Guidestars and guiding - is my thinking correct??

[scoobymike]

I have recently bought an st80 and a LVI smartguider (from Quatermass after his problems with it)... I have only used it once, and whilst it works admirably with a bright guidestar, it does seem to need something fairly bright in the FOV to register a guidestar. So if I can't find something with it all perfectly aligned, then I need to adjust the guidescope to bring something suitable into view. Now if my thinking is correct the following should apply, but is this correct? :-

If I adjust the scope in the lateral plane (ie left/right if mounted on top of the main tube rings) relative to my main scope it will slowly introduce guiding errors, but if I adjust in the vertical plane relatively then all should be fine.

I am thinking of making a hinge assembly for the front of the st80 mount to allow vertical movement, but want to be sure I am not wasting my time and very limited machining skills...

Thanks,

Mike

[hemihaggis]

you should not have a problem finding any star to use as a guide

[scoobymike]

You (and I) would have thought so, but it failed to find a star on a few attempts (I was only out for a couple of hours, so a good poportion). I believe the camera is the same as a QHY5, so am wondering if it's a camera sensitivity issue, or if I really was just looking at starless skies. If so, then I would still need to move the guidescope, hence the question...

I know skywatcher sell a guide mount that lets you move the scope around, but its expensive, so as such I would have a go at making my own version.

[Ant]

Moving the guide scope in relation to the main imaging scope shouldn't give you any issues...

Having two scopes either Side by Side or one on top of the other will introduce flexure (slight movement between the two scopes) - there isn't much you can do about this except make sure everything is as tight as it can be.

Ant

[themos]

Actually, I am convinced that as you choose a guide star further and further from where you're imaging you introduce more and more field rotation whose magnitude depends on your polar alignment error. Use the calculators here to figure out how bad it can get:

On Polar Axis Alignment

[scoobymike]

Thanks!

I've just noticed hot_ice's post underneath mine on exactly the same subject. Maybe hemihaggis has it right, and I just shouldn't have the problem anyway. I'll go and check the focus properly (if we ever get a clear night) and see if that resolves the issue.

Mike

[hot_ice]

Hi Mike

Just read your post. Initially, I have problem guiding with a fainter star using the LVI as during DSO imaging stars around the main scope is fainter. However, I have now resolved the problem by doing micro adjustment to the plastic ring of the smarteye lens it was not in parfocal with the LVI camera.

Since them I am able to focus on a fainter stars. I also emailed LVI in Italy last Saturday and Paolo replied almost immediately and confirmed my findings. He has advice me to adjust 0.2mm at a time until i get both parfocal. This is very tedious way of getting both in same focus,but i have discovered a quicker way of doing this and managed to parfocal in less then 30mins. Paolo also confirmed to me using a diagonal instead of straight tube is not a problem with the smartguider. It can suss out the orientation of the AR/DEC axiles regardless of the optical train

I know you issue is to be able to move the guide scope. I have bite the bullet and order a SW mount from FLO and awaiting delivery to test it out

Kim

[Spikey]

I used to have my ST80 clamped tight with one ring so I could move it left and right if necessary. I never did, but I never failed to find a star. I've now got it in guiding rings but have never had to adjust those either. Sounds like the sensitivity could be the issue.

[scoobymike]

Thanks guys,

Kim, you said you had a quick way of getting the eyepiece par-focal.. Would you mind letting me in on you secret please?!!! It definitely looks like focus is the issue..

Thanks again,

Mike

[hot_ice]

Hi Mike,

There is no secrets I do not want to write a long reply to you post. Here is how I did it.

1. Measure the distance of the smart eye Plastic band from the

rim of the smarteye lens and write it down. Mine was initially

1cm

2. Focus the Guide scope on a bright star using smarteye lens

and centre the star

3. Remove the lens and attach the camera and see the camera

can guide with the bright star.

4. If it can then the parfocal is only slightly out

5. Focus the Guidescope this time on a fainter stars ( I usually

look around the Orion belt area) and centre this star

6. Once the smarteye lens is in focus replace with the Camera -

You should get a message Star not found.

7. Remove the camera and insert the smarteye and check the

smarteye is still in focus.

8. Now slowly move the focuser inwards until the star is blurred

with the smarteye lens.(You may have to move it outwards if

inwards does not work)

9. Replace the lens with the camera if after about 90 secs Star

found appears let it start guiding and then stop and exit

10. Now replace the camera with the lens - It should shows the

star as blurred.

11. Carefully move the lens slightly out of the tube until the star

is in focus. I used diagonals for this procedure as it is easier.

12. Move the lens in and out and note the distance when you

have achieved focus, It should be very small amount

probably about 2mm. It is difficult to judge the distance but i

managed to hold the lens when it is in focus and noted the

distance in my mind.

13. Move the plastic band either forward or backwards

depending on when you have received focus but only very

tiny amount. You may have to centre the star again if it has

move while you are adjusting. It is easy to check and

recheck the focus until you get it right

14. Once the smarteye is in focus replace the camera and start

star searching It should still find the star

15. Now both the smarteye and the camera is Parfocal.

16. I then find another faint star and check the lens and camera

are both parfocal

17. Use a fine metal ruler to measure the plastic band distance

from the rim and make note in case it goes out again in the

future . I used a barlow lens to even the plastic band all

round once I have make note of the distance

Good Luck and hope this help

Kim

[scoobymike]

Thanks Kim,

Now if only I could see some stars...

Mike