Venus, diffraction spikes

[daz]

Viewing Venus with recently purchased Mak 127, using a 30mm plossl & barlow, Venus is quite clear, small, but has 3 spikes in each of the 4 cardinal directions radiating outwards.
Is this a collimation issue?

[AstroJunk]

Probably not. The mak has all spherical surfaces which makes it good as it never needs collimation. If you are seeing spikes, then it is probably atmospheric turbulence, seeing, causing them, especially low down on Venus. Try near the zenith and see if that improves the image.

I have a beautifully corrected scope and sometimes for months on end I feel like chucking the thing out because it cant come to a sharp focus due to seeing!!!!

[mura_gadi]

Hello,

If you were viewing Venus, had the scope been given time to acclimate?

Were you viewing from a balcony? Or carpark? Ie, a large warm slab of concrete? Looking over heated concrete buildings etc? All these things will make atmospheric heatwaves which can reduce seeing dramatically.

I have no idea on a mak, I do know they take longer to acclimate due to being a close tube. If your out around sunset the temperature may be dropping to quickly to get the scope acclimated.


Steve

[Startrek]

2 possible explanations

1/ What brand Plössl eye piece ?
Could be a low cost $30 Plössl which have poor coatings on the optics and poor anti internal reflective paint in the barrel
Could be light scatter ??
Try a more expensive eye piece if using a cheap Plössl, cheap Plössl are notorious for introducing internal optical reflections

2/ Venus is very low at the moment to observe therefore the atmosphere can play havoc depending on your location and geography
Are you observing over ocean ( large waterways ) or land , are you observing from a suburban light polluted environment or rural dark skies ?
Could be atmospheric dispersion or refraction ?
Even expensive good quality eye pieces can be affected when observing at altitudes below 30 deg
Test your eye piece and Barlow combo on a bright Star up nice and high

[Startrek]

If your keen on planetary observing ( Maks are well suited for planetary) try Saturn or Jupiter which are up nice and high after 11.30pm at the moment. This would be a better test in lieu of a bright star.
Stellarium will provide you exact co ordinates if using a non Goto mount or if using an EQ Goto mount just select the planet on the handcontroller if polar aligned

Good luck !!

[daz]

Thanks for your help guys. Possible reasons include (before getting to eyepieces)



1. light pollution here on the highway in Surfers. it really is terrible.


2. atmospheric heatwave, although the image I had last night seemed clearer than last week.


3. collimation. I noticed last week when observing Venus, i had it out of focus at one point and Venus looked like a ufo i.e. oval. I can't imagine that being optimal. Found a good guide last night on collimating orion maks, which have the same setup, so will work through that first.

[astro744]

Venus has a gibbous phase at present so it will look oval.

To test your optics and your eyesight properly, select a star. Rack in and out slowly. Is there any oval shape to the out of focus image? Does it change 90 deg the other side of focus? This could be mirror astigmatism. Does it rotate with you as you turn your head 90 deg? This could be eyesight astigmatism.

As for the spikes try increasing the power. Are they still there? The lower power will produce a larger exit pupil which passes through more of the edge of your eye so if you have any defects there they will show. At higher powers you have a smaller exit pupil passing through less of the outer region of your eye. When you look at the stars in the sky unaided, do they have spikes?

[daz]

Wasn't aware of the gibbous phase, that could explain it. I couldn't replicate the oval shape last night, so really not sure at this stage. Conditions did seem clearer last night.



Was diagnosed with astigmatism as a youngster, not sure how bad it is. I haven't noticed any spikes on stars either unaided or through the scope (to this point).

[astro744]

Have a look at Stellarium (or any other planetarium app) and zoom in on Venus. It will show the phase. It is coming from around the other side of the Sun and getting closer to USA do appearing larger. It is waning from full to new and soon will be 1/2 full and then crescent.

I only mention eyesight because so many in this hobby exhaust all avenues trying to get their telescopes to perform only to have issues with their own eyesight that is contributing to the optical flaws observed.

Wait a couple of weeks for Venus to get further from the Sun (angular separation) and then try and find Venus in the daytime. (You can do this now since Venus is following the Sun so the Sun won’t drift into your field and the separation at present is quite good). You won’t have brightness issues in the daytime and you may even observe the chevron cloud feature on the planet which can be clearly seen when gibbous.

[daz]

Last week when I observed Venus looking like a ufo, as I said, it was oval, but appeared to have no center. Which I think probably isn't the gibbous phase causing that.

My approach is to go slowly, eliminate as many possible causes as possible, before making any judgements.

[mental4astro]

There is another source of these "diffraction spikes".

You say that there is a lot of light pollution where you are. Venus is quite low in the sky for us here in Oz and in the west. I actually suspect that what you saw was internal reflections of stray bright lights entering the OTA, say from local floodlights or streetlights. If you were pointing the scope low in altitude, it is very likely this was the source of your spikes.

If your scope did not have a dewshield fitted, these stray light internal reflections are common. The stray light enters at an angle and reflects off all manner of surfaces inside the OTA: baffle tube, baffle tube stops, secondary mirror baffle. These are the main sources for this sort of aberration with Maks & SCTs as there are no spiders to create these spikes. Even the Moon can create these internal reflections when it is kept just out of the field of view of the eyepiece, as can Venus and Jupiter because of their brilliance.

This lack of control of internal reflections is a major source of loss of contrast with many scopes regardless of design, refractors and reflectors (chose your flavour of reflector). It can also affect high end scopes as these internal reflections can be very sneaky in the way the are produced. Even the design of the baffle tube on how it is supposed to control these reflections may be undermined because the paint that the baffle tube is finished with is just too reflective on a sharp edge.

It is possible to improve the control of these internal reflections, but it takes time to chase down the various offending surfaces and a willingness to open up the OTA and apply a suitable paint (such as Black 3.0) on these surfaces. I am considering doing this on my own 9" Mak after seeing the stupendous contrast of a friend's 10" Mak! Same eyepiece and diagonal, same object (Moon), and the difference in contrast was staggering, and it was entirely because of the design of the baffle tube of that 10" Mak and the paint used in those critical areas. And the contrast in my 9" Mak I thought was pretty good!

Alex.

[daz]

Thanks for the post, Alexander, I believe what you say could well be the cause. Do you think a dew shield by itself will improve the situation, as I really don't think I'm at the point to do the rest of it just yet?

[mental4astro]

To quote Dirty Harry, "A man's gotta know his limitations".

A dewshield is a quick and easy starting point that will help. The dewshield can also double as an insulating wrap for your Mak. The old school way of treating a Mak and SCT was to wait for the OTA to cool after setting up. The issue being that a heat plume would develop inside the still warm scope which would disrupt the quality of the image at high magnification. The problem with this is the wait time. Depending on the size and type of scope you have, this can take anywhere between an hour to three or even longer.

But there is a way to avoid needing a wait period and start ripping high magnification straight away from setting up, by insulating the OTA.

As the scope cools, the metal tube cools quickly and much sooner than the warm air, primary and baffle tube inside the OTA, so a heat differential is created between these and hence a heat plume is generated. But if the OTA is insulated then the tube won't cool quickly, no heat differential is created and so no heat plume, meaning you can use the scope straight away.

I have been insulating my SCT's and Maks for several years now. I have used old yoga mats for these and right now I have made insulating wraps for my Maks using Coreflute, and even a car sunshade. Other people have used other insulating materials for this. The way that you design this wrap will allow you to make it dual purpose of wrap and dewshield.

Below are pics of the wraps I've made, for an old orange tube SCT (yoga mat), 9" and 127mm Maks (Coreflute) and a friend's 6" Mak (car windscreen sunshade). A dewshield for this last scope is a separate item rather than being all in one.

Alex.

[astro744]

Try Venus in the daytime as you will not have any issues with any diffraction spikes since from any source as they will be considerably subdued yet Venus is bright enough to give good contrast against the background sky. At present Venus is about 2 hours behind the Sun (following). This is plenty of separation and if you determine when Venus is on the prime meridian at your location (due North) and find it’s altutude above horizon, then you can easily find it in binoculars and unaided and then telescope (low power first).

I use the Stellarium app to do locate Venus. Sky Safari (free version) will do the job too but I like the way you can drag the sky with your thumb in Stellarium after tapping on the clock and then tap on the sky to lock again.

Venus is a planet best viewed and studied in the daytime as it offers sharper views by being higher in the sky and it won’t set on you too soon if you observe earlier in the day. Also brightness is subdued in daylight but contrast is ample for studying surface features. Try it and you’ll be amazed how easy I it is to see unaided against the background daytime sky. Enjoy!

Note at present Venus is east of the Sun (following) and visible in the evening after sunset, the “evening Star”. When it is the “morning star”, it is rises before the Sun and is west of it. The Sun then follows it and depending on the angle of separation, one must be careful when observing so as not to have the Sun enter the field of view in your telescope, especially when your telescope is not tracking.

[daz]

O.k., so I've today been collimating the mak, have it much more centred now, but will still need to do a star test.
Quick question, before I started the collimation procedure, the 3 large screws were fairly tight, I'm guessing to hold the collimation in place?
Currently, they're not tight, is this otherwise o.k.?

[mental4astro]

Have a loot at this link. It discusses collimating a 127mm Mak made by Synta (ie Skywatcher, Celestron and Saxon branded Maks):

https://www.manualslib.com/manual/221751/Celestron-127slt.html?page=31#manual

The only way to properly collimate your Mak is by star testing because this also sets the scope in an position that you will be using it in which is important because of the mirror shift that these scopes have. If you attempt to collimate the scope while it is set in a horizontal position the primary mirror will not be experiencing the same forces as when the scope is pointing up.

Alex.

[daz]

Thanks, Alexander, haven't heard of mirror shift before.

[mental4astro]

Mirror shift happens because the focus is achieved by the primary mirror being moved back and forth along the baffle tube. The shift happens because of the way that the mechanism is designed - being a poorly designed but cheap system - so the result is slop when the mirror is moved back and forth. Celestron and Meade SCT's were notorious for showing mirror shift (also called mirror flop), and Synta Maks can show it too. You will notice this mirror shift in your scope when focusing at high magnification, seen as a small shift in the image from one side to the other as you adjust focus. The thing is this shift will affect collimation. If you collimate in one position of the primary mirror, then it will be out of collimation when the primary mirror is shift in the other direction.

Because of this mirror shift, when focusing and hence also when collimating, you must get into the habit of moving the focus knob in a set pattern all the time so that it sets the mirror back into position for when its focused position is the same as the collimated one. So you defocus say by turning the knob anti-clockwise, and focus only be turning the knob slowly clockwise. If you go too far your repeat the afore mentioned pattern and do not try to focus by turning the knob anti-clockwise.

When collimating, you need to follow the following (following the how-to instructions of the screw manipulation)
1, Use high magnification - you cannot collimate accurately enough with low power
2, Do gross collimation with a large "doughnut". Wind the focus knob anti-clockwise to get a large doughnut and then turn it a little clockwise as if focusing just a touch but leaving that large doughnut. Tweak the colli screws are required.
3, Now do fine collimation with a very small doughnut. Wind in the focus knob to reduce the size of the doughnut so it is very small. This is because a large doughnut will eventually see whatever error remains in the collimation process be evened out. A small doughnut will allow you to check for whatever small tweak needs to be done.

This may all sound very dramatic, and it can lead some people to distraction, but while it really should not be there, it is easy to work around it. My little 127mm Mak does show some mirror shift, but it does not bother me as I am well accustomed to working with mirror shift.

Mirror shift does not affect all Cassegrains that focus with primary mirror movement. Russian made Maks show no mirror shift and they had this figured out over 30 years ago. But these Russian made Maks are very special beasties! Mass production SCT's are not in the same league as these scopes.

Alex.

[daz]

Gee, thanks so much, Alexander, you are a huge help.


Looks like I've managed to to get it collimated to some extent. Much easier on my eyes, now, Venus is much larger, have managed to use the 10mm plossl.
Seeing blue and yellow at opposite ends of Venus (top & bottom), the diffraction spikes are coloured blue and yellow, now there are another 16 diffraction spikes, fainter than the 4 cardinals. Same with both eyes.
Outside of the Mak is noticeably cooler to the touch, so may need insulation.

[daz]

Just found Mars, much, much smaller & dimmer than Venus.

[mental4astro]

Try aiming the scope at a bright star closer to zenith & look for the same aberrations. Better still would be Jupiter but only with it high in the sky (which right now means around 1am). This will ensure as much as possible less stray light getting in - & try to set up somewhere without strong lights impinging on your scope.

If these strong spikes keep on appearing it will mean looking at things more carefully. I have not seen any such spikes in my 127 Mak. Could also be the eyepiece. Is it very modest in cost? This isn't a certainty as a source, just part of the investigation.

Oh yes, Mars is flaming tiny right now. Earth is leaving it very far behind now in our orbit around the Sun. It won't be a good target for another 18 months or so. Jupiter and Saturn are coming up to opposition next month. Uranus and Neptune later on again, and they present their own challenges to identify.

And don't forget DSOs and the Moon!

Alex.

[astro744]

Mars is 3.8 arcs at present, almost as small as it will appear. Saturn is about to rise on the opposite side of the sky with Jupiter soon after. Next Mars opposition will be Dec 8, 2022 at 17 arcsec. The last one was 22 arcsec so it’s going to be quite a few years before Mars will be approx 25. Very close oppositions are 15-17 years apart. Oppositions are 26 months apart.

https://www.nakedeyeplanets.com/mars-oppositions.htm

[daz]

Thanks for your advice, Astro744, hopefully will get a chance to do some daytime observing.


Alexander, I'm pretty sure you're right about the spikes being stray light. There are a couple of highrise not to far from the viewing angle, one of which has a big blue neon sign on top. The yellow could be apartment lights.
Seems the most obvious explanation.


Once again, thanks for everyone's help. :thanx:

[astro744]

You mentioned 4 cardinal spikes in your Mak. Does it have a 4 vane spider? I thought the secondary was on the back of the thick primary front element but I understand there are many different Mak designs.

You shouldn’t be seeing 4 cardinal spikes at all through a central obstructed telescope unless there are some vanes in place or square obstruction (unlikely). An SCT with a huge central obstruction produces no spikes but changes the Airy disk in terms of which rings are brighter.

[daz]

No spider vanes, Astro744.

[daz]

Alexander, I saw the vixen eyepieces you have for sale, I'm not really in the market for an eyepiece atm, but out of curiosity, would you recommend the 10mm or 14mm?

Looking through the 10mm plossl last night seemed a step up in magnification to what I was using, but have read that often comes with a tradeoff in sharpness?

[daz]

Rained out last night, but o.k. tonight. Managed to bodgie up a shield to keep out stray light. Vast improvement.

Using 10mm plossl, I'm still seeing diffraction spikes on Venus, but nowhere to the same extent. Same colours, blue & yellow.
The Moon, no diffraction spikes, but some aberrations around the illuminated area, best guess is scope needs more collimating. (haven't done star test yet, as no bright stars visible to the naked eye in the west, and currently sans finder).

[daz]

Seems there is a small problem with the scope. The rubber o-ring that fits between the back plate and the tube isn't sitting properly. There is a gap for about 1/3 the circumference where the o-ring should be.

[mental4astro]

Sent you a PM regarding the EPs as it is beyond the scope of this thread.

Alex.

[daz]

Apparently, the rear end plate screws off:


https://stargazerslounge.com/topic/364781-mak-90-is-this-fixable-or-knackered/


however it seems plenty tight, so not sure how to. Shouldn't need to screw it all the way off, just enough to pull the rest of the o-ring out.

[daz]

Having second thoughts about attempting this. I'm assuming the o-ring's main function is to stop dust from entering the ota via the thread, so might just tape it up in the interim.

[daz]

Alex,


Seems like I can't pm atm. In regards to the Vixen SSW's, the 82 deg fov is probably ideal for a mak, yes?

[raymo]

If it is super tight and has a gap part way round, maybe it is cross threaded,
which wouldn't do much for the quality of your images.
raymo

[daz]

I'm pretty sure it's threaded o.k. Raymo, but the o-ring has been somehow caught in the threading, leaving a gap where the o-ring should sit.

I've contacted Saxon, & have an o-ring on the way, but no eta.

[daz]

Finally loosened the back plate and pulled the o-ring out, the o-ring was covered in a black, sticky substance which I managed to get on quite a large portion of the back plate.


The o-ring, surprisingly, was in tip-top condition; I was expecting a mangled mess.

[daz]

Unfortunately, despite having gotten the collimation pretty close, still have massive diffraction pattern.

When I defocused on Venus tonight, I had a perfectly round object thingy, with a dot right in the centre, the way it is supposed to be.
Really don't have a clue as to what's causing the diffraction.

[daz]

Well, I think I may have found the culprit. In the image just to the left and below centre is a highly reflective object that appears to protrude approximately 1mm from the surface of the tube.

Now to open the bugger up and see if it is easily detached.

[daz]

Well, I've opened the mak, taking the front off (which needed a special spanner), to have a look at whatever it is that is reflecting.



Turns out to be a round, metallic aberration in the tube. I'm not sure if this is normal, or not. Possibly an artifact of the manufacturing process.


The problem is, it is highly reflective, so I'm thinking the solution is to paint the inside of the tube.


Next step is to get a hold of some Black 3.0.


Speaking of which, does anyone know if it can be sourced in Australia?

[raymo]

Before going looking for special paint I suggest that you paint the protuberance matt black and see if that fixes the problem. There is probably no need to paint the whole tube interior.
raymo

[daz]

Might as well do it properly seeing as I've gone to this much trouble, Raymo.