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Guidance on Guiding


Shibby

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That's right, I'm joining the darker side of the dark side... Naturally, I have many questions for you sith lords.

I already had the (unmodified) webcam, so bought myself an Orion Mini Guidescope (one of these). Orion claim that it is suitable for focal lengths up to 1500mm (my FL is 750mm).

Gave it a first initial test a few nights ago. It was extremely windy and I was dodging clouds all night, so I don't know how valid a test it was - I can't be sure that the PHD calibration worked properly under these conditions?

I took this image of NGC 2419 (1st attached) which consists of 5x5min subs, 3 looked ok and 2 didn't. (note that I can normally image unguided up to around 4mins). The stacked image doesn't have perfect star shapes but this shape does seem to match the vibration direction caused by the wind. The image is cropped, representing around 40% of the frame.

The 2nd attached image is a 100% crop of a poor sub. Is this backlash? If so, it appears to have occured 1/2 way through the sub - Shouldn't guiding jump to the rescue with this?

I was able to guide on stars up to around mag 5.5, not ideal but the scope can be moved around a bit so will do for now. I understand that modding the camera for long exposure would be worthwhile in the future.

Should such a small scope (FL 162mm) really be good enough for up to 1500mm guiding??

I'm sure I'll have many more questions to come, as I don't have a clue what I'm doing really!

Thanks.

PS: NGC2419 is a globular cluster 250,000 light years away that apparently may have escaped the Milky Way! The small galaxy nearby is NGC2424

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Welcome to the (very) dark side :)

It's tricky to say what would cause those star shapes. Could be a number of things. Backlash is usually accounted for in PHD and should be taken up by the weights when you balance the scope, try to keep it slightly heavy one side so the mount is always pulling against it.

Usually poor polar alignment would give those stars, but you say 4 mins unguided (that seems a lot for 1500mm FL ) so that should be ok.

How does your guiding graph look in PHD? You want it to read fairly flat, or as best as you can. Try to work out what direction the stars are skewed in, whether RA or DEC.

At the end of the day though, I suspect that the culprit here is differential flexure. It really starts to show after 5 mins, and the difference in focal length between your scopes wont help. You can do your best to clamp everything tightly and rigidly, but flex is a real problem when using a separate guidescope, especially at longer focal lengths. Normally the stars are pretty much all stretched in the same direction, but the guidestar doesnt appear to have moved from its original position.

The cure is to use an Off Axis Guider. These completely eliminate flexure and you can take subs that are 2, 3 or 4 hours long if you so wanted, with round stars (its true, I do it regular ;) ).

First things first though, try to keep an eye on the guide graph for a while, and if it shows real wobble, post it up for us. Flexure wont show up in the graph, just in the results.

Cheers

Tim

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Also, out of interest, how do your stars look with just a short sub, 1-3 seconds? Choose a bright star field for best results. If they look a bit triangly anyway thats another issue.

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mmm, 160mm for 1500mm FOV... I think you may be exagerating any guding errors you may have at this ration. These include as Tim says, in my order, top being worst

1. Ratio / resolution of guiding : Related to the refresh rate on PHD : Shorter the better and your are using an unmodified webcam and not a proper guide cam.

2. Polar alignment

3. Flex

4. Balance of rig.

I may be wrong, but had similar issues when trying to guide at 1/2 ratio let alone the ratio you are using almost 1/10th. Mine turned out to be a combination of weight / balance and polar alignment that shifted as I bumped into the obs tripod one night whilst drunk :)

I would start with basics.

1. Check polar alignment and get it as close as you can (forget drift for the time being).

2. Ensure the guide scope is as rigid as it can be, including the alignment screws.

3. As Tim says, use shorter lengths for testing and use PHD defaults. Ensure calibration runs smoothly and set the time to about 1.5 seconds refresh. Remember your ratio : You will need to find a brighter star for shorter PHD refreshes.

Hope that helps.

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Thanks for all the tips guys. I was hoping to have another trial run last night but once again the winds were hopelessly strong! :)

I should just clarify that my FL is 750mm, so a ratio of ~4.6; should I be expecting acceptable results with this sort of ratio?

I will ensure to capture the guide graph next time; it was pretty messy as I remember, but again very windy so hard to tell. Darn weather!

Also, out of interest, how do your stars look with just a short sub, 1-3 seconds? Choose a bright star field for best results. If they look a bit triangly anyway thats another issue.

I don't have any short exposures to show you yet, but here's a 1min unguided sub from a previous session. What do you make of the star shapes? I believe the focuser can be seen on the brighter stars. Would perfecting focus resolve this?

post-17708-133877712683_thumb.jpg

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Also, out of interest, how do your stars look with just a short sub, 1-3 seconds? Choose a bright star field for best results. If they look a bit triangly anyway thats another issue.

Sorry to side step, I've had triangly stars in the past. Do you know what exactly causes it?

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Triangly stars usually results from primary mirror clips that are too tight.

I'll have to check that!

Shibby, is there any way you can prevent the focuser tube being in the light path?

I don't think so, unfortunately :( Even though it's the PDS scope, I still have to have the focuser almost all the way in to reach focus with my DSLR.

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How about moving the mirror up the tube, with better springs and longer screws?

How easy is this to do, and can you point me in the direction of the replacement screws & springs that I would require?

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How easy is this to do, and can you point me in the direction of the replacement screws & springs that I would require?

I removed the primary mirror, cut an inch off the end of the tube and then re-drilled the holes to hold the mirror cell and bobs your uncle.

The hardest part is making sure that you cut the end of the tube square.

Good luck.

Cheers

Ant

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  • 2 weeks later...

Had another, more successful go with guiding on Monday night. Here's the result (3x5min subs, full field of the camera):

shibby-albums-deep-sky-picture15254-test2c.jpg

And here's a 100% crop, they don't look perfect, do they? but not too bad...

shibby-albums-deep-sky-picture15255-test2c100pc.jpg

Ant, I'm not sure I fancy sawing my telescope in half to be honest with you! I think I might just have to live with those star shapes for now, the problem is minimised with good focus.

Is it odd that a telescope designed specifically for DSLR imaging still has this problem with the focuser?

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