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Hi, I have changed the focuser on my newtonian but its too short.
I realise that I can just add an extension tube to it but I’m not sure if that is the best solution.
I also seem to have have a stray light problem (I’m getting a hazy image on a 5mm eyepiece).
A third problem is that I find it quite cumbersome that the focuser has to stick out so far (155mm+eyepiece).
I wonder if I might solve the stay light problem by extending the main telescope tube instead of the focuser and therefore getting the eyepiece closer to the secondary. And also maintain more precision in the focuser by not extending it.
As it stands the focuser would have to come out 265mm from the centre of the secondary mirror in order to focus. The secondary is 50mm wide. From primary to secondary is 950mm approx.. The primary is 200mm Thanks for reading. Any advice will be much appreciated.
I'm aiming for the best possible field illumination for my Explore Scientific PN208 f/3.9 imaging newtonian. The scope is 5 years old now and while doing maintenance I wanted to do something about the design flaws as well.
With the current secondary holder design it's impossible to center the mirror beneath the focuser. My question is: is this really an issue when the entire primary is visible? I read about optimal field illumination when the secondary is centered, but this isn't achievable with this telescope.
I attached an image showing the (very bad aligned and already degrading) secondary at the moment. This is as far down as I can go, I would need to replace the holder screws with longer threads to go further but I don't know if it would make any difference for field illumination?
Thanks in advance,
The focuser on my SkyWatcher 150i is a basic rack-and-pinion, unsurprising for the price point, but sometimes a bit of a pain to control finely enough. I’m not looking to spend any serious money upgrading it, but I did want to see what I could tweak.
The first thing I did was to slacken off (slightly) the screws holding the plate against the spindle, as the operation was very tight when new – that helped a bit (and I think that without doing this first, the “friction fit” approach described below wouldn’t have worked). I will eventually get around to taking it all off as per AstroBaby's tune-up.
Improving the fine control without a major change means doing something with the focusing knobs – they’re quite small, so the effective “gearing ratio” when you operate them is on the harsh side. Some folk have described fitting larger diameter replacements, either bought or made, and even using ones with a planetary-style mechanism to achieve a reduction in the ratio. I didn’t fancy this, as I couldn’t see how the existing knobs were attached to the spindle without trying to prise them apart (possibly terminally). The other option is to increase the effective diameter of the existing knobs, for which purpose a clothes peg is apparently quite popular, but I’ve also come across descriptions of chop sticks inserted into holes drilled at intervals into the circumference, and punctured lids from peanut butter jars.
I wanted something that was cheap, relatively tidy and non-destructive. The answer seemed to be some sort of thick sleeve that I could fit over the knob. It would need to be a tight fit so as not to slip in use, to be not so large as to foul against either the focuser tube or the main OTA, and to be thick enough that it didn’t flex sideways when grasped. I thought I might find some larger rubber washers that would do the job, but none were thick enough to be rigid in use. However, a bit of searching found these spacers that are apparently used in vehicle shock absorbers.
My calipers said the diameter of the focuser knobs was around 29.5mm, and the nearest spacers that were available had an internal hole 30mm and outside diameter 60mm. I ordered one that was 10mm thick, not quite as deep as the knobs, but which allowed a bit more space on the inside edge for free operation. I’d hoped the internal hole might be a but undersized when it arrived but it was spot on, so I wound five or six turns of masking tape around the knob first. To avoid taking the tape off when fitting the spacer, I positioned one side first and stretched it across the face as I pushed. When it’s flush with the knob’s outer face, it’s just clear of the focuser body and OTA. There might be enough room to stick some kind of friction surface around the outside to improve the grip, but I don’t think it’s going to be necessary.
I decided to do only the one knob, so I now have a very Noddy “dual speed” affair. Because the clearances around the fitted spacer are quite tight, it’s worth checking the positioning of the spindle in the focuser body first – mine was fractionally off centre, so there was more room one side than the other (assuming you have no preference).
Hello, I am having a problem with a relatively expensive flat field eyepiece in a combination with a newtonian reflector. I am unable to achieve proper focus with that eyepiece because when I focus it on axis, the edges are blurry. When I focus it at the edge, the center field is out of focus. I don't want to mention the concrete type of this eyepiece because I received it with a spot on the bottom barrel, therefore I cannot be sure whether it has been dropped. This eyepiece has very good reviews on this forum. Optically it looks good and there is no rattle. My question is whether these flat field eyepieces are usable with newtonians as these telescopes doesn't generally suffer from the field curvature. Many thanks for help.
By Anthony RS
I'm selling my my 70mm secondary mirror I used with my 150mm F4 newtonian for 100$ (final price). Shipping expenses (from Beirut, Lebanon) on me for free provided I use LibanPost for shipping. We can discuss using other shipping methods and maybe splitting the cost.
The mirror is brand new, still have the original package and box, bought the telescope a month ago and used only twice. No scratches, not even dust (images attached). Excellent mirror, no issues whatsoever, 94% reflectivity and optimized for the offset of f / 4 Newtons, but it can be used up to f / 5. It's from TS, made in Taiwan, original price is 134 euros (including VAT) excluding shipping and custom fees. Link below:
It does not come with the tilt screws and center screw though unfortunately.
I can also sell you the spider vanes (1mm thickness) for 6 inch newtonian for 25USD. Also brand new but without the tilt screws and adjustment screws. https://www.teleskop-express.de/shop/product_info.php/info/p11056_TS-Optics-Spinne-fuer-6--Newtons---164-mm-bis-180-mm-Tubus-Innendurchmesser.html
Let me know if interested.
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