Coronado PST imaging

maksutover · #1 08-03-2008, 10:08 PM

Hi all

I really need help in focusing my webcam for solar imaging. Ive tried using barlows but they still dont focus! Can anyone suggest a simple answer to this problem???

sheeny (Al) · #2 08-03-2008, 11:53 PM

Are you using a barlow, or are you taking the lens off the barlow and screwing it into the end of the 1 1/4" EP adapter like a filter?

I had no luck with a barlow as is, but I think I could achieve focus with the lens screwed into the adapter. I was still strugling with focus, but I'm sure it was much better, and I think my main problem was simply the rapid drift across the FOV as I was using a fixed tripod. Now that I have an adapter to fit my PST onto my CG5 I'll be able to give oit a proper go (when I get my PST back...).

Al.

Suzy_A · #3 09-03-2008, 12:12 AM

You need to do some surgery to the PST...

The problem is that the focal point is not far out enough to allow imaging, so you need to remove a bit of the PST.

Ther black eyepiece holder needs to be removed. This then comes apart into two pieces. Remove the piece that is closest to the body of the PST and then put a 3 or 4 mm washer or spacer or something between the eyepiece holder and body and screw the eyepiece holder back in.

If you don't put in a spacer the metal part of the eyepiece holder will hit the prism in the body of the PST and put a big scratch across it....

The threads are sealed with lock-tite or something like that so you need to use a bit of strength and a lot of care when you do this or you will scratch the aluminium, although this is only cosmetic. If you can't do it by hand, wrap some masking tape around the tube of the eyepiece holder and then use some multi-grips or something to undo it.

Obviously, if you do this it's all up to you. I had to find this out the hard way, but if you are careful and make sure theer is enough clearance between the metal bit and the prism, then it's no problem.

Merlin66 (Ken) · #4 09-03-2008, 03:05 AM

Suzy_A is a 100% right!! FOR THE ORIGINAL PST WITH THE GOLD COLOURED front ERF. If your's has this golden colour read on.....
If you look into the eyepiece holder you'll see the "red" blocking filter; this is the important part and MUST be retained.
To prevent scratching during removal I made a "oil filter" type clamp by putting a couple of turns of a narrow nylon tie-down tape around the eyepiece holder then inserted a long flat stick ( 12" wooden ruler in my case) into the tape and then held the end of the nylon tapeat the other end of the ruler. By positioning the ruler about two inches over centre you can get a good "torque" on the tape/ ruler which will grab the eyepiece holder and allow it to be un-screwed.
Remove the spacer piece as Suzy_A says and re-attach the eyepiece holder.
Maybe I had adjusted the internal prism ( which focusses the PST), don't know, but I didn't have any interference condition.
You may find the eyepiece has to be positioned a little out of the eyepiece holder to achieve visual focus, but no big deal.

On the newer or "repaired" PST's with the BLUE front ERF I found there's an additional filter ON THE BOTTOM of the eyepiece spacer tube; so the above doesn't work!!!! This additional filter MUST be retained!!!
Interestingly I found focussing a webcam on the original impossible without removing the spacer, but after repair ( with the blue ERF+ additional filter) it now DOES focus with the webcam??????
Screwing a x1.5/ 2 barlow lens in the front of the webcam Mogg adaptor should work either way, as suggested by Sheeny.
I'd be interested in knowing whether or not you have a "Gold" or "Blue" PST

maksutover · #5 10-03-2008, 06:26 PM

Hey Merlin

Ive got a blue PST. To be honest ive already tried using a barlow (both with and without the lens) to focus but to no avail! So your saying that focus with a webcam is impossible for my model???

Merlin66 (Ken) · #6 10-03-2008, 07:21 PM

Good question! Don't know yet. If I get the chance I'll see what can be done.
Makes me wonder if there's a difference between the repaired PST ( ie change of ERF and adding the extra eyepiece filter) and an new PST (ie does it incorporate a different blocking filter???)
Can anyone with a "new" blue PST comment???

sheeny (Al) · #7 10-03-2008, 07:34 PM

Quote:

Originally Posted by maksutover

Hey Merlin

Ive got a blue PST. To be honest ive already tried using a barlow (both with and without the lens) to focus but to no avail! So your saying that focus with a webcam is impossible for my model???

OK, but have you tried with the barlow lens - taken off the barlow - and screwed into the EP adapter of the camera? That would be the first thing I'd try before having a go at the mods...

Quote:

Originally Posted by Merlin66

Good question! Don't know yet. If I get the chance I'll see what can be done.
Makes me wonder if there's a difference between the repaired PST ( ie change of ERF and adding the extra eyepiece filter) and an new PST (ie does it incorporate a different blocking filter???)
Can anyone with a "new" blue PST comment???

I can't comment... my PST is currently away being fixed so presumably it isn't a new one.

Al.

Zuts · #8 10-03-2008, 07:47 PM

Quote:

Originally Posted by maksutover

Hey Merlin

Ive got a blue PST. To be honest ive already tried using a barlow (both with and without the lens) to focus but to no avail! So your saying that focus with a webcam is impossible for my model???

Someone correct me if i am wrong but...

The focal plane of any telescope is fixed, the reason eyepieces and ccd's focus at different distances is because of the distance of the lense elements from the end of the barrel or the distance of the chip from the end of the assembly.

To see if it can focus, calculate the rough distance of the ccd to the closest point of where the barrel attaches to the scope. Then see where the focal plane of the pst is. This can be done by pointing at the moon without an eyepiece and seeing how far back you can get a crisp image on say a piece of grease paper.

If the two coincide you should be able to acheive focus.

Paul

sheeny (Al) · #9 10-03-2008, 07:53 PM

Quote:

Originally Posted by Zuts

Someone correct me if i am wrong but...

The focal plane of any telescope is fixed, the reason eyepieces and ccd's focus at different distances is because of the distance of the lense elements from the end of the barrel or the distance of the chip from the end of the assembly.

To see if it can focus, calculate the rough distance of the ccd to the closest point of where the barrel attaches to the scope. Then see where the focal plane of the pst is. This can be done by pointing at the moon without an eyepiece and seeing how far back you can get a crisp image on say a piece of grease paper.

If the two coincide you should be able to acheive focus.

Paul

Correct, Paul... just a minor technical point though... it might be necessary to use the sun rather than the moon for a PST. I doubt a moon image would be bright enough

.

Al.

Merlin66 (Ken) · #10 10-03-2008, 08:31 PM

Haven't actual checked the prime focus position; should do that!
I'm thinking, as my webcam is modded the distance from the adaptor shoulder to the CCD chip may be shorter than the "standard" distance??
When the "worse storm this year, gusts up to 80mph" passes (!!??) I'll get the PST out and check.

Merlin66 (Ken) · #11 10-03-2008, 09:26 PM

In the "lull between the storm fronts", set up the PST. It's impossible to image the sun on a card or transparency, there's not enough light!
I put the webcam in and it focussed OK!!

Measuring the position of the focal plane on my eyepiece and then inserting into the PST, without re-adjusting the PST focus knob gave me the following dimensions:
Distance from the PST body to focus: 70mm
Distance from PST body to the end of the focus tube: 57mm
Therefor it's possible to have up to 13mm between the end of focusser tube and the webcam CCD.
I tried to move the focus further "outward" and only got, maybe a 1mm at the most. At this point the focus knob on the PST was turned all the way out ie max protruding from the PST body.
Certainly before "repair" I had to remove the spacer ( as per Suzy_A) to get good focus, so somthing has changed.
Hope this helps???

Merlin66 (Ken) · #12 18-03-2008, 03:01 AM

http://www.astro-engineering.com/CCD...ce.html#webcam

The AC378 model seems like the one I have (Mogg type) and the flange limits how far the webcam can be "pushed" into the focusser.
The AC624 has this retaning flange removed, so at least in theory should allow the webcam to move in closer to focus. This might be a solution for the PST users???
Anyone tried one???