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  #1  
Old 30-07-2018, 12:20 AM
Jasp05 (Aaron)
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How to know if OTA is a decent scope

Hi guys. I bought an eq3 mount a while back that came with an 8 inch reflector for 100 bucks.

However the mount nor OTA has any markings as to brand or specs.

I pulled the mirror out of the ota and measured focal length. (Set up a lamp with a piece of white card next to it. And moved the mirror in till i got sharp focus on the paper). Measured 1500mm or there abouts. So i assume its an 8 inch f4 scope.(half the distance for focal length - approx 750mm. Seems odd so assumed 800mm fl).

Mirror has seen better days, and focuser is only single speed. Is it worth upgrading the focuser to a dual speed and try using it for some astrophotography. Ive tried to find focus with my dslr attached and its a very fine threshold to find. I think too fine for the single speed focuser to get. (Put it this way.i haven't been able to snap an infocus image of the moon with it yet. (Although i have only tried with a quick collimation).

And how much would collimation play into being able to get sharp focus on such a "Fast" scope. (There is no centre mark on the mirror and i am yet to mark it. So have eyeballed collimation with a laser collimator for now).

Do you think i've picked up a bargain and would be worth throwing some cash at it to spruce it up? Will be perched upon an heq5pro if it gets used.
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  #2  
Old 30-07-2018, 12:28 AM
glend (Glen)
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Sounds like junk to me, sorry. You need to consider the cost and aggregation of trying to get it to work. Put it out for the council cleanup day, someone might pick it up.
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  #3  
Old 30-07-2018, 06:35 AM
Tropo-Bob (Bob)
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A photo would give others a better idea of what you are dealing with.
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  #4  
Old 30-07-2018, 10:46 AM
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mental4astro (Alexander)
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Aaron,

I don't understand how you come to the conclusion that the mirror is f/4. If the image came to focus at ~1500mm, then that's the focal length, and f/7.5. But this is ONLY an approximation as the actual focal length is determined by the light source being at infinity - having the light source so close and the focal point is shorter! A better way to do this is take the mirror out of the tube (you can wash it at the same time is it's dirty - there's an article about this in the Articles section in the left margin) and do the same thing that you did with a lamp but with the Moon this time. This will give you the true focal length of the mirror.

Yes, a photograph or two will help us with sorting things put.

A laser alone won't allow for true collimation. It won't deal with the secondary mirror! You may think that the secondary is set up properly because of the laser beam is on the primary, but this is actually a false reading. The laser will not tell you if the secondary is properly centered in the focuser nor if it is square with the focuser. This last point is most critical as it needs to be correct before the primary mirror is tweaked. This was a lesson that took me a LONG time to figure out. I too relied entirely on the laser when I first got it, but the "collimated" image was still poor and getting worse until I figured out what the problem was - and unfortunately this part very few people ever mention!

If the mirror had seen better days, then just use this OTA for practice. Cut your teeth with it ith your photography, ad that's all it is good for if the primary isn't in great shape. And if the mirror isn't brilliant, don't do your money on a new focuser as the investment isn't a good one.

We can't really offer much more help unless we know what your gear looks like.

Alex
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  #5  
Old 30-07-2018, 10:51 AM
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Merlin66 (Ken)
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Alex,
He seems to have tested the mirror at the centre of curvature which would yield a distance of twice the focal length......
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  #6  
Old 30-07-2018, 11:04 AM
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mental4astro (Alexander)
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Ah, thanks Ken,

As for figuring out if your scope has good optics, first it is best if the optics are clean - the condition of the silvered surface isn't as critical to determining the quality of the figure, but of course if the silvered surface isn't too good, this will reduce the reflectivity of the mirror, and introduce a lot of scatter making the image "hazy" and with reduced contrast. This can be fixed though by re-silvering/re-aluminizing the mirror.

The figure of the mirror is what determines "quality". And the easiest and quickest way is by doing a star test. Seeing conditions really need to be very good in order to get the best evidence. Collimation is CRITICAL here too!!! Unless this is done properly, with the secondary mirror* looked at first and only then the primary mirror, then a star test is useless.

* To get the secondary mirror right, the easiest tool to use is the humble Cheshire eyepiece. Full collimation can be totally done by using a Cheshire, but a laser helps things with the primary mirror and tiny tweaks of the secondary. But remember, a laser alone will not get the secondary mirror right, and the secondary NEEDS to be looked at first before the primary.

Once the scope is properly collimated, and seeing conditions are stable, bring up a very bright star in the scope, and using a medium to high power eyepiece you defocus the image of the star by moving the drawtube in and out to produce a large "donut". A good set of optics will show an identical round and symmetrical donut with the eyepiece inside and outside of the focus point. Variations in the symmetry can indicate problems. This is why collimation needs to be spot on as a star test will also show any collimation errors! However, star testing also needs some understanding of what to look for. You can do a search for star testing to help you with this. Star testing is just one tool for gauging optical quality, and not comprehensive by itself. There's other bench-top ways to do this testing, and I'll leave it to others to add to this.

Alex.
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  #7  
Old 30-07-2018, 06:41 PM
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doppler (Rick)
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Seeing that that the mount and ota are unbranded, some pics of the setup would be best to determine what you actually have.

A few years ago there were a lot of cheap black tubed 8" f4 scopes with green painted eq3 styled mounts on the market, these had spherical mirrors and were impossible to get to focus with a camera.
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  #8  
Old 31-07-2018, 10:53 AM
Jasp05 (Aaron)
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Quote:
Originally Posted by doppler View Post
Seeing that that the mount and ota are unbranded, some pics of the setup would be best to determine what you actually have.

A few years ago there were a lot of cheap black tubed 8" f4 scopes with green painted eq3 styled mounts on the market, these had spherical mirrors and were impossible to get to focus with a camera.
Hi Rick,

I do believe you have hit the nail on the head there. The mount is a light pastel green colour with a black tube. The person I bought it from had an invoice for like $600 - $700 brand new as a setup. there was a company that sold them, which I found through googling, although with no labels could never confirm if it was it.

But buy the sounds of it its a visual scope (at best) and will invest funds towards a new scope.
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  #9  
Old 31-07-2018, 02:09 PM
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sil
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It'll be a common generic consumer telescope. It'll be nicely built and branded it it was a quality product and would have cost much much more new and second hand. Doesn't mean its crap, the problem with the low end telescopes is not the tube usually (especially with reflectors) the problem is with the eyepieces. its certainly usable but not a wise purchase to begin with. My first astrophotos of planet was with something cheaper and nastier, but with good Baader eyepieces it gave crisp spectacular views.

The biggest problem with scope (any many will disagree) is "optics", meaning the mirrors and lenses that bend and focus the light. each optical part degrades the quality of the light that ends up in your eyes, most of which are inside the eyepiece. people assume its the big tube at fault instead. On a reflector its mostly just mirrors the photons touch and making reflective surfaces is fairly easy at consumer grade sizes so there is typically good transmission of light with those. However the clear/magnifying elements (the "lenses" ) have to allow photons in the front and out the back but all transparent substances refract light (ie want to split white light into rainbows), glass is better than plastic which cheap optics systems often contain. plus different chemical formulations of the glass perform differently. More elements in the train are often used to try to recombine the photons from rainbow to white again as light travels through them. Same as in camera lenses. So usually the front element refracts the light a lot and the light at the edge gets bent more and starts to split quicker than in the middle. the next element tries to correct that and get the photons travelling parallel again to the next focusing element with another corrector etc. the goal is for photons to be travelling parallel (at the same speed) when they exit the end of the optical train just as they were entering the front so the image is crisp and undistorted wshen it reaches your eye or a camera sensor.

A cheap reflector is mostly mirrors (easy cheap to make) and very few lenses (very difficult and costly to make) then the eyepiece is all lenses. A cheap refractor scope is all lenses plus the eyepiece so theres nothing much possible to improve those.

so the let down with a bargain reflector is likely to be with the eyepieces, a quality eyepiece will vastly improve the views and I think a camera should work fine too (but it'll be very high magnification as it acts like a 6mm eyepiece and atmospheric distortions come into play in a big way when you go below 10mm.

If you're serious buy a Baader eyepiece (very good value for money, they dont make any crap budget eyepieces at all. Pick up a 10mm followed by something around 25-30mm and thats all you need mostly. Eyepieces come in 1.25" and 2" standard sizes, so your current scope probably uses 1.25" as do most scopes under $1000 I think, so the eyepieces will work in any future scope you buy pretty much. Or you can waste money on cheap eyepiece sets assuming lots of eyepieces is good value and you'll spend heaps and a ton of time coming to the conclusion if you'd ignored the shiny new toy impulse and bought something of good quality straight away you'd have been up and running with great viewing years earlier. You'll notice most people have Televue eyepieces because they know the truth of good eyepieces and TVs are high end quality at a good price. There are better technically which can cost many times more (there is no limit to spendng in astronomy) but its the plateau everyone typically reaches, just up to you how many times you want to "upgrade" on your way to the same finish and what that'll cost. Baader had a great midrange set when I started and grabbing a few of those and their Zoom eyepiece elevated my astronomy greatly and with my failing eyes I didnt progress to the Televues. Baaders are down in price these days and have better quality range, morpheus i think its called, And I think this would be a wise buy right now. a 10mm first to give you crisp high magnification under most "seeing conditions", and something around 25mm to 30mm for a wider view of large items and just to help you star hop and navigate with visual (very hard to do with 10mm) and i think those two cover the biggest needs of everyone for their lifetimes so buying smart on these will suit you forever. any scope you buy will have kit lenses I bet are worse than the Baaders. Ideally a baader zoom but it can have problems getting to focus on some cheap scopes with poor quality focusers, but their regular eyepieces should be fine. Optically the zoom is not noticably worse quality to my eyes and its easier than swapping around eyepieces. With a zoom you can keep note of the ranges you use all the time and then use those to guide your purchase of the right size Televues for your needs.
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  #10  
Old 31-07-2018, 02:55 PM
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doppler (Rick)
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Quote:
Originally Posted by Jasp05 View Post
Hi Rick,

I do believe you have hit the nail on the head there. The mount is a light pastel green colour with a black tube. The person I bought it from had an invoice for like $600 - $700 brand new as a setup. there was a company that sold them, which I found through googling, although with no labels could never confirm if it was it.

But buy the sounds of it its a visual scope (at best) and will invest funds towards a new scope.

I have one of those in my spare parts cupboard, mine was in pristine condition and I paid about the same as you. Here's a link to my thread.
http://www.iceinspace.com.au/forum/s...ght=dud+mirror
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  #11  
Old 31-07-2018, 05:06 PM
Jasp05 (Aaron)
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Hi sil. I hadn't bought the setup for the ota. I bought for the tripod and eq head more than anything. (I was messing with a homemade tracker for a while). Ota was a bonus.

I have an 8 inch dob for visual and have some half decent plossl eyepieces.

But i will keep playing with it. See if i can get it to focus with the dslr and just play around. Its been sitting in the shed for the last 6 months untouched.
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