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PostPosted: Tue Jul 24, 2018 2:07 pm 
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Hi @ the SPA forum

I have been playing around with a Celestron First Scope (FL = 300 mm; Apert = 76 mm), my iPhone 6 & a Svbony Metal Universal Camera Mount Telescope Digital Camera Adaptor (which is very fiddly to set up!).

I took out my 2x Barlow lens, inserted a Celestron 20 mm FL len with adapter/iPhone attached only to discover a black blob (please see dreadful images attached) in the centre of the image. I could also see the blob if I looked directly into the EP & looked 'around' the FoV, but it wasn't centrally placed. I cleaned everything I could get my hands on, but the problem remained: no amount of tiddling with the focus got rid of it. After faffing around & trying a Meade 40 mm FL EP & getting the same result, I decided to try again plus Barlow lens - the blob 'disappeared'. I've never seen this blob before, but then never used the scope without the Barlow.

Am I right - probably not - that this is some kind of weird, optical, focussing issue/abberation (sorry, not a very technical explanation) with 'long' FL lenses plus such a short 'scope? I've done an internet search & the nearest thing I can find is that the blob is a 'image' of the secondary mirror.

Did O-level physics 40-ish years ago, so have forgotten much of the optics stuff - I'm so embarrassed :roll:

I'm keen to learn anything astro-related so can anyone shed some light on this mysterious blob?

Many thanks in advance.

BTW: I do a have a larger Newtonian reflector in storage awaiting the dark winter skies :)
The little scope is for playing with &looking at the moon :P


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PostPosted: Tue Jul 24, 2018 2:30 pm 
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Location: Lancashire
Here's a daft reply. There was a scifi film in the 1950s called "The Blob", but I can't remember how they got rid of the wobbly alien. Maybe the desperate townsfolk added custard and ate it. :lol:
Sorry to seem to be frivolous Sue, but it's a bit of a mystery. Maybe it's due to the exit pupil being too big and producing the equivalent of the "kidney bean" effect.

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Last edited by brian livesey on Fri Jul 27, 2018 9:56 am, edited 1 time in total.

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PostPosted: Tue Jul 24, 2018 2:43 pm 
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Hi Brian

I think you might be right about exit pupil effect :)

Otherwise I might spray it with sea water as per the Quatermass :roll:

Thanks for taking time to reply.

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PostPosted: Tue Jul 24, 2018 3:43 pm 
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The Blob is a famous movie because of the early appearance of actor Steve McQueen.
The special effects are very basic.

The blob in the scope could be a UFO or maybe an eclipse.

More likely the diagonal.

I think that these small scopes have big diagonals relative to the objective.

An 80mm f/5 refractor may work better.

Regards,
David


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PostPosted: Tue Jul 24, 2018 6:24 pm 
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Location: Wellingborough
Is this a reflector you are using? So no diagonal? From what you say I think this is the case. Looks to me like its a focusing problem, the background is not in focus and the blob should not be seen at true focus.

Found this:
https://www.youtube.com/watch?v=CiQoAxby8Rk

regards,

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PostPosted: Tue Jul 24, 2018 6:37 pm 
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Hi Brian

yes, it's a reflector... shall check out thUtube thing later.

Thank a lot :)

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PostPosted: Wed Jul 25, 2018 5:56 am 
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Location: Portslade, Sussex Lat 50deg 51min Long 0deg 13mins West
Low magnification on very small instruments with a secondary mirror obstruction , either in Newtonian or Maksutov-type configuration, always produce this very annoying shadow. Using a higher magnification improves the problem but the image will them be poor. Even on a large reflector with a secondary obstruction it is impossible to use a very low power eyepiece without this annoying blob appearing. Regards maf.


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PostPosted: Wed Jul 25, 2018 6:43 am 
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Hi Mike
Thanks for your input - much appreciated.
I noticed the shadow ‘disappear’ at higher magnification.
Just glad that there’s nothing gone wrong with my ‘scope.

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PostPosted: Wed Jul 25, 2018 6:50 am 
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Hi Mike
Thanks for your input - much appreciated.
I noticed the shadow ‘disappear’ at higher magnification.
Just glad that there’s nothing gone wrong with my ‘scope.

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PostPosted: Wed Jul 25, 2018 10:17 am 
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There is NO SUCH thing as a DAFT question, just remember that.

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PostPosted: Wed Jul 25, 2018 11:37 am 
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Thanks Skyhawk :) I was just a bit embarrassed in case I was doing something stupid :oops:

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PostPosted: Wed Jul 25, 2018 4:52 pm 
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Three weeks ago (I am a PRO photographer and NUJ REGISTERED journalist) and shoot for a glossy magazine, I went out with the intent to shoot Kites (birds).

D4s, D810, 150-600 Sport lens, arrived 40 miles away, NO CF card in cameras.

AND my son had taken the spares out of the car when cleaning

And YOU may feel STUPID :) :) :)

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Celestron 8" Edge HD Evolution, Esprit 120mm triplet, 72mm APO, Sky Tee 2, 6" reflecting scope, William Optics Binoviewer, Quark Daystar Ha Chromosphere on 72mm ED, LVW8mm eyepiece and Celestron 19mm Axiom, matched W.O 10 and 20mm, and a few others, D4s, D810,


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PostPosted: Wed Jul 25, 2018 5:18 pm 
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:lol: :lol: :lol:

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