vlaiv

3 minutes ago, Xilman said:

A Newtonian can be, but rarely is, corrected for SA with lenses in the same way as its coma (absent in a spherical mirror) can be and usually is in fast systems.

Well, people do have good opinions on telescopes like Maksutov Newtonian and Schmidt Newtonian. Granted, those are full aperture correctors and not sub aperture correctors, but can be quite fast systems as well- often F/4-F/5.

2 minutes ago, Xilman said:

Not without corrective elements, they don't.  My  0.4m Dilworth contains nothing but spherical surfaces. A train of spherical lenses corrects for spherical aberration. Not sure of the focal ratio of the primary but guessing from the length of the OTA it is probably around f/2.5.  Take a look at http://www.astropalma.com/equipment.html to see what I mean.

For newtonian with single curved mirror - it is true that spherical aberration increases rapidly with faster optics.

https://www.telescope-optics.net/reflecting.htm

For paraboloid - it's equal to 0 but for spherical with K=0, we can see that it is inverse of third power of F/ratio, so telescope has to be really slow, or have small diameter.

10 minutes ago, TiffsAndAstro said:

Big aperture shorter tube length would suggest not spherical/catadioptric? Or the opposite? Or neither

Catadioptric has refractive glass element in optical train next to mirrors - usually as front aperture corrector - making tube closed. Sometimes this element sits inside focuser or in front of secondary - but then you'd have very short tube (Bird-Jones design) or Ruten Maksutov type (but you have newtonian, so it's not that).

If it's a longer tube - greater chance that it's spherical mirror. Fast telescopes and spherical mirrors don't get along. If it's F/8 or slower - then it might be spherical.

7 minutes ago, TiffsAndAstro said:

If I happened to stumble across an old newt with the idea of messing around with collimation *just for the lulz* is there an easy way to tell if it has a spherical mirror?

Two things come to mind - first is to check for off axis coma and second is to get Ronchi eyepiece:

https://www.firstlightoptics.com/specialist/gerd-neumann-ronchi-eyepiece.html

2 minutes ago, Nigella Bryant said:

I knew I was a nerd but not how much, lol 😂 

Ha-ha!

Less nerdy

(at least no one is as nerdy as I am - 0% scored the same )

34 minutes ago, Ags said:

One thing that puzzles me is that the gotos require so much correction from plate solving. I am almost perfectly polar aligned, so why doesn’t the goto get close. With a regular go to, you expect the target to be somewhere in the field of a wide eyepiece, but the HEM15 and AIR  are not even close. Plate solving papers over the errors, but it is puzzling. 

How fast is your slew to position?

Looking at the specs - stepper motors could be quite a bit under powered. This can result in skipped steps at high slew speeds. Steppers loose their torque with higher RPMs and since this mount does not use counterweights - it instead relies on stepper motors to provide torque to keep the weight in position.

From specs - max amperage when slewing is 0.8A - that is very low, we can say that it uses probably 0.3A per motor (some of current for electronics and some for motors - two of them).

Anyway, it slews at max 6 degrees / second - with reduction ratio of 360:1.

That makes one whole revolution in 60 seconds - or 1 rpm, so motor needs to spin at 360 rpm.

Here is example of 0.4A Nema 17 motor showing dependence of torque to RPM - at 360rpm pull out torque is 1/10th that of slow speed (motor is rated at 26N/cm). This is with 24V and 1600 micro steps (or 1/8th micro stepping) - things get only worse with finer micro stepping and when using lower voltage to power motors - like 12V

Anyway - try setting slew speed to 1-2 degrees / second instead to see if that helps with goto accuracy.

5 minutes ago, Xilman said:

If one star is a white dwarf, it could be moving towards, away from, or be stationary relative to the Earth.

A strong gravitational field also creates a red shift which may be larger than the Doppler-induced one.

 

I always wondered how large this effect is - particularly on galactic scales.

For example - when computing Hubble's law - do we have to take into account relative difference between galaxy masses? Origin galaxy and MW?

When light leaves origin galaxy it will be red shifted, but then when it "falls into" MW it will be blue shifted - difference between those two will be some percent of total red shift - but how large is the effect?

1 minute ago, mikeDnight said:

Unfortunately Vlaiv, my tablet won't allow me to open your link, which I'm sure would be highly informative as usual. 😆

I was aiming more on funny side rather than informative, but hey ....

(I edited the post and inserted the actual image instead of hot linking it ...)

35 minutes ago, mikeDnight said:

 I've never been so insulted - middle class nerd indeed!  I'll have you know I'm a working class nerd and proud of it. 😉 

Just to be sure on terminology there ...

17 minutes ago, ollypenrice said:

I agree that redshift/blueshift don't need acceleration (as Jim also points out) but a body's proper motion does, surely, need to originate with an acceleration? What I'm getting at is that the OP's question is about movement since he asks in which direction the stars are moving. Is there not a fundamental difference between a body 'moving' because of the expansion of space and 'moving' because it has, at some point, been accelerated?  If not, why do cosmologists distinguish between Doppler and Cosmological redshift?

Olly

I'm not sure that OP is concerned about origin of motion, but rather the fact that there is no preferred direction.

If you look at this post:

7 hours ago, Space Traveller said:

Consider this, Star B is moving to the right at a speed of 6 units.  The earth is moving to the right at a speed of 4 units and Star A is also moving to the right at a speed of 2 units, thus the stars show a Red-Shift.

However when we observe the stars, we are studying them from what I would describe as 'Thumb-Tack Earth', and there is nothing we can do about it.

I believe it shows what is the heart of the question. We tend to study stars from earth and study their motion relative to us.

Someone else might see different motion - after all motion is relative, and their Doppler shift would be in agreement with what they observe - as movement is relative.

22 minutes ago, ollypenrice said:

But where did this uniform motion originate?

Olly

It does not matter.

Consider this case:

We have a source of waves moving on a line from our right to left like in image above. At first - it is approaching - so there is component of motion that is in our direction. Source is blue shifted. At the mid point - source is still moving at constant speed - but component of motion in our direction is zero - so we receive normal frequency. After it passes this point - it starts moving away from us - now there is component of motion in our direction that is away from us - source is red shifted.

No acceleration takes place in above case - but we have 3 different scenarios happening: from blue shift thru no shift to red shift. This shows that Doppler effect in its essence is not related to acceleration (that once must have happened for object to be moving with respect to us) - but rather to component of its relative speed in our direction.

This is regular red shift - and what we observe when we look at for example rotational curves of galaxies (stars moving from/away depending if they are on one side or other side of the galaxy). It also happens when ambulance passes by - we first hear high pitched siren, then regular, then low pitched siren ...

There are other two sources of red shift - gravitational (or the one related to acceleration / curvature of space time) and cosmological red shift - which happens due to expansion of space. Last one is "equivalent" to regular red shift - as if galaxy is really moving away from us at certain speed. There are only a few indicators that the space is actually expanding - like fact that we can have different places recede faster than the speed of light - which would not be possible if galaxies were moving thru the space instead of space expanding.

27 minutes ago, ollypenrice said:

1) The Doppler redshift. This is created when the star or the observer (or both) are accelerated by some force. This kind of acceleration must have a direction and we are familiar with 'movement' in this sense.  In this case we would talk of the star's proper motion. (It's own motion.)

No need for acceleration - just uniform motion is enough to produce Doppler shift.

On the other hand, red shift (or blue shift for that matter) - can be due to acceleration - but we might not need force for that either - curvature of space time is enough to produce the effect. Light emitted from vicinity of large mass will be red shifted to observer that is far away, while light produced by observer floating in intergalactic space and observed near massive body (or even inside the galaxy) will be blue shifted due to it "falling" into gravitational potential well of mass concentration.

This is closely related to time dilation effects by the way (think of laser producing exact wavelength of light and those oscillations being slowed down due to time dilation and thus producing longer wavelengths - red shift).

3 minutes ago, Stu said:

Did I pass the test then Vlad?? 🤞🤞

Only if you do the analogy with moving train, three people on the train - one seated, one exiting car on one side and other exiting car on the other side versus bystander watching the train go by

Who will see what?

19 minutes ago, Space Traveller said:

Sure they are moving away from us but in which direction.

Based on the basic diagram is Star A moving to the right and Star B moving to the left.

Consider this, Star B is moving to the right at a speed of 6 units.  The earth is moving to the right at a speed of 4 units and Star A is also moving to the right at a speed of 2 units, thus the stars show a Red-Shift.

However when we observe the stars, we are studying them from what I would describe as 'Thumb-Tack Earth', and there is nothing we can do about it.

Depends on where you put observer.

If you stand on earth - then earth is moving with 0 units of speed relative to you and thus star A must move to the left and star B must move to the right.

For some external observer, standing near star C - it can indeed appear that all three are moving to the left or to the right or some other combination - because motion is relative to the observer.

2 minutes ago, 900SL said:

Thanks Vlaiv. I'm tempted to take a chance on the Bresser. I have a 10% off voucher, and it won't need a coma corrector. Might make a good lunar/planet/galaxy scope with my existing set up.  It's a long aluminium tube so focus might change a bit over the course of an evening..

That's 150mm F/8 newtonian right?

Aluminum has about thermal expansion of about 22um per meter Celsius. You have about a 1.2 meters, so you get something like 26um of expansion for every C.

F/8 system has critical focus zone of about 156um, so you should be good for about 3C change in temperature (give or take - it might be even a bit more, depending where you landed with original focus).

Slower scopes are harder to get knocked out of focus due to temperature change because critical focus zone grows as square of F/ratio.

Btw, Coma free zone of F/8 newtonian is about 5.7mm radius, so 11.4mm diameter circle. That is shy of 16mm diagonal of ASI533, but you'll likely not notice much coma to the edge of the sensor as it is very small at that F/ratio.

Just now, 900SL said:

With my 90mm f6 I can get 1.4 "/px, so presumably not a lot of point in binning to use the f8 newt at 1200mm?

Point is faster system with 150 F/8.

Both setups will have roughly the same pixel scale 1.4"/px vs 1.3"/px - but 150mm will have much more light gathering surface over 90mm.

That and, if both scopes are of decent optical quality - 150mm will produce ever so slightly sharper image.

Maybe get USB sniffer and look at the traffic between device and driver?

On 12/04/2024 at 11:27, 900SL said:

 

@vlaiv is my maths right there buddy?

I would think so.

Don't be afraid to bin, and also - do pay attention that you need larger aperture to hit certain pixel scale. I would not go below 1"/px, and even approaching that, you would need 8" or more.

6" is more suited for ~1.4-1.5"/px range as upper limit of sampling rate.

5 hours ago, John said:

Bresser 70mm F/10 achro on an EQ5 Pro GOTO mount ?

There seems to be an erecting eyepiece and possibly a barlow in the diagonal - not much use for celestial observing !

To me it looks like draw tube of focuser is closer 0.965" than 1.25"?

Also, having trouble figuring out how the scope attaches to dovetail or how it's mounted to the mount itself.

And yes - no CW bar or counterweights for that matter

4 minutes ago, fwm891 said:

Dakota Starry Nights reviews on YT are quite in depth for these Dobs...

That review seems to be from 2015.

As far as I know, there has been at least two revisions of these truss scopes. Don't know if latest revision resolved issues with original?

3 minutes ago, Louis D said:

FLO is definitely offering a wider range of focal lengths in this line than let's say CN's sponsor Astronomics in their Astro-Tech PF line.

It is actually SkyRover PF line and they included new focal lengths:

https://skyroveroptics.com/products/sky-rover-55-10-5-15-5-19-25mm-eyepieces

1 hour ago, heliumstar said:

Are these new design? If not can you please share the equivalent eyepieces on the market?

Take look at this:

https://www.cloudynights.com/topic/826223-sky-rover-premium-flat-field/

Very similar if not the same specs, although some focal lengths seem to be missing - like 3.5mm and 7.5 and some specs being slightly off - like 25mm being 5/3 vs 4/3 when it comes to lens elements.

Just went to check it - and sure enough:

What do you think is scope / mount combination in the image?

FIY, we now live in age where AI generated stock content can show some rather strange combinations

Don't be surprised to see very odd combination of gear in images or perhaps misused / misconfigured setups (Like SW rings on TV scope with unknown mount)

I had a chance to use 12" version of ES scope few weeks ago.

A friend of mine just got it in time to do Messier marathon with it this year. Three of us were in the team and we took third place. Rules of marathon state only one scope to be used per team - so we went with new scope. Probably not the best choice as we only had ES68 28mm eyepiece as our lowest power EP - which gave us ~ x54 magnification and somewhat narrow FOV for quick finding of the objects.

General impressions that I got from that night:

- scope gathers a lot of light

- we had lower contrast due to truss design and no proper shielding (there was no time for "sock" to be made / ordered).

- mirror box is quite heavy, and is two man job to comfortably transport / set it up. Rest of the scope is very light weight. Putting top cage on truss poles is again better done with a helping hand (because it needs to be held securely while tightening screws and aligning everything).

- Collimation was a breeze (with laser collimator) and scope held collimation well for the duration of the night.

- it was fairly easy to use / point - much like any other dob of similar size.

- balance was issue at low altitudes. Scope comes with counter weights and counter weight bar - but due to excessive top loading - 50x9 RACI finder, red dot and telrad (although red dot was redundant next to telrad) and 500g eyepiece - scope slipped down if not held by hand when pointing low above horizon.

Friction mechanism to stiffen up altitude motion was rather flimsy in my opinion.

- focuser and finders are at an angle I was not used to - but I managed to use the scope without issues

- scope gives very nice sharp image at low to medium powers. We did not do any high power work with it yet so I can't tell how good the optics really is.