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SCT post clean issues

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#1 DeanCN

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Posted 08 August 2020 - 12:05 AM

Meade lx90 8" SCT made 13 years ago, according to the print mark on the corrector plate edge.

After watching many vids, reading in the forums etc. and being annoyed with black specks and dust on the

primary and secondary mirrors and corrector plate surfaces, I did the deed and took the plate off

and gently cleaned (pec pads, distilled water, iso alcohol etc etc).  I put it back together aligning

the factory marks of the corrector plate, no shims were moved or lost and I tightened down the plate gasket

ring with even torque to the screws in a non-circular pattern.

I tried to collimate using Deneb as best I could and then did some test images of Deneb while

pointing near direct overhead, focused manually CCW and got this :

 

deneb_sct_20200808.jpg

 

image is with ASI385MC-cool 1.5 sec at 240 gain, -15C, SCT has a Meade 4000 0.63x FF/FR mounted inside

a Moonlite CS focuser.  

Not trying for a pretty image : I focused while standing right beside the scope which sits on a balcony.

What is causing the flare and the purple ring just  above Deneb?  I can focus that purple

ring to a dot at the expense of everything else in the image going out of focus.


Edited by DeanCN, 08 August 2020 - 12:05 AM.


#2 james7ca

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Posted 08 August 2020 - 12:46 AM

It's not unusual to get reflections when imaging a very bright object with an SCT. Did you ever try to take a similar image before you cleaned the corrector? Similarly, the "flare" on Deneb looks like a combination of diffraction and reflections, perhaps from the tube baffle that juts up from the primary mirror.

 

Also, if this image is from the center of the field or is the entire field (just reduced, not cropped) then it looks like your focus was off and/or you have a collimation problem. However, if it is significantly off axis (cropped and not from the center of the field) then the situation may not be as bad as it looks.


Edited by james7ca, 08 August 2020 - 12:48 AM.

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#3 DeanCN

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Posted 08 August 2020 - 12:55 AM

i never noticed this before, and for the first time imaged without a dew shield since

I now have a dew heater strap. Im trying to reduce payload on the mount

and also image later into the night when or at locations where dew conditions arise.

The flare I get could be a reflection, but what about the purple ring that can be focused?

Deneb was at the center of the field.


Edited by DeanCN, 08 August 2020 - 12:56 AM.


#4 Kokatha man

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Posted 08 August 2020 - 12:58 AM

...looking at all the comet-like appearances of the stars in the fov it's pretty obvious that your collimation attempt was not successful, unfortunately. frown.gif

 

Btw, I hope you tightened the corrector retainer ring only minutely past the point where the undersides of those screws' heads contacted with the retainer ring - they should never be "tightened" in the normal sense..!


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#5 james7ca

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Posted 08 August 2020 - 01:03 AM

...Deneb was at the center of the field.

 

If that's from the center of the field then you've got a pretty serious alignment or aberration problem.



#6 DeanCN

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Posted 08 August 2020 - 01:24 AM

short of buying a Hotech SCT laser collimator or shipping off to Dr Clay, I'm not sure where to begin to address

the issues.  I took 3 digital caliper inner measurements from the outer circumference

of the secondary to the inner circumference of the lip that extends past the end of the OTA

and perpendicular to each collimation screw location. They were no more than 1 mm

in deviation at that precision of measurement technique.  The gasket screws are "snug +"

and not as tight as when I first broke them loose.  Pointers to articles/other posts or

continued responses would be much appreciated.



#7 Kokatha man

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Posted 08 August 2020 - 09:05 PM

<"The gasket screws are "snug +" and not as tight as when I first broke them loose.">

 

Be mindful that when you first loosen each retainer-ring screw it is quite likely that they will appear to require more force than what you need to re-tighten them..!

 

I use my thumb & next 2 fingers to lightly hold a screwdriver's handle as I "tighten" each one in the manner you tighten auto wheel nuts...but I emphasize that "tighten" is not really an appropriate word! lol.gif

 

When each screw seats I go onto another opposite screw until they are all seated - "seated" meaning the underside of the head has made contact with the retainer ring!!!

 

Once all are seated then apply a meagre amount of additional turning such that the light hold you have on the screwdriver handle with the thumb & 2 fingers allows slippage between them & the handle - that's it!!!

 

Forget about those caliper measurements btw - they're fairly useless tbh.

 

And forget about Hotech collimators or anything else along those lines - SCT collimation is about as easy as it gets & to me it appears patently mis-collimated: in my signature I give some collimation tips for planetary imaging where we are dealing with very high f/l's (6000mm Vs your approx. 12/1300mm)...you might find something useful therein & try to emulate some of the suggestions on Deneb & see how you go! wink.gif



#8 DeanCN

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Posted 09 August 2020 - 01:44 PM

yep, I tightened in a pattern similar to what one does with car rims.  Thanks for the links.

I'll re-post when I get a chance to collimate (successfully).




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