Altair Wave 115 Triplet Apo

[Adam J]

On 24/01/2022 at 15:13, Chris Willocks said:

So I've finally received the scope I must say it's a beaut; quite a bit bigger than I thought in real life, but they always are. Makes the FSQ-85 look tiny:

The focuser is pretty large as well. Should be good for imaging:

I like the fact that they include the optical report. Strehl ratio is 0.980, which I presume is quite good?

Anyway, just installed it in the dome with the FSQ piggybacked. Hopefully I can get imaging with it soon, when the weather picks up.

Chris

That is a very nice lens you got a good one if that report is accurate about 1/8th wave. You should be very happy. 

[Adam J]

6 hours ago, Deadlake said:

Looking at the performance report is this poly Strehl or at a particular wave length?
Also PV of the lens has an effect. But now Far East is showing performance reports hard to ignore compared to top tier.
Having said that I know some AP owners rate that brand high then the Takahashi's they also own......

No it won't be Polly or you would have a strehl Vs wavelength chart. It will be green light almost certainly. Nothing has a Polly of 0.98 over the visible range lol. 

[vlaiv]

3 minutes ago, Adam J said:

Nothing has a Polly of 0.98 over the visible range lol. 

Good reflector?

 

[Adam J]

4 minutes ago, vlaiv said:

Good reflector?

 

In theory the mirror could be tested as perfect. In reality the in use value is lower due to diffraction from the central obstruction and so it would never reach 0.98 in use. 

[vlaiv]

5 minutes ago, Adam J said:

In theory the mirror could be tested as perfect. In reality the in use value is lower due to diffraction from the central obstruction and so it would never reach 0.98 in use. 

Oh nooo - but off axis, slow spherical mirror to the rescue!

No central obstruction!

[Louis D]

1 hour ago, Adam J said:

In theory the mirror could be tested as perfect. In reality the in use value is lower due to diffraction from the central obstruction and so it would never reach 0.98 in use. 

What about with an off axis aperture mask between the secondary spider vanes?

[Adam J]

7 minutes ago, Louis D said:

What about with an off axis aperture mask between the secondary spider vanes?

Don't see how that would help, can you show me what you mean? 

[Louis D]

1 hour ago, Adam J said:

Don't see how that would help, can you show me what you mean? 

There's no central obstruction when using a parabolic mirror off axis via a circular mask.  Here's a discussion of the concept on CN.  Here's a technical description of the optics.

There are still aberrations as discussed in the latter treatise.

Here's how to get a 14.5" unobstructed telescope using a 36" primary mirror:

DGM Optics used to make and sell the OA-4.0 Off Axis Newtonian, among other sizes, using off axis circular cores from larger mirrors, IIRC.

[Stu]

Let’s keep this on topic please, Chris’ 115mm Altair. Thanks!

[alan potts]

Well said Stu,

Alan

[Mr Spock]

Lovely looking scope. Performance should be good too. 

[Chris Willocks]

Thanks everyone, not had chance for first light yet with it, as the weather has typically gone downhill (always the way when you buy a new scope).

Also been trying to get everything set up equipment-wise in the dome. Unbelievable how many cables can congregate when you have a two-scope setup. Nearly there though. Last issue I'm having is trying to use two SX filter wheels simultaneously. According to SX, you have to use one with USB and the other with the serial port. So hopefully I can get that sorted.

Anyway, I'll post some images when I manage to get any. I'm looking forward to imaging some rarer galaxies and other smaller objects.

Chris

[Chris Willocks]

Hi everyone,

I've had chance to use the scope a bit now and here is the first image I've managed to obtain with it in combination with my Atik 428EX Mono camera; NGC 2841 in Ursa Major:

Quite pleased with the result. Quite a low image scale at 1.16"/px (although I have done a x2 drizzle on this particular image which is probably pushing it a bit). Strangely I can see a bit of lateral chromatic aberration on some of the stars if you look closely enough (blue on one side and green on the opposite side). I wasn't sure if this perhaps might be caused by tilt or something else in my imaging train? I've had to use quite a long extension tube (about 55mm long) in front of the filter wheel to get it to focus, so this might be flexing slightly? The stars still look round though. Anyone have any experience with this? It's only minor, but the perfectionist inside me is struggling to cope with it. I initially stacked it in DSS, but noticed it was improved and not as visible after stacking in PixInsight.

Anyway, apart from that, it's a great scope. The stars are nice and round. I was debating with myself on whether to add a field flattener to it, although the sensor on the 428EX is quite small, so might not be necessary? Perhaps it may improve the stars though still? The focuser is very sturdy and has many connection options e.g. 2", M63 etc. So it's great for full-frame sensors. It also has the rotation ring which is good.

I've not had chance to look through the scope yet visually, as I plan on using it mainly for imaging. However, I will have to at some point.

Chris

 

[Adam J]

4 hours ago, Chris Willocks said:

 Anyone have any experience with this? It's only minor, but the perfectionist inside me is struggling to cope with it. I initially stacked it in DSS, but noticed it was improved and not as visible after stacking in PixInsight.
Chris

 

Green and Blue fringing is an odd one, you would normally expect Red / Blue if it was something like a decentered lens element. You should be able to see this in each of the RGB stacks as a change in the shape of the star between each channel, like coma but shifting direction from red to blue.

If I split out the green channel in PS I do kinda see a little of that with the a tiny bit of something left of the red and to the right of the blue and the green being correct, but its more in the halo than anything else. Its also notable that this is seen across the entire image in the same orientation and so is not typical lateral CA (usually caused by a corrector in any case and you are not using one), there are a few other things it could be though. Its almost like there is a flare on the top left of the Red channel and if you removed that then it would look quite similar to the blue. So all in all I would not panic right away as there could be something else going on here.

1) RGB, 2) RB, 3) R, 4) G, 5) B:

 

You dont say if the camera is an OSC 428 or a Mono 428? You mention a filter wheel so am guessing mono.

Also if you are using filters, what filters are you using / what brand size etc?

Red and Blue, filters not sitting flat in the wheel could cause this kind of effect.

What order did you shoot the RGB in? The flair in red could be as a result of the scope not being fully cooled down (for that matter the whole thing could be that).

Two things to try here are to rotate the entire imaging train relative to the primary (use a shim so it tightens in a difference spot or the rotator if the scope has one) if the aberation remains fixed relative to the camera then its not the primary, if the aberation rotates as you rotate the camera it becomes more likely its the scope optics.

The other thing to do it to perform a star test  (try and get an image with between 3 and 5 diffration rings in each RGB channel).

It could be a simple colimation error and that will show up on the star test.

In any case dont be tempted to try and mess about with the lens centring yourself.

All in all I would be better off taking a look at the linear stack for each channel as opposed to splitting an RGB image.

The other thing you could do is just forget about it and move on as for me it is not detracting greatly from the image when viewed at a reasonable scale and you are pushing the saturation quite hard for limited SRN.

Adam

 

Edited February 13, 2022 by Adam J

[Chris Willocks]

45 minutes ago, Adam J said:

Green and Blue fringing is an odd one, you would normally expect Red / Blue if it was something like a decentered lens element. You should be able to see this in each of the RGB stacks as a change in the shape of the star between each channel, like coma but shifting direction from red to blue.

If I split out the green channel in PS I do kinda see a little of that with the a tiny bit of something left of the red and to the right of the blue and the green being correct, but its more in the halo than anything else. Its also notable that this is seen across the entire image in the same orientation and so is not typical lateral CA (usually caused by a corrector in any case and you are not using one), there are a few other things it could be though. Its almost like there is a flare on the top left of the Red channel and if you removed that then it would look quite similar to the blue. So all in all I would not panic right away as there could be something else going on here.

1) RGB, 2) RB, 3) R, 4) G, 5) B:

 

You dont say if the camera is an OSC 428 or a Mono 428? You mention a filter wheel so am guessing mono.

Also if you are using filters, what filters are you using / what brand size etc?

Red and Blue, filters not sitting flat in the wheel could cause this kind of effect.

What order did you shoot the RGB in? The flair in red could be as a result of the scope not being fully cooled down (for that matter the whole thing could be that).

Two things to try here are to rotate the entire imaging train relative to the primary (use a shim so it tightens in a difference spot or the rotator if the scope has one) if the aberation remains fixed relative to the camera then its not the primary, if the aberation rotates as you rotate the camera it becomes more likely its the scope optics.

The other thing to do it to perform a star test  (try and get an image with between 3 and 5 diffration rings in each RGB channel).

It could be a simple colimation error and that will show up on the star test.

In any case dont be tempted to try and mess about with the lens centring yourself.

All in all I would be better off taking a look at the linear stack for each channel as opposed to splitting an RGB image.

The other thing you could do is just forget about it and move on as for me it is not detracting greatly from the image when viewed at a reasonable scale and you are pushing the saturation quite hard for limited SRN.

Adam

 

Thanks for the detailed reply and analysis Adam.

I'm using the mono version of the 428EX with the Starlight Xpress MIDI Filter Wheel. I'm using Astronomik 1.25" Deep Sky RGB filters also: https://www.astronomik.com/en/photographic-filters/deep-sky-rgb-colour-filters.html. I've always used Baader filters, but the Astronomik filters have had good reviews from what I've read.

I shot the colours in standard R, G, B order and over the space of one night; 10x frames for each filter at 600s seconds each. The cooling of the telescope is a good point. I do have it permanently housed in a Pulsar dome, but obviously the optics themselves need quite a bit of time to cool down, being a triplet. I did start imaging within 10 minutes or so, so this may be a possible cause. Another thing that crossed my mind was the fact that when I started imaging at around 7pm, NGC 2841 was quite low in the sky and towards the end of the night was near the zenith. I've read that atmospheric distortion can cause this type of issue?

Perhaps I could try combining the last frame i.e. frame 10 from each filter to see if the issue still exists, as these were taken later on in the night when the scope would have cooled down and the object would have been higher in the sky also.

I'll also try your other suggestions.

Thanks,

Chris

[Adam J]

1 minute ago, Chris Willocks said:

Thanks for the detailed reply and analysis Adam.

I'm using the mono version of the 428EX with the Starlight Xpress MIDI Filter Wheel. I'm using Astronomik 1.25" Deep Sky RGB filters also: https://www.astronomik.com/en/photographic-filters/deep-sky-rgb-colour-filters.html. I've always used Baader filters, but the Astronomik filters have had good reviews from what I've read.

I shot the colours in standard R, G, B order and over the space of one night; 10x frames for each filter at 600s seconds each. The cooling of the telescope is a good point. I do have it permanently housed in a Pulsar dome, but obviously the optics themselves need quite a bit of time to cool down, being a triplet. I did start imaging within 10 minutes or so, so this may be a possible cause. Another thing that crossed my mind was the fact that when I started imaging at around 7pm, NGC 2841 was quite low in the sky and towards the end of the night was near the zenith. I've read that atmospheric distortion can cause this type of issue?

Perhaps I could try combining the last frame i.e. frame 10 from each filter to see if the issue still exists, as these were taken later on in the night when the scope would have cooled down and the object would have been higher in the sky also.

I'll also try your other suggestions.

Thanks,

Chris

You could try looking at early red frames vs late red frames, the main thing that made me think cooling was that flair on the top left of the red, like tube currents.

If you mean that you shot in a cycle of 10 frames per filter and then returned to the first filter then I would do as you say and look at the last group vs the first group.

Atmospheric refraction can cause this, but its less likely in a mono camera as the RGB channels are independently registered to each other,  its something you would notmally see more often in a OSC camera.

Adam

 

 

[Chris Willocks]

11 hours ago, Adam J said:

You could try looking at early red frames vs late red frames, the main thing that made me think cooling was that flair on the top left of the red, like tube currents.

If you mean that you shot in a cycle of 10 frames per filter and then returned to the first filter then I would do as you say and look at the last group vs the first group.

Atmospheric refraction can cause this, but its less likely in a mono camera as the RGB channels are independently registered to each other,  its something you would notmally see more often in a OSC camera.

Adam

 

 

I've had a look at the 1st, 5th and 10th frames for each colour to see how the stars change and strangely the aberration seems to get worse, rather than better. Here are the individual frames for each filter:

1st red frame:

 

5th red frame:

 

10th red frame:

 

1st green frame:

 

5th green frame:

 

10th green frame:

 

1st blue frame:

 

5th blue frame:

 

10th blue frame:

 

Here are the combined RGB images for the 1st, 5th and 10th frames only:

1st RGB:

 

5th RGB:

 

10th RGB:

 

I've tried to balance the colour channels as best I can for each RGB image. As far as I can tell, the aberration appears worse in the 10th image than the 1st.  Here are the other red frames for reference:

2nd red frame:

 

3rd red frame:

 

4th red frame:

 

6th red frame:

 

7th red frame:

 

8th red frame:

 

9th red frame:

As you can see on these later frames, the issue seems to be worse; especially if you look at the brighter stars. The only variables that changed were the location of the galaxy i.e. higher up in the sky and the temperature.  Tilt is another possibility that came to mind, however the other filters aren't affected as much as the red channel. The other thing I forgot to mention was the moon was quite bright that evening, so this may have affected the images possibly? Although there shouldn't be any reflections inside the scope, as it is inside a dome that shields the scope from it.

Chris

 

 

[Adam J]

10 hours ago, Chris Willocks said:

I've had a look at the 1st, 5th and 10th frames for each colour to see how the stars change and strangely the aberration seems to get worse, rather than better. Here are the individual frames for each filter:

1st red frame:

 

5th red frame:

 

10th red frame:

 

1st green frame:

 

5th green frame:

 

10th green frame:

 

1st blue frame:

 

5th blue frame:

 

10th blue frame:

 

Here are the combined RGB images for the 1st, 5th and 10th frames only:

1st RGB:

 

5th RGB:

 

10th RGB:

 

I've tried to balance the colour channels as best I can for each RGB image. As far as I can tell, the aberration appears worse in the 10th image than the 1st.  Here are the other red frames for reference:

2nd red frame:

 

3rd red frame:

 

4th red frame:

 

6th red frame:

 

7th red frame:

 

8th red frame:

 

9th red frame:

As you can see on these later frames, the issue seems to be worse; especially if you look at the brighter stars. The only variables that changed were the location of the galaxy i.e. higher up in the sky and the temperature.  Tilt is another possibility that came to mind, however the other filters aren't affected as much as the red channel. The other thing I forgot to mention was the moon was quite bright that evening, so this may have affected the images possibly? Although there shouldn't be any reflections inside the scope, as it is inside a dome that shields the scope from it.

Chris

 

 

Huum, its clearly mostly an issue with the red. I really dont know that tilt would cause something like this to be honest.

Take another image, rotate the camera and imaging chain and see if you get the same result.

Adam

[Chris Willocks]

13 hours ago, Adam J said:

Huum, its clearly mostly an issue with the red. I really dont know that tilt would cause something like this to be honest.

Take another image, rotate the camera and imaging chain and see if you get the same result.

Adam

I'll give it a try with the camera rotated as you say and will report back.

Chris

[Chris Willocks]

On 13/02/2022 at 23:44, Adam J said:

Huum, its clearly mostly an issue with the red. I really dont know that tilt would cause something like this to be honest.

Take another image, rotate the camera and imaging chain and see if you get the same result.

Adam

Okay, so I've finally had chance to test the scope again. The weather hasn't been great up to now.

I've since added a TS Optics 2.5" Field Flattener to the scope. A little excessive perhaps.

Here are some images I took of the airy disk of Alpha Leonis for each filter:

Luminance:

Red:

Green:

Blue:

How do these look? There is a slight notch on the left-hand side which is likely caused by the OAG prism. I have it orientated to the side of the sensor. I've tried to position the prism so that it causes minimal interference.

Here is an LRGB (x1 frame per filter) image of Alpha Leonis in focus:

Again, I think the slightly strange diffraction pattern on the left may be caused by the OAG. It is quite a bright star afterall.

I've tried orientating the camera at a different angle (OAG remaining at same angle relative to the scope) and it doesn't move with it.

Chris

[Chris Willocks]

Can anyone help with this? I still seem to be getting bloating in the red channel, but not the others as can be seen in this quick and scrappy LRGB image of NGC 2903 I put together:

Initially I thought it might have been the focus in the red channel, but I'm using auto focus in SG Pro and everything looks fine with the graph.

Could it be the RGB filters I'm using perhaps? Does anyone have any experience of the Astronomik Deep Sky RGB filters?

Edited February 27, 2022 by Chris Willocks

[Adam J]

On 25/02/2022 at 18:18, Chris Willocks said:

Okay, so I've finally had chance to test the scope again. The weather hasn't been great up to now.

I've since added a TS Optics 2.5" Field Flattener to the scope. A little excessive perhaps.

Here are some images I took of the airy disk of Alpha Leonis for each filter:

Luminance:

Red:

Green:

Blue:

How do these look? There is a slight notch on the left-hand side which is likely caused by the OAG prism. I have it orientated to the side of the sensor. I've tried to position the prism so that it causes minimal interference.

Here is an LRGB (x1 frame per filter) image of Alpha Leonis in focus:

Again, I think the slightly strange diffraction pattern on the left may be caused by the OAG. It is quite a bright star afterall.

I've tried orientating the camera at a different angle (OAG remaining at same angle relative to the scope) and it doesn't move with it.

Chris

Might be the off axis guider might be pinching. The presence of some fainter notches in the outer ring of the star test suggests some mild pinching (Which could get worse with decreasing temperature) if it was a cold night when you took the original image then I would say that is likely the cause of the issues. Especially so as you mentioned that it got worse as the night progressed. Did you use a dew heater?

In terms of NGC 2903 I would not be unhappy with that result at all. You do have to expect a little bloat in comparison to an FPL53 scope. But I would say that is very acceptable. If you are going to design a lens and know that a little lost of Polly stehl is inevitable in either blue or red then the correct place to shove it is into the red channel. 

Adam 

Edited February 28, 2022 by Adam J

[GalaxyGael]

These 115 scopes are quite nice. Maybe an oldish thread, but my Tecnosky version 3 iteration of this scope had a similar issue. It has the 2. 5" focuser which was perfectly square (still don't know why I overpaid for the 3" 0.79x reducer flattener!). 

A Cheshire (crosshairs removed) test showed a shift in reflected rings. These are great lenses for the money, no collimation error of the individual n-p-n arrangement themselves but lens cell tilt error was there. The three sets of push-pull allen screws on the focuser side of the lens cell are very easy to manipulate until all reflections are centered and it solved my half-and-half stars, even though the airy disc looked OK beforehand. 

Just a thought, might be worth sanity checking lens cell tilt. 

Edited April 8, 2022 by GalaxyGael
typos

[rnobleeddy]

On 07/04/2022 at 19:59, GalaxyGael said:

These 115 scopes are quite nice. Maybe an oldish thread, but my Tecnosky version 3 iteration of this scope had a similar issue. It has the 2. 5" focuser which was perfectly square (still don't know why I overpaid for the 3" 0.79x reducer flattener!). 

A Cheshire (crosshairs removed) test showed a shift in reflected rings. These are great lenses for the money, no collimation error of the individual n-p-n arrangement themselves but lens cell tilt error was there. The three sets of push-pull allen screws on the focuser side of the lens cell are very easy to manipulate until all reflections are centered and it solved my half-and-half stars, even though the airy disc looked OK beforehand. 

Just a thought, might be worth sanity checking lens cell tilt. 

Out of curiosity, did you settle on a reducer? 

I have this scope and would love to get a reducer, but the one that altair supply is out of stock and they appear not to reply to emails, so would like to source something for the autumn.

In other news, I too probably see a hint of red fringing with this scope - I saw a similar result to the one above. I can't be 100% sure as I've tended to capture LRGB on different nights, and on at least a couple of sessions, the R had worse seeing conditions.  I've got a backlog of data to process so I'll post back with any conclusions.

I haven't had a refractor very long - I have a chesire - but don't really understand how to use it to check for tilt

[GalaxyGael]

I've not seen the red fringing on mine. I have the 3" TSRED379 reducer/flattener. Unless using full frame camera, its a bit overkill. The 2" version would be fine, any mild vignetting correctable with flats. At least with the 3" version, there are no internal reflections I can see and 55.5 mm is the right backfocus. I also got a second hand Hotech 2" flattener to image at 800 mm f/7 when I want to.

when testing for tilt with the cheshire, put in into the visual back as normal without too much compression and leave the lens cap on. Remove the cross hairs on the cheshire too, might be screw out like mine. Use a flashlamp to give a good brightness to the side angled mirror on the cheshire and look through to see all greenish circles reflections are concentric with a single centre. Best to do this without any reducer in. The smaller inner circle should be centered and no displaced north, south , east or west (or some other direction) away from the centre. Start this way and see if you see anything off centre first.