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Hi everyone, Help please!!

krobbo

By krobbo
in Getting Started With Imaging

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krobbo

krobbo

Posted
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Hi everyone need some help please !!

resurrecting my old astro gear, a Celestron C8 Celestar deluxe, equatorial wedge, f6.3 field flattener. imaging will be on a DSLR for starters.

I'm having another go at CCD imaging ( tried it a few years ago with a Starlight Xpress MX5C CCD camera) with mixed results.

I've just bought a OVL off axis guider (clone I think but is of really good quality) & it came supplied with the T ring adaptors. 

The problem I've got is finding a focuser for the guide camera? The platform has a T male thread of 42mm & I would really like to fit a helical focuser but I can't find one !! The more I research T adapters the more confusing it gets !! It seems there's several sizes of T adapter?? I've found a Baader one that fits but even though it says helical it states that it rotates which seems a no no for focussing on the auto guider. 

I've 2 cameras to try for auto guiding, a Xbox type USB camera that a friend has given me with filter removed (C mount type) & a Opticstar SL-131C CoolAir (has a bigger chip, better for autoguiding??)

So I'm looking for a a focuser that'll fit the T male thread & have a 1.25" adapter the other end, helical preferably, &  not cost an arm & a leg !!

Very grateful for any help please!!!

Very sorry Admin if I've missed this topic elsewhere !

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krobbo

krobbo

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Hi Steve & many thanks for the reply. This is the one I came across but it says it rotates when focusing? I always thought true helical focusers didn't rotate, I can't see how I can focus with this one if it's rotating the guide star I'm trying to focus on?? Can't seem to find any others !!

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steppenwolf

steppenwolf

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I don't let Amazon sell my book BUT the cheeky blighters list it as 'unavailable' (even though they have never had the opportunity to sell it and never will!!) so that they can offer alternatives. I guess this is a compliment that they NEED my name and book title to sell other lesser titles but it is a rather cheap shot I feel!!

The book can be bought from more REPUTABLE sources like FLO or directly from me!

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Oddsocks

Oddsocks

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3 minutes ago, krobbo said:

can't see how I can focus with this one if it's rotating the guide star I'm trying to focus on?? Can't seem to find any others !!

Depends what software you are going to use for auto guiding, most don't care what the angle of the guide camera is with relation to the RA and DEC axis, the calibration routine tells the software how to interpret a move of the guide star and will move the appropriate drive. For help diagnosing problems it is always better to have the guide camera aligned exactly RA - DEC but it is not essential.

You just focus the main camera, lock the main scope focus, adjust the guide camera focus, don't worry about rotation of the camera, lock the guide camera focus, calibrate the guider for the current rotation position, start guiding.

What is more important is to consider the focal path length of the main camera with respect to the guide camera, this will determine whether you actually have the focal length to fit a helical focuser.

You need to measure how far back the main camera sensor will sit behind the OAG pick-off prism and then determine the same length of path from the prism to the sensor of the guide camera, both have to be the same, most likely you won't have the space to fit the helical unless you also add spacers to the main camera mounting and then you begin having problems with flexure as the weight of the main camera hangs further away from the back of the OAG and the focuser.

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Waldemar

Waldemar

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32 minutes ago, krobbo said:

Hi Steve & many thanks for the reply. This is the one I came across but it says it rotates when focusing? I always thought true helical focusers didn't rotate, I can't see how I can focus with this one if it's rotating the guide star I'm trying to focus on?? Can't seem to find any others !!

Normally you do not use a focuser for the guide cam. Just achieve focus with spacers. Will never change focus again with a specific camera combination, so spacers will do and are a lot cheaper...

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krobbo

krobbo

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Hi Oddsocks & thanks for the info. I was going to try PHD2 with Backyard EOS for the camera? They seem to be really popular. I've read about getting the "film planes" for both cameras the same (oldschool!) just the hassle at the moment getting the hardware together !!

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Oddsocks

Oddsocks

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1 hour ago, krobbo said:

I've just bought a OVL off axis guider

If the OVL guider you bought is this one:

https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html

The focus for the guider port is adjusted by loosening the grub screw on the T mount and sliding it up and down the stalk, you just need to ensure the stalk does not push up into the guider camera sensor, if this is a problem add a small spacer to the main camera mount to push the focal point further out, or if no other choice, you take a hacksaw to the stalk and shorten it.

The guide star does not have to be perfect focus and for some guide cameras you get better results if the focus is a little soft. The guide software does not look for the guide star to occupy a single pixel but looks for the light to be spread over several adjacent pixels, the software then calculates the average centroid for the star from all the surrounding pixels.

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Waldemar

Waldemar

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9 hours ago, Oddsocks said:

If the OVL guider you bought is this one:

https://www.firstlightoptics.com/off-axis-guiders-oag/off-axis-guider.html

The focus for the guider port is adjusted by loosening the grub screw on the T mount and sliding it up and down the stalk, you just need to ensure the stalk does not push up into the guider camera sensor, if this is a problem add a small spacer to the main camera mount to push the focal point further out, or if no other choice, you take a hacksaw to the stalk and shorten it.

The guide star does not have to be perfect focus and for some guide cameras you get better results if the focus is a little soft. The guide software does not look for the guide star to occupy a single pixel but looks for the light to be spread over several adjacent pixels, the software then calculates the average centroid for the star from all the surrounding pixels.

 

9 hours ago, steppenwolf said:

Quite the opposite is actually the case! A slightly blurred guide star gives the centroid algorithm a better chance of working .....

Absolutely correct.

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