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Topics - Scott Bentley

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It seems quite a few premium eyepieces have very sensitive eye placement issues with blackout and kidney bean. My 11 DeLite was picky, and so is my 17mm ES 92. Conversely, my Panoptics, Plossls, and 12mm Nagler Type II are extremely easy to look through and have no eye placement issues.

Is it just something inherent with long eye relief eyepieces?

Hi CN community again,

I have a son who will soon turn 5. I was planning on introducing him to the wonderful world of astronomy, and thought that it would be a wonderful way to also learn about optics. Does anyone in the community have experience with introducing kids this age to how these things actually work (I mean telelscopes etc.). What resources have you found to be useful for discussion of such topics?Update 2016-04-03:

Since I had the question in the beginning, I started looking as well. The wonderful world of the internet means that someone has thought of these things before and so ideas can spread quickly. Following, I am listing some sites I have found that may be useful for other parents who may do the same: That is to say leisurely and playfully teach optics to kids.

Websites with optics material for kids:

Khanh Academy:
This guy has never disappointed me. Hi is really good in explaining things.

Michael Biezen:
More traditional lectures which I am not sure is too kid friendly but IS cool refresher material for the grown-up kids.

Eyepieces Questions & Recommendations / Need to try a Brandon
« on: January 03, 2018, 03:27:08 PM »
I've been waiting patiently to try one of these before plunking down some serious dough for the set. I have always wanted to own the set because... 1. They are made in the USA. 2. They look good.  and 3. Some have said the views are better than Zeiss and the Supermojos!!

I attend at least 1 star party a month and the story is always the same... "You don't happen to have a Brandon eyepiece in that case do you?" . "Never heard of them... Are they nice??" or "I have a friend with a few, but he's not here tonight.. "

So far, to me it seems these eyepieces are as rare as their users.

Mounts Questions & Expirience / SkyWatcher Star Adventurer Polar Alignment
« on: December 31, 2017, 06:29:11 PM »
Hi guys,

I just acquired the SkyWatcher Star Adventurer but has no clue as to how to do the polar alignment accurately with this mount. I have used the iOptron SkyTracker and I find polar alignment with that mount more straight forward. For one, because the polar scope of the Star Adventurer rotates along the R.A axis, how do I make sure the 0 on the reticle is aligned with the one on the app (I use PSAlign on iPhone)? I think I am supposed to be using the dials on the eye piece, but I am not sure how to use them. Any advice?


What happens if you put too heavy of a scope on a mount? If one balances the setup appropriately and the mount/tripod combo is steady enough for visual use (high power), what's the downside? It's not like the tracking motors are working too hard, or is it?

In my case, I'm mounting a 5" triplet (f/6) weighing in at 22# with finder and rings on an old Takahashi EM-2s mount (non go to) rated at 7kg. The current EM-11 is about the same size (but supports go to) and is rated at a recommended 6kg (9kg max) or even 25# on some websites. Tripod is the Tak fixed length wooden model used with the EM-200, which, by the way, appears to use the same dimension legs as the wooden tripod for the EM-400.

If what I'm doing is a no no, why is that?

Please advise.


ATM, Optics and DIY Forum / The Dual Fibre point diffraction Interferometer
« on: December 29, 2017, 11:48:29 PM »
The dual fiber point diffraction interferometer is an interesting interferometer that uses a pair of coherently illuminated single mode fibers to produce diffraction limited spherical wave test and reference beams.
It uses no optics other than the system under test:

It uses a pair of coherently illuminated single mode fibres, one forms a spherical test beam that is used to illuminate the optic under test. The other forms a spherical reference that illuminates the image sensor, in effect forming a dual point source version of a Young's interferometer with the tip of the single mode reference fibre forming one source and the image formed by the system under test of the tip of the single mode test fibre.
Since ther are no optics to image the test system exit pupil onto the image sensor, diffraction artifacts are present. If the size, geometry and location of the edge of the mirror under yest is known the effects of diffraction can be removed mathematically, leaving the purely geometric optics effects from which the figure of the mirror under test can be deduced.
There is no retrace error as their are no auxiliary optics at all.
Either the laser used needs a sufficiently long coherence length to allow fringes to be formed despite the large OPD between the test and reference beams, or a means of reducing the OPD to a value well within the laser coherence length is required. Traditionally, as the Japanese did, an trombone style variable optical line is inserted between the beamsplitter and the reference fibre whilst the delay from the splitter to the test fibre is fixed.
If one starts with a fibre coupled laser diode followed by a single mode fibreoptic coupler/splitter one can compensate the OPD between the test and reference paths with a suitable length of single mode fibre optic patch cable inserted in the reference path. Such a custom length patch cable is considerably cheaper than a optical trombone delay line (or an interferometer reference sphere). Any residual OPD can be exploited to implement phase shift interferometry by modulating the laser diode current to vary the laser wavelength.
One issue (common with most interferometers) is the effect of reflections from the surfaces of any image sensor cover glass distorting the fringe pattern. If the window is AR coated then these effects are relatively small unless subnanometer accuracy is the goal. If the laser coherence length is reduced to 300um or less these effects disappear. However one then has to match the test and reference optical paths to a hundred microns or less requiring a small range optical trombone delay line as well as the patch cord.

The biggest challenge is in designing a suitable mount for the pair of optical fibres.
To permit bonding of the fibre tips without thermal expansion issues, I intend to use single mode fibres with an angle polished borosilicate ferrule on the tip (the other end uses a standard FC connector) bonded to a small tungsten alloy bar. The Zeiss instrument used an intricately machined invar fibre mount head. I have devised a simple to machine mount for the fibres which allows either clamping with a spring flexure or bonding of the fibre tips to the fibre mount head.

A Japanese paper on this is:
<a href="">http://cdn.intechope...ical_fibers.pdf[/url]
A thesis on this:
<a href="/">http://repository.tu...a-76934129ce27/[/url]

This Interferometer is intended for use in measuring the figures of a couple of 24" F/4 primary mirrors the figures of which are far from perfect. I intend to start with a small test mirror for which I have a 5 axis mount so as not to be distracted by test stand issues.

Beginners Forum / Beginner Getting Started!
« on: December 28, 2017, 11:54:47 PM »
Hello everyone!
My name is Leo and i have just discovered this amazing forum! Well done on everyone for the amazing work you have done all these years!
I have just purchased my first telescope which is the POWERSEEKER 127 EQ.To be honest my intention was to pledge more money on the telescope, however many of you here (in various threads) recommend to save money and
spend them on accessories for the telescope. As a beginner i will start observing moon and then move to other planets like saturn, venus etc.
Any recommendation from the more experienced members? What accessory you thing that i should purchase next?

Eyepieces Questions & Recommendations / Meade 14mm UWA
« on: December 28, 2017, 04:29:42 AM »
Hi, Everbody,

According to some reviews{not all} this eyepiece wasn't supposed to perform in the last 20% of the field of view.
This eyepiece{for me} is very good to the last 3-5 degrees. You would only know if you turned your head and looked.

The reason I don't like ES eyepiecs is because the eye relief isn't long enough. I see my eyelashes in the eyepiece and it's so distracting. The Meade UWA series have and a rotating top for a more flexable eye relief.

If your like me and want something at least good but around $100 give these a try. I like mine untill I can afford a TV
12mm with the adjustable top.

Clear Skies, De Lorme

Reflectors Telescopes Forum / Mounting a tablet to a Dob
« on: December 28, 2017, 02:39:03 AM »
I've seen a couple photos of people who have mounted their tablets to Dobs for either tracking control or just a nice quick reference to star charts. I love the access to star charts idea and I happen to have an old tablet that would work well. Unfortunately, most of the photos don't show the critical "money shot" of the actual attachment to the base and/or to the tablet. I haven't found a thread in the forum that seems to already have addressed this.I know there are thousands of ways to do this, but before I get started down a path, I'd like to learn from the mistakes of others. For those who have done this, what are the do's - don'ts and general things to watch out for? Are the inexpensiveE-bay mic stand tablet holders garbage or do they work well enough?

Thanks in advance, Clear skies...

Eyepieces Questions & Recommendations / Need Eyepiece Opinions
« on: December 28, 2017, 12:37:08 AM »
Hi CN...

First post from a total newbie and novice to Astronomy here. I just picked up my first telescope, a Celestron NexStar 130 SLT. This accepts both a 1.25" and 2" eyepiece. I'm looking for advice on eyepieces and filters.

Assuming I have a max to spend of $100 on a given eyepiece, what would you recommend in terms of brand/model, focal length, size tube, AND why/for what type of viewing? I am thinking 2-3 focal lengths to get started, but you tell me where to start.

Also, seems a ND moon filter is a no-brainer, but what other filters, if any, should I consider purchasing?

Do you need any other info in order to make recommendations?

Thanks in advance for your assistance!

- - clayshooter25

Sort of a theoretical question:

I was reading the specs for a field flattener for refractors, and I've noticed that the only parameter that decides the optimal distance between it and the camera sensor is the focal length of the instrument.
I thought it's odd. Does field curvature depend only on focal length? Or am I missing something? Is it different for doublets, vs triplet APOs, vs ED doublet pseudo-APOs, etc?

I am assuming that the strength of the correction depends on the distance from the field flattener to the camera sensor. Is this assumption wrong? Or is this one of those things where "it's complicated"?

And now the practical aspect: I've tested the FF linked above with an Orion ED80 refractor (F=600mm, pseudo-APO doublet) and an APS-C sensor. Based on the specs on that page, I've used a backfocus distance of 57.5mm. It definitely improves aberrations very noticeably, but it does not completely remove them. There's some residual aberration hidden in the extreme corners.

So, what's going on? Are those backfocus specs only approximate, and some experimentation is required? Are the residual aberrations due to other factors (though they do look like field curvature to me)? Or is the field flattener not supposed to remove field curvature 100%? (they claim an aberration-free field of 30mm, which is larger than the APS-C diagonal)

Mounts Questions & Expirience / CEM25 Discontinued?
« on: December 27, 2017, 11:45:05 PM »
Seems this mount is discontinued according to several retailer sites.

Is it because this has been a problematic mount?

Thanks to a very kind member of Cloudy Nights I am the happy owner of a 23mm Celestron Luminos 2 inch Eyepiece.
The build quality is very nice as is the fit and finish.

As I am gearing up for Mars and with my 4 inch refractor plan on concentrating on mostly Lunar observing,has anyone used this line of eyepieces for mostly Planetary and Lunar observing. If so how did they do?

Thanks for the help as always everyone.

Mounts Questions & Expirience / Help me with my SW AZ-EQ5 shakes problem
« on: December 27, 2017, 02:36:02 PM »
I have had this mount for a couple of years now. It's GoTos and tracking are fine. the problem is it shakes like crazy whenever I touch the scope to focus or in the slightest breeze.
I usually mount a classic C8 on it or the ES 127mm apo refractor. It's impossible to just focus while touching the focus wheel. The mount shakes like crazy. I have to focus - stop and wait a few seconds for the shakes to die down- look- focus- stop and wait...etc. The slightest breeze shakes things so that at all but the very lowest powers the mount is difficult to use.
A few points. When I first got the mount it had rather bad backlash in the azimuth plane as has been discussed in other posts. I was eventually able to adjust that out. There is no moving or clicking between the stops in azimuth now. (I only use the mount in alt-az mode). I have the lightest weight tripod the mount comes with. I don't know what that is but definitely not 2" legs.
I've been exploring what happens when the wind doesn't blow and the mount is steady. Even a slight tap on the tripod legs with my fingers sends it shaking. But the same thing happens if I tap the body of the mount, too.
Do I need a better tripod? Is there something wrong with the mount? I really need a fix as I observe in the desert where the wind almost always blows and gusts at speeds of 8-15 mph and this vibration problem makes the mount a disappointment most of the time..
/Ira, The Starman of Mitzpe Ramon

I have been trying really hard to get my 6-inch SCT to do imaging of deep sky objects. I like the objects I can get at around 1000mm focal length, such as planetary nebula and small galaxies. And it's a nice, compact, light scope, with good image quality, and rarely needs collimation.

With the reducer-corrector, I get it to f/6.7. At least that is what the field of view tells me. I've checked, and re-checked, and based on many images, I have 1005mm focal length. It is 150mm aperture, so that makes it f/6.7, right?
But the images all seem so dark. Even with 300 second exposures, and ISO 1600, my luminescence is less than 20%.

On the other hand, when I use an 80mm APO, at its native f/6, The images seem much brighter. I get about 33% luminance, or even more. And that's with a filter that really knocks the brightness down, especially the background, that contributes much more to the total luminance, than the actual object (at least for small objects).

So what the heck is going on?

F/6.7 shouldn't be that much slower than f/6? (only 25% in exposure time, by my calculations)

Am I getting so much vignetting, that only the very center of my field, with the SCT, is the speed of an f/6.7 optical system? But even the center seems much darker?

Look at the attached images, that I brought into APT, which shows the luminosity of the raw files. Even with the moon &amp; skyglow filter, the 80mm f/6 APO is much brighter than the f/6.7 SCT.

Both images were taken from home, so the same background sky darkness, etc. Yes, different parts of the sky, but both were rich with stars, and close to the zenith when imaged.

I think I've decided to give up on the SCT for deep sky objects, unless they are very bright. It seems I am losing too much light. Maybe it is death by a thousand cuts?

- two mirrors @ 90% is 81%, so a 19% loss
- secondary obstruction = 14%
- f/6.7 versus f/6 = 25%
- And throw in the vignetting from the small hole / central light baffle tube, and that's the final nail in the coffin...

I do own a really nice 80mm APO now (not the one used for the image). I think I am going to try that for a while. But so far, my best imaging scope has been a cheap 6-inch f/4 Newtonain astrograph.

Any insights as to why the SCT images are always so dark are appreciated!Attached Thumbnails

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