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CELESTRON NEXSTAR 127 SLT SHARPNESS ISSUE

Astrophotography Beginner Cassegrain Celestron Collimation Equipment Reflector Optics Imaging Maksutov
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#1 marcymonta

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Posted 06 December 2021 - 01:37 PM

Hi everyone.
I'll say right off the bat that I'm an astroNoob, reason why I'm looking for help and can't manage to find the answers I need by myself.
I recently bought a second hand celestron 127slt.
I'm not much interested in observations, for I entered this hobby through the photography gate. So my main trouble is now that I have 6 days of testing warranty in which I can take my time to test both the electronics and the optics. I'm in urge for tips on how to set this up, as I know I might be doing something wrong..

I got the adapter to attach my nikon dslr to the telescope, and it all combines, focuses smoothly and takes photos without vignetting nor anything else.
My question is: why do I get unsharp results? Its like everything is a slight bit off focus but the focus knob has actually room to go way past and before the subject I'm focusing on.

I've browsed online looking for similar issues but couldn't find any, so I'm starting to be worried about the glass itself not being as good as I thought it was (is this possible?! Has anyone noticed heavy sharpness degradation with such scope?).

I sincerely hope it's me messing up, and you can help me figuring out how as I don't want to return it.

Thanks in advance to anyone

#2 WarmWeatherGuy

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Posted 06 December 2021 - 02:28 PM

The scope may need collimation. If you're imaging terrestrial things you will probably get a less sharp image than you were expecting. That's just what happens when imaging through too much air. Try the Moon when it is well above the horizon. When you take multiple pictures and stack them things will be sharper.

 

You can test collimation by looking at a bright star near the zenith and defocusing the star to get concentric rings. If they're bunched up on one side you need collimation.

 

You can learn more here.



#3 vtornado

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Posted 06 December 2021 - 02:39 PM

Are you allowing time for the telescope to cool off?

 

Have you spent time just testing the focus?

 

I don't do much picture taking, but  I do know a camera is much more sensitive

to focus than eyeballs are.  The view screen of the DSLR is not accurate enough

to judge sharp focus at 1x.   (does your camera has the option to magnify view

preview screen), or can you connect it to a computer so you can view the

picture enlarged in real time?  I normall take several pics and tweak the focuser ever so slightly.  Just like bracketing the exposure.  One is in perfect focus.

 

Can you run a star test on your mak?  Polaris works best.  At a slight defocus,

you should see a set of perfect rings.  If they are skewed it means your scope

needs to be collimated.



#4 marcymonta

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Posted 07 December 2021 - 01:48 AM

Okay, I'm going to reply to the both of you.
I knew about cooling times thanks to the research done while trying to find this problem online, so my guess is that its not this case, as I let the scope cooling on my balcony for like an hour.. but if necessary I will definitely let it sit there longer.

I also knew about collimation, which was something I was afraid of when looking through used scopes. But thats the first thing I checked inside the store and the rings are very symmetrical and circle shaped.

I don't know how to post samples, but I'd like to show you the rings of defocused light sources and maybe a couple of magnified pictures to compare lenses sharpness to scope sharpness in comparable conditions.

Also my camera offers the chance to magnify more than 1:1 ratio in live view, which is already where I don't understand whether I'm pitch focused or not. I'll try to take some daytime shots tomorrow as I unfortunately can't test it in daylight because of work.

Many thanks again, and I'd send you those test shots if you need, just tell me how

#5 maroubra_boy

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Posted 07 December 2021 - 02:34 AM

If you are not able to focus sharp then you may be needing a focus mask, such as a Bahtinov Mask.  These go over the aperture of the scope and it generates a diffraction pattern.  When the pattern is perfectly symmetrical then focus is as sharp as it can be.  This is done via the camera on your monitor.  See this:

 

https://en.wikipedia.../Bahtinoav_mask

 

You can buy these specifically for your aperture size or you can make your own.

 

It can be difficult to focus a scope.  This mask and others like it take much of the guess work out of this.  Once focused you remove the mask and away you go.

 

Collimation may be a cause, for which there is plenty of info on how to check and deal with it.  Cooling is overcome easily by insulating the OTA - no cool down wait period, use the scope straight away.  Plenty of info on this in the Cats and Casses forum.  I have a 127 Mak and a 9" Mak and I have insulated both - no wait period.  I can rip as high magnification as seeing conditions will allow.

 

Alex.


Edited by maroubra_boy, 07 December 2021 - 02:35 AM.


#6 marcymonta

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Posted 07 December 2021 - 07:28 AM

I'm 90% sure its not a matter of collimation.

And I already own a bahtinov mask, but for my manual lenses, which means that it doesn't fit such a larger aperture. I will obviously consider buying a bigger one in case I won't return the scope.

Thing is, I think I can't manage to get that mask within the ongoing week, meaning that I need different ways to test for the focus precision :/ are there any?

I'll try harder tomorrow anyway.. in plain daylight hoping to face acceptable weather, no fog nor haze. Let me know if there's any chance to mimic the bahtinov mask role (or emulate a diaphragm mechanism with an aperture mask forcing a bit of diffraction to occur, hoping to scale it down right enough to ensure that its not a matter of glass conditions.. even though I dont know how for it is a catadioptric, would it be a donut + a circle? just one donut? just a circle? Is this absolute nonsense? ahahah probably pls help this is driving me crazy)



#7 nighty

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Posted 07 December 2021 - 11:51 AM

Use cardboard, paper plate or anything to adapt the mask you have to the bigger scope. For testing 3 hour cool down is about the minimum for all parts to reach the same temp as the air if the inside temp and out side temp are different by a lot. Maks take a long time to cool to perfection. For visual an hour may be enough. For critical testing more time.

 

Also what about camera shake? Can you drop the mirror before opening the shutter? 




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Also tagged with one or more of these keywords: Astrophotography, Beginner, Cassegrain, Celestron, Collimation, Equipment, Reflector, Optics, Imaging, Maksutov



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