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First light ready...just need the Sun

Posted: Sun Feb 22, 2015 11:05 pm
by Starry Jack
I am pleased to say that I am ready for first light...I just need the Sun to come out from behind the clouds.

Here is my project:

-PST Stage 1 Mod
-Celestron Nexstar 100mm donor scope
-100mm Lunt ERF mounted in front of the objective
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First I took the objective off of the PST. Loctite made it a bit tough, but I won in the end. I failed to get the gold tube apart from the etalon, but according to my calculations, the gold tube is close to the correct size of the light beam at that point in the donor OTA so I used the gold tube minus the PST objective as my nose-piece for the PST's black box. The gold tube also fit perfectly in my 2" donor stock focuser. I did have to remove several of the donor OTA baffles, but I believe the gold tube makes up for those missing items.

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I took the donor OTA outside and used a full moon projected onto a piece of white paper to determine prime focus. I trimmed first 200mm off the OTA but could not reach focus (barely) so I cut about 250mm total from the OTA.

The ERF cell turned out to be easy to make. The Nexstar dew shield has an internal ridge that is about 107mm in diameter. The Lunt 100mm ERF is actually 105mm diameter. So I took two strips of felt and lined the inside of the dew cover and the ERF nestled in PERFECTLY. Nice and snug but soft enough to provide room for thermal expansion. I did place two small pieces of felt on one edge so I gave the ERF a 1-2mm tilt to avoid ghosting. Reading another post on this forum I ensured the Lunt ERF had the slightly purplish/blue/red tinted side facing the sun. At this time the ERF is kept from slipping out of the cell by three small black binder clips. One day I will clean that up, but in the meantime it works beautifully.

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The PST-100 is piggy-backed on my 6" achromat on an AVX mount. I think that pushes the weight limit of my mount so I plan on just swapping the scopes depending on day or night viewing.

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I made a Sol Finder by taking a piece of PVC pipe and shaving off 1/8" from one long side of the pipe using a mitre (chop) saw. This gave it a flat bottom to adhere to an old dovetail mount. On one end of the pipe I glued an old black plastic film canister lid with a small hole drilled in the center. On the other end I glued a translucent white plastic disc to receive the projected image of the Sun. Works like a charm.

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First light yesterday had the sun behind 90% cloud cover so all I could do was verify that I could reach focus. I had almost zero detail. Today is rain and tomorrow as well.

Thanks for all your guidance,

Starry Jack

Re: First light ready...just need the Sun

Posted: Sun Feb 22, 2015 11:11 pm
by Merlin66
Sounds good!
If the PST etalon is positioned correctly at -200 inside prime focus, the final focus at the eyepiece of the black box will remain unchanged.

Re: First light ready...just need the Sun

Posted: Sun Feb 22, 2015 11:25 pm
by Starry Jack
Does this rig seem sound enough for visual or is that simply too risky?

Thanks,
Jack

Re: First light ready...just need the Sun

Posted: Sun Feb 22, 2015 11:26 pm
by Merlin66
Should be 100% OK for visual.

Re: First light ready...just need the Sun

Posted: Sun Feb 22, 2015 11:44 pm
by Starry Jack
Super!
I'll report back at first "real" light.

Jack

Re: First light ready...just need the Sun

Posted: Sun Feb 22, 2015 11:46 pm
by Starry Jack
Is there a way to post Flickr pictures? My images did not seem too happy in the post and I want to share solar photos as well.

Jack

Re: First light ready...just need the Sun

Posted: Sun Feb 22, 2015 11:48 pm
by marktownley
Excellent!

Re: First light ready...just need the Sun

Posted: Mon Feb 23, 2015 12:25 am
by Starry Jack
This is only a test post. I'm trying to find a photo service that will display my images inline.

Dropbox test: Image

Re: First light ready...just need the Sun

Posted: Mon Feb 23, 2015 12:31 am
by Starry Jack
TEST POST: Attaching the photo rather than linking.
IMG_5530.JPG

Re: First light ready...just need the Sun

Posted: Mon Feb 23, 2015 12:38 am
by Merlin66
BTW, I do recommend fitting the Vegemite lid "fine" focuser knob to the PST focuser.
(If you don't have any Vegemite to hand, I suppose you could use a peanut butter lid ;-) )

Re: First light ready...just need the Sun

Posted: Mon Feb 23, 2015 12:39 am
by Merlin66
Jack,
For "normal" images you can reduce the size to 800 x 400 approx. jgp
That's usually good enough.

Re: First light ready...just need the Sun

Posted: Mon Feb 23, 2015 5:24 pm
by swisswalter
Hi Jack

that looks great. I can see only the last pic "test post"

Re: First light ready...just need the Sun

Posted: Mon Feb 23, 2015 6:14 pm
by Starry Jack
I have resized all the images and will attach them tonight. I read the sites FAQ and thouhjt linking to images would work. Looks like attachments are the way to go. FYI, I called lunt and they confirmed that the ERF will work either way but they generally apply the purplish side towards the sun.

Best,
Jack

Re: First light ready...just need the Sun

Posted: Mon Feb 23, 2015 6:57 pm
by swisswalter
Hi Jack

you can't go wrong. The LUNT ERF's have an arrow on it pointing to the SUN

Re: First light ready...just need the Sun

Posted: Tue Feb 24, 2015 4:23 am
by Starry Jack
Interestingly enough, my Lunt ERF does not have any arrow indicating direction that I could discern. Oh well, at least the thing is in there properly now!

Jack

Re: First light ready...just need the Sun

Posted: Tue Feb 24, 2015 9:15 pm
by Starry Jack
I took the scope to work today and got first light visually during lunch. The image is nice and bright but the PST sweet spot seems even more narrow and very crescent shaped. Is that to be expected?

The tripod was manual and shake but the filaments and proms I did see had good detail and were sharp.

Best,
Jack

Re: First light ready...just need the Sun

Posted: Tue Feb 24, 2015 9:19 pm
by marktownley
Starry Jack wrote:The image is nice and bright but the PST sweet spot seems even more narrow and very crescent shaped. Is that to be expected?
Something is not optically square then...

Re: First light ready...just need the Sun

Posted: Tue Feb 24, 2015 9:34 pm
by Merlin66
+1 from me.
As Mark says, something not square. Could be the PST tube to focuser fit allowing some sag/slop?

Re: First light ready...just need the Sun

Posted: Tue Feb 24, 2015 10:52 pm
by Merlin66
The amount of OTA "cutting" depends on whether or not the focuser (which can't be used !!) is retained or not.
The bottom line is that the final focus position in a Stage 1 DOESN"T CHANGE FROM THE ORIGINAL PST POSITION. It will still be difficult to fit and focus some cameras.
Yes, for optimum performance, the PST etalon must be accurately positioned at -200mm from the donor prime focus.

Re: First light ready...just need the Sun

Posted: Wed Feb 25, 2015 5:54 am
by Starry Jack
Interesting points, all!

Let me begin by saying that in the 30 years I have built telescopes, my tried and true motto is, "Measure 3 times and STILL cut wrong!" ;-) In other words, I am sincere and enthusiastic but sometimes my skills lag a bit. :-)

So. A while back I had trimmed 1/8" from the eyepiece holder on the PST. Many folks try this when trying to reach focus on a standard PST and using a camera. It worked for me, but did introduce some slop in the eyepiece holder which I am trying to correct.

Secondly, I found 2 other places of slop so I am trying to correct those as well tonight.

Third, take a look at the light path I have drawn out on my workbench using tape and lines. In my calculations, I show that the etalon is indeed exactly at -200mm (yes...I cut way too much off my OTA and now have to rack the focuser out). I also mapped out where the drawtube is, it's aperture, the light width at that point, where the gold tube starts within the draw tube and the light path width at that point.

The good news is that the light path is smaller than the openings at every point by about 2mm. It seems to me that I don't need to worry about vignetting.
IMG_5557.JPG
I have tightened up a few slop points and will try again tomorrow visually.I can't try the camera until this weekend.

Best,
Jack

Re: First light ready...just need the Sun

Posted: Wed Feb 25, 2015 1:34 pm
by Starry Jack
Theo,
You read my mind. The prism was my task for tonight. Good point about axial alignment as well. I don't have the funds at the moment for a nosepiece so if you have a clever device to share I am all ears.

Also fascinating is your description of uneven brightness. I will check to see if I am experiencing the same...sounds about right.

Jack

Re: First light ready...just need the Sun

Posted: Wed Feb 25, 2015 4:12 pm
by Starry Jack
Good idea. I wonder if I still have my laser...

What should my expectations be for clarity across the FOV? A PST has the famous sweet spot. Should I expect the same with my mod or should the entire FOV b consistent?

Jack

Re: First light ready...just need the Sun

Posted: Wed Feb 25, 2015 5:08 pm
by Starry Jack
I just got a few minutes observing. Most slip was removed though I don't know if axial alignment is spot on. I found a huge amount of leopard spots in the FOV and discovered the objective had fine watermarks on it perhaps dew formed while in the observatory.

Again, the entire image is nice and bright but the sweet spot is very narrow. I could see a long north/south filament nicely but nothing to the east or west of it so you know the sweet spot was pretty narrow.

Are there other symptoms I should be observing to help diagnose?

Thanks,
Jack

Re: First light ready...just need the Sun

Posted: Thu Feb 26, 2015 5:39 am
by Starry Jack
I cut the gold tube, and in so doing it unleashed the loctite so I could finally get the tube and etalon and black box all apart. I could not remove three of the screws (more loctite) from the prism box so I could only assess alignment of the prism by removing the etalon and BF.

I did not have any good way to measure, but it sure seemed to my eye it was a tad off, but that is only anectdotal and not based on measurements. I did turn the PST focuser all the way so that the prism was at the top of the box and while it seemed equal in space to the top of the box, it seemed to my eye a micro bit skewed to the front of the black box. But I could discern no way that would be possible so I left my assessment as, "not real valid".

I wrapped aluminum tape around several times around the shortened gold tube so that there was a very snug fit into the focuser. The only slop I can find now is a bit of slop in the focuser itself and I am not sure how to fix that quite yet...but I will. :-)

Lastly I cleaned up the ERF from the watermarks and I will give it another go tomorrow at lunch.

Thanks all,
Jack

Re: First light ready...just need the Sun

Posted: Thu Feb 26, 2015 6:53 am
by marktownley
slop / sag in the focuser is normally the culprit...