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 Post subject: Cheap and Cheerful Scope
PostPosted: Fri Jun 24, 2016 9:31 am 
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Joined: Sun May 11, 2008 6:11 pm
Posts: 3260
Location: Portslade, Sussex Lat 50deg 51min Long 0deg 13mins West
During a long period of unpleasant weather, I was looking for some project to fill the time until the sky was clear again. A Heritage 76mm Newtonian mini-Dobs. had been in the display case at the shop for some time and then was moved to the window. Finally it was in open-display and after giving the once-over, I decided to buy it to investigate just how good or bad it was, especially as was now it in the sale at £35.00. It looked almost unused and came complete with box and two eyepieces. These were the standard two wide-angle 10mm and 25mm supplied with most cheap telescopes.
Once home, it required no setting-up, and was more carefully examined and seemed reasonably collimated and the mirrors were untarnished. The first, obvious problem was that it was designed to be table-standing and with no tables in the garden or window-sills in the house, it meant either balancing it on the rubbish bin or dragging a small cupboard to in front of the window. Even then, getting height right meant standing on a box or stool as the eyepiece location was on the top of the tube. Recently a similar problem occurred when I tried to use a hide clamp mount for a scope to project the solar image, and that was even worse because this needed an horizonal surface just at the right height and thickness!
The final solution with the Heritage Dobsonian was to remove the wooden base plate and discarding this, and having found a metal plate with screw-holes of the standard size that I had rescued from on old scrapped instrument, I bolted this onto the bottom of the upper plate and was able then then to simply fix it to the top of a standard, solid photographic tripod. It was now no longer a mini-Dobsonian but I could now use it without moving the furniture around or noisily trundling the rubbish bin around the garden!
The next problem concerned the Newtonian design itself . Using the 25mm low-powered eyepiece supplied with it, giving a magnification of x12, the image was clear and bright but the large size of the obstructing secondary mirror meant that, in the bright daylight, its ghostly out-of-focus shadow danced across the field of view however closely I positioned my right eye to the lens. I had a similar experience with a 10x30 Mak monocular many years ago. Replacing the 25mm eyepiece with the high-powered 10mm, giving x30 , the shadow disappeared but the image was quite poor. I looked through my box of eyepieces and tried them all in turn. The 15mm at x20 was reasonable when viewing the landscape in sunlight. Once the sky was fairly dark, the shadow was no longer a problem. The Gibbous Moon was nicely shown using the 20mm + Moon filter which revealed clearly the features on the lunar globe, sailing in perhaps a 3.5 degree fov and at x15. The image was of course laterally transposed (back-to-front) and, whether inverted or (almost) erect depended on how one was located at the eyepiece! Obviously not a high-quality 'scope but have spent a number of enjoyable hours playing with it! M13, M31, M92 found easily even in Full moonlight and the nesting Herring Gulls on the roof opposite nicely shown. Cheap and cheerful you might say, but once the initial problems are overcome, not just a toy! regards maf


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PostPosted: Fri Jun 24, 2016 9:56 am 
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Joined: Mon Jan 09, 2006 11:05 am
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Location: Lancashire
Like yourself Mike, I did a bit of a DIY job on an ALDI 76mm/f.4 Newtonian ( reduced from £55 to £25 ), by removing the 'scope entirely from its alt/az mounting and attaching it to a camera tripod.
The supplied Huygenian eyepieces were a joke ( made worse used at f.4 ), but with good eyepieces the 'scope gave reasonable low power views at the centre of the field. I've since dumped it.

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PostPosted: Fri Jun 24, 2016 12:59 pm 
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You could have had a good shaving mirror.

Regards,
David


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PostPosted: Sat Jun 25, 2016 8:41 am 
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Joined: Sun May 11, 2008 6:11 pm
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Location: Portslade, Sussex Lat 50deg 51min Long 0deg 13mins West
I came across a true story once of an astronomer who stayed at a motel and when he went to sink in the morning, found that the "shaving mirror" on the wall was indeed quite a large parabolic telescope mirror! I rarely rarely shave as such- only tidy-up from time to time, being one of those old "grey-beard astronomers" that apparently haunt all the astronomy-group meetings! regards maf
PS: even worse that the eyepieces were.....yes you guess it........the finder! One of those straight-through, plastic jobs. The "X"-hairs were in focus but the objective was a horrible single plastic lens stopped down to about half-an-inch! These appear on most small, cheap telescopes! There was little need for it anyway with the large fov and, even if it was a super-quality finder, it would have been hard to use anyway on the small dobs stand! maf


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PostPosted: Sat Jun 25, 2016 1:30 pm 
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Location: Lancashire
The ALDI 'scope also had one of those useless plastic finders, but all was not lost. I junked the finder, but adapted the plastic pillar that held the finder into a tiny pinhole finder for solar observing.
Half-way down the pillar, there's a little tube that's part of the plastic molding; it probably adds strength to the pillar I glued a small piece of black card on the front aperture of the tube and made a pinhole in it with a needle. At the back of the tube I glued a piece of thin white paper to act as a tiny screen. I then mounted the pillar onto the Stage 2 PST-modded 70mm refractor. The pinhole finder works just as well as the PST's obstructed built-in pinhole finder.
The PST's finder can't be used because the added diameter of the 70mm refractor gets in the way by blocking the light to the pinhole.
I did the same thing with an identical plastic finder pillar on the LUNT/PST double-stack. The LUNT etalon housing is quite a bit bigger in diameter than the PST objective cell, so it too blocks the light to the built-in PST pinhole finder.

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PostPosted: Sun Jun 26, 2016 8:10 am 
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Location: California
Great review for a cheap scope. Thanks for sharing.


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PostPosted: Sun Jun 26, 2016 9:55 am 
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Joined: Sun May 11, 2008 6:11 pm
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Location: Portslade, Sussex Lat 50deg 51min Long 0deg 13mins West
Having learnt a lot (see below) and entertained myself over the last couple of weeks investigating this cheap scope, I finally decided to follow Brian's example and dump it! Or perhaps disassemble it would be a better description in my case ! Basically undid all of the screws, saved the eyepieces, the main mirror and various bits and pieces which I added to my "box of bits", so they may be useful in the future. The mirror would indeed make a good magnifying shaving mirror....and can be amusingly used to scare members of the household by asking them to look in it........"no do not hold it in the distance, put it in until you face is in focus!" said I........this was followed by "aargh........" and various other expletives. Also I noticed that I really must get that cracked tooth fixed very soon!
The personal points I must remember for the future are
(1) Table-top telescopes should be avoided (unless to stand as an ornament on you desk or bookcase.)
(2) Avoid small reflectors with obstructed secondary,as the floating ghost-shadow of the secondary is very annoying in daylight unless one uses a high-power eyepiece, and then a small cheap scope cannot cope with this.
(3) I personally disliked the transposed image, having used terrestrial-type spotting scopes recently.
(4) The use of standard size eyepieces was plus and other eyepieces could be used (unlike most spotting scopes) and this meant that a screw-in moon-filter could be used but why all cheap scopes come with a cheap 10mm eyepiece I do not know.
(5) The position of the eyepiece more or less on the top of the short tube was quite handy, especially if one wanted to look high in the sky, but the transposed image and sometimes inverted image made it difficult for me to move around familiar star-fields.
(6) I imagine that the large secondary mirror must considerably reduce the amount of light hitting the main mirror and dims the image.
and so on! regards maf


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