Long focal length + small chip, finding objects

[themos]

Hello, planetary imagers! I hope you can help me with an annoying problem I have every time I try to image with my ToUCam and a Barlow. I find the planet easily enough with an eyepiece, then add the Barlow and center it in the eyepiece, then put in the webcam in place of the eyepiece. At that point I'd like to slew to a star so that I can work on my focus and then slew back to the planet. Problem is, I can't find a star and return to Mars (say) without taking the webcam off because my FOV is tiny otherwise. I use an EQ6 and EQMOD but don't bother with a GOTO alignment. I tried noting the RA/DEC coordinates, slewing somewhere else and then manually returning to the noted RA/DEC but Mars was not there anymore. This is driving me up the wall, there must be some clever procedure that works all the time. I have a finderscope but I need to align it VERY accurately if it's going to help. I also have a piggy-backed DSLR with a 300mm lens on it so maybe that will need to be used. Has anyone cracked this?

[George]

Why not just zoom in with whatever package your using for capture and fine tune your focusing that way?

Works for me, I'm using K3CCD.

[themos]

Can you use computation to help you focus on Mars? With a star target, the software can compute measures (such as FWHM) and then you KNOW if you're focused or not.

[blinky]

There is (to quote) a clever procedure to return to the same place...... Carry out an alignment with EQMod! Seriously..... Do an alignment and then for your last star get Mars in the eyepiece then swap to the webcam or whatever then centre it in that, then accept that as an alignment star. You should now be able to goto a nearby star and back again.

[George]

Dunno if FWHM will work with a big target like a planet, atmospheric boiling would play havoc I should imagine.....might be worth trying tho.

[brianb]

I have a finderscope but I need to align it VERY accurately if it's going to help.

That's the way. Use the Moon to align the finder, it's easy enough to get something on the chip, zero in on a prominent feature (Plato or Tycho are usually good) & carefully adjust the finder. Try slewing to a different lunar feature using the finder to give yourself confidence. Then go for it.

[themos]

blinky, you mean do a triangle alignment using barlow+eyepiece for three stars around Mars, then slew to Mars and "accept that as alignment star"? I do alignment by using the Sync button from CdC always. I am not sure how I do the "accept that as alignment star" step. Select Mars in CdC and sync to that, once I have Mars in the eyepiece?

[themos]

I was using an RDF last time, so maybe that's why I got so lost. I should be using my optical finder. I think that makes sense. Anyone make finderscopes with zoom eyepieces?

[Merlin66]

You can fit a standard 1.25" eyepiece ( including a Zoom I think//!)

on the Antares type finders......

[lukebl]

I use an illuminated reticule eyepeice in my guidscope.

1: I line the main scope up with the object in the centre of the field of view.

2: I roughly align the guidescope until the object is somewhere in the field of view of the reticule eypiece (doesn't need to be exactly in the middle). I note exactly where the object is in the eyepece field of view of the guidescope using the reticule markers. That way the guide scope doesn't need to be exactly lined up.

3. Slew to a bright star, locating it in the guidescope using the reticule marks; focus on using a Bahtinov Mask.

4. Slew back to the object, locating it in the guidescope using the reticule marks.

Obviously won't work if you're already using the guidescope for guiding at the time, but I assume you're not using it for planetary imaging.

Works for me, even when imaging at 6m focal length.

Best wishes,

[dark knight]

What about using the spiral search function in eqmod?

[arad85]

What about using the spiral search function in eqmod?

Goes by far too quick for me... Tried that on Sunday (or perhaps I should have set the rates to be very low - I assume it uses the ra/dec rates to figure out where to search).

I find it a pain to do too, but I end up focusing on Mars itself, judging focus by seeing when it snaps in/out of focus on a 10:1 crayford...

[themos]

I tried the spiral function and I did see a bright streak after a few seconds of holding down the button. Then what do I do?? :-)

I think the hyper-aligned finderscope is the way to go on this.

[lunator]

Hi Themos

I haven't imaged Mars this way, but I do take pics of faint stars. I have found the finderscope alignment is absolutely critical. It can be frustrating but it does work.

Cheers

Ian

[jgs001]

When I did mars the other night with my Skymax... I went to the moon first, and tried to align the finder... although a 6x30 is not easy to align with a Mak and small sensor camera... Then slewed to mars, centreing in the finder, even without barlow, it was hard to find, so centered using an ep, then used the camera to centre, and increased the barlow magnification in steps (x2, x4, x5), centreing after each change, until I had it centered with the 5x barlow.

[beamish]

I only use a Telrad with my 8", I go thru the alignment process, using either a 25,20 or 17mm EP ,centre in the EP, check the Telrad, then move to gradually shorter fl EP's until eventually I've got the star centred in a 4mm EP and check the Telrad again,then continue with the alignment with a longer length EP. Using three stars to make up a triangle within which Mars appears or as suggested make Mars one of the alignment objects.

Once you Mars in the EP, work back thru the EP's to shorter fl to check centring, when your happy -stick your cam in !

Sounds a bit long winded but TBH, more often than not I rarely need to use too many EP's back and forth cos the alignment procedure and Eqmod are pretty good. Telrad's fine as a finder even at the silly faocal lengths ! Occasionally I've even managed to just drop the cam in.

[themos]

I'm estimating that the entire chip takes up something like 3.5 arc-minutes on the shorter side. That's about 1/10 of the full moon. So my finder needs to be aligned to that order of accuracy. Sounds doable.

[brianb]

So my finder needs to be aligned to that order of accuracy. Sounds doable.

It is.

I'm using the standard Celestron 9x50 finder; accurately focused, Mars just disappears behind the crosswires, but if you throw the focus off slightly, you can get the out of focus disc big enough so that it just peeks past the wires - slew until the brightness of the 4 "pizza slices" is the same, it's very sensitive.

[Pete Presland]

now thats a very good idea! pizza

[themos]

I used the pizza method last time and it worked well.