed and how a more refined approach would cope with the imperfect response of the mount hardware.

Unfortunately I don't think that the idea of turning off tracking while the spiral runs will really help much - I think it would probably just increase the total drift overlaid onto the spiral movement.

Right now, if you can't make the spiral search work reasonably by a choice of movement speed then it may be best to try a different approach to planet finding, possibly including :

* Plate solving on the guide scope - if you can work out the position in the guide scope view that the main scope is looking at, then plate solve, sync and put the planet on that point

* Defocus and set exposure to ~1s, max gain, max display stretch - this should pick up the defocused light from Jupiter when nearby - then move the mount to brighten the image to close in on the planet

* Place a bahtinov mask over the telescope - this will create spikes and all you need to do is get one of the spikes into view then follow it back to the planet.

cheers,

Robin

[kaymann]

...
Right now, if you can't make the spiral search work reasonably by a choice of movement speed then it may be best to try a different approach to planet finding, possibly including :

* Plate solving on the guide scope - if you can work out the position in the guide scope view that the main scope is looking at, then plate solve, sync and put the planet on that point.

I do this and it requires a little work but ultimately gets you centered. One other little tip: To make alignment of the guide/finder scope easier to adjust is to add a tripod leveler ($20-30) to the base of the guide/finder holder that way you have two independent axis' to mess with instead of three screws that mess with both axis' at the same time.

* Defocus and set exposure to ~1s, max gain, max display stretch - this should pick up the defocused light from Jupiter when nearby - then move the mount to brighten the image to close in on the planet

Done this before but not with the specifics of your suggestion which make it sound a lot easier

* Place a bahtinov mask over the telescope - this will create spikes and all you need to do is get one of the spikes into view then follow it back to the planet.

Never thought of that! What an awesome idea. I don't see how that can not work -awesome!

cheers,

Robin

[takuya_t]

Thank you for your quick response. I understand the situation well. Also, thank you for suggesting a solution.
Until now, I have been using the method in your second suggestion to pick up defocused light to bring the planet into view. However, even with a focal length of 6000mm, I still sometimes get lost.
When that happens, I sometimes have to remove the Barlow lens, bring it into view, and then put the Barlow lens back on.
I wanted to eliminate the hassle and stress of that sort of thing, so I wanted to establish a method of introduction using spiral search, but it seems difficult at the moment.
I guess your first suggestion is effective if I make good use of SharpCap's read function. I'll give it a try. Thank you.

Regards,
Takuya

[carlomuccini]

Hi

Sorry to butt in on this interesting discussion.
But I wonder: why not point Jupiter and THEN insert the 3x Barlow? Why put it first?
It would also be easier to focus with Jupiter in the frame ....

It's just an idea...

Carlo

[takuya_t]

Hi

Thank you for your opinion.
That is correct, but there are times when it is possible to install it with 3x Barlow from the beginning, so I operate it that way. This time, I was consulting about try and error to see if it would be possible to install 3x Barlow from the beginning and still have the planet in view surely & smoothly.

Regards,
Takuya

[admin]

Hi,

I leave the barlow in for a couple of reasons...

1) It's just easier - especially with a permanent setup

2) No need to worry about focus being way off each time - you will at least be close

But I guess if finding the target is more of a challenge than focus, you might do it the other way around.

cheers,

Robin