30 replies to this topic

[Megawatt]

Whenever I read that refractors are hassle-free instruments compared to Newtonians and SCTs, I laugh bitterly. For me, this has not been the case at all.

 

After struggling, and failing, to obtain decent star shapes with my secondhand William Optics triplet, I decided to update my scope to a brand new model from a different manufacturer. I understood from the outset that any refractor can have issues with pinched optics, etc., but after reading some promising reviews, I crossed my fingers that this would be a good one. 

 

After a couple of nights of imaging, I'm getting asymmetrical diffraction spikes and distorted star shapes. The first night, I miscalculated my back spacing so I didn't read too much into that, but the second night I had the spacing nailed down. My stars have little devil horns, with one horn being more prominent than the other, giving the stars a sort of teardrop shape. Temperature was between 0C and -4C in the examples below. 

 

Telescope is a Starfield GEAR80 EDT with a 0.8x reducer/flattener. I have not tested without the reducer/flattener yet. Camera is an ASI2600MC Pro. Gain100, offset50. 120" subs on an HEQ5. I've read numerous reports from other manufacturers where lens spacers can cause such diffractions. Looking through the focuser, I can see two small tabs protruding a tiny bit around the objective. Doesn't look like they are in the light path, but hard to tell.

 

Hoping this Onedrive links will work. These are single subs with autostretch applied. Just looking for opinions and feedback as I decide on my next step. 

 

https://1drv.ms/i/s!...vcvq1Q?e=tKBGSR

https://1drv.ms/i/s!...k0o1AA?e=9Myls2

[starhunter50]

i have the Starfield 90MM F5.5 ED (500MM)  and always got spikes on my stars and never figured it out. 

specially on ther brighter stars.

the smaller one seem to be triangular ish and figure i have pinced optics.

Edited by starhunter50, 19 April 2025 - 01:40 PM.

[deansjc]

I had hoped to find a simple chart available online that helps do diagnose aberations such as yours.  Instead, I am emailing a link to much more technical analysis of a variety lf types of aberations.  https://www.handprin.../ASTRO/ae1.html

 

To aid analysis, try much higher ISO and shorter exposures to rule out any movements of the image.  Image at several points of rotation to see if the aberation goes along for the ride.  Most likelyt it will.

[RichA]

The stars do indeed look like what you described.  You have to work it out by removing the field flattener and seeing if the primary lens is at fault.  Also, stop down the lens from the front with a small mask that obscures the spacers you see so they can be factored out.  Additionally, two spaces is odd, there should be three, equi-distance from one another IF they are in-fact, lens spacers.  I'd thought most lenses went to full ring spacers that you can't see.

[starhunter50]

Whenever I read that refractors are hassle-free instruments compared to Newtonians and SCTs, I laugh bitterly. For me, this has not been the case at all.

 

After struggling, and failing, to obtain decent star shapes with my secondhand William Optics triplet, I decided to update my scope to a brand new model from a different manufacturer. I understood from the outset that any refractor can have issues with pinched optics, etc., but after reading some promising reviews, I crossed my fingers that this would be a good one. 

 

After a couple of nights of imaging, I'm getting asymmetrical diffraction spikes and distorted star shapes. The first night, I miscalculated my back spacing so I didn't read too much into that, but the second night I had the spacing nailed down. My stars have little devil horns, with one horn being more prominent than the other, giving the stars a sort of teardrop shape. Temperature was between 0C and -4C in the examples below. 

 

Telescope is a Starfield GEAR80 EDT with a 0.8x reducer/flattener. I have not tested without the reducer/flattener yet. Camera is an ASI2600MC Pro. Gain100, offset50. 120" subs on an HEQ5. I've read numerous reports from other manufacturers where lens spacers can cause such diffractions. Looking through the focuser, I can see two small tabs protruding a tiny bit around the objective. Doesn't look like they are in the light path, but hard to tell.

 

Hoping this Onedrive links will work. These are single subs with autostretch applied. Just looking for opinions and feedback as I decide on my next step. 

 

https://1drv.ms/i/s!...vcvq1Q?e=tKBGSR

https://1drv.ms/i/s!...k0o1AA?e=9Myls2

Light path stars right at the objective, so yes that will introduce spikes on some stars.

i need to find the link to that info online.

[rob1986]

Do higher end scopes also suffer from overtightening to keep the objective from rattling in the cell durring shipping?

[starhunter50]

im not sure but when i was working for CanadianTelescope therer was an issue with some Takahashi APO refractors showing spikes on some stars.

Never go to the bottom of it.

 

Mitch

Edited by starhunter50, 19 April 2025 - 02:45 PM.

[starhunter50]

Whenever I read that refractors are hassle-free instruments compared to Newtonians and SCTs, I laugh bitterly. For me, this has not been the case at all.

 

After struggling, and failing, to obtain decent star shapes with my secondhand William Optics triplet, I decided to update my scope to a brand new model from a different manufacturer. I understood from the outset that any refractor can have issues with pinched optics, etc., but after reading some promising reviews, I crossed my fingers that this would be a good one. 

 

After a couple of nights of imaging, I'm getting asymmetrical diffraction spikes and distorted star shapes. The first night, I miscalculated my back spacing so I didn't read too much into that, but the second night I had the spacing nailed down. My stars have little devil horns, with one horn being more prominent than the other, giving the stars a sort of teardrop shape. Temperature was between 0C and -4C in the examples below. 

 

Telescope is a Starfield GEAR80 EDT with a 0.8x reducer/flattener. I have not tested without the reducer/flattener yet. Camera is an ASI2600MC Pro. Gain100, offset50. 120" subs on an HEQ5. I've read numerous reports from other manufacturers where lens spacers can cause such diffractions. Looking through the focuser, I can see two small tabs protruding a tiny bit around the objective. Doesn't look like they are in the light path, but hard to tell.

 

Hoping this Onedrive links will work. These are single subs with autostretch applied. Just looking for opinions and feedback as I decide on my next step. 

 

https://1drv.ms/i/s!...vcvq1Q?e=tKBGSR

https://1drv.ms/i/s!...k0o1AA?e=9Myls2

Have a look :

 

https://www.dehilste...g_artefacts.php

[Megawatt]

Have a look :

 

https://www.dehilste...g_artefacts.php

Great info, thanks!

[Megawatt]

The stars do indeed look like what you described.  You have to work it out by removing the field flattener and seeing if the primary lens is at fault.  Also, stop down the lens from the front with a small mask that obscures the spacers you see so they can be factored out.  Additionally, two spaces is odd, there should be three, equi-distance from one another IF they are in-fact, lens spacers.  I'd thought most lenses went to full ring spacers that you can't see.

I have no idea if these tabs are spacers. Here's a crappy picture where I've drawn little circles over the tabs. This is lookin through the back of the scope; I can't see them at all from the front.

 

https://1drv.ms/i/s!...6CKiTw?e=qzwtC0

[starhunter50]

ED elements shim spacing.( foil sometimes) 0.01 mil

If the hold down rings on the front of the glass are to tight, you will get strange star shapes...

 

Whats this?

 

 

Edited by starhunter50, 19 April 2025 - 03:11 PM.

[starhunter50]

try @deansjc idea,

to in fact lower the gain on your 2600 and try 15 sec guided image then we can pixel peep.

Hi Gain can stretch/warp the star light by making the sensor noisy.

[Megawatt]

I also notice that the first image, taken earlier in the night, looks better. The smaller stars are almost perfect while only the larger/brighter stars are affected. Whereas in the second image, when the weather was a few degrees colder, virtually all the stars have horns. 

[Megawatt]

ED elements shim spacing.( foil sometimes) 0.01 mil

If the hold down rings on the front of the glass are to tight, you will get strange star shapes...

 

Whats this?

 

 

this.jpg

Something I didn't notice, evidently! I will take a closer look.

[starhunter50]

So there no frost / dew or your TEC settings way low = -15C ?

[starhunter50]

Something I didn't notice, evidently! I will take a closer look.

We do want spikes on only the larger stars . ( no we dont, but hey!)

im looking for my B33 image with my wifes AT72ED so you can see those spikes.

[starhunter50]

 there it is  on the horse head Neb ... why ?  we dont really know.

 

Edited by starhunter50, 19 April 2025 - 03:26 PM.

[Megawatt]

So there no frost / dew or your TEC settings way low = -15C ?

Camera was cooled to -5C. I think frost or condensation would have been apparent in the subs. Incidentally, I don't recall seeing a setting for turning on the camera's dew heater in NINA...?

[WadeH237]

Do higher end scopes also suffer from overtightening to keep the objective from rattling in the cell durring shipping?

I would not consider it normal for any refractor to require adjustment of any part of the cell when the scope is new.

 

I've owned 5 or 6 refractors over the years, and I only had one that needed an adjustment.  It was an older Astro-Tech AT102ED.  It showed a cross shaped diffraction pattern when it was new.  To fix it, I had to remove the sliding dew shield and slightly back off 4 Delrin screws around the perimeter of the primary cell.

 

I believe that this scope was built by the same folks that built the Stellarvue 102ED, which was known at the time to have some units with this problem.  When I first encountered the problem, I could not find any references to Astro-Tech scope having the problem, so I did my research on the Stellarvue scopes.

 

My other refractors (including another Astro-Tech) worked perfectly right out of the box.

 

So I would say, that it's not normal, but it's also not a particularly difficult problem to correct.

 

I would just add a note that while I am comfortable adjusting radial screws to back them off a tiny bit, I would not attempt to collimate a refractor.  I would definitely send it back if it were out of collimation.

[rob1986]

I would not consider it normal for any refractor to require adjustment of any part of the cell when the scope is new.

I've owned 5 or 6 refractors over the years, and I only had one that needed an adjustment. It was an older Astro-Tech AT102ED. It showed a cross shaped diffraction pattern when it was new. To fix it, I had to remove the sliding dew shield and slightly back off 4 Delrin screws around the perimeter of the primary cell.

I believe that this scope was built by the same folks that built the Stellarvue 102ED, which was known at the time to have some units with this problem. When I first encountered the problem, I could not find any references to Astro-Tech scope having the problem, so I did my research on the Stellarvue scopes.

My other refractors (including another Astro-Tech) worked perfectly right out of the box.

So I would say, that it's not normal, but it's also not a particularly difficult problem to correct.

I would just add a note that while I am comfortable adjusting radial screws to back them off a tiny bit, I would not attempt to collimate a refractor. I would definitely send it back if it were out of collimation.

Cheaper achromats often have their lens cells overtightened to reduce risk of damage during shipping. SCTs often have similar problems.

Mine took all the strength i had to loosen the ring without violently shaking it.

However, there was a definite improvement in views. Particularly slight triangle stars which rounded out.

I would have thought the more expensive offerings would use better packing instead

Edited by rob1986, 19 April 2025 - 05:06 PM.

[KGoodwin]

There should be no adjustment required on any quality refractor. I have never had an issue like this on any refractor I’ve owned: 2 explore scientifics, a Takahashi, and an AP. I would not accept something like that if I purchased a new scope. The only “adjustments” I’ve ever made to refractors are to the focuser or rings.

[Megawatt]

Cheaper achromats often have their lens cells overtightened to reduce risk of damage during shipping. SCTs often have similar problems.

Mine took all the strength i had to loosen the ring without violently shaking it.

However, there was a definite improvement in views. Particularly slight triangle stars which rounded out.

I would have thought the more expensive offerings would use better packing instead

I'm hoping there will be an easy fix for my issue, but it is disappointing when you realize what a crapshoot it is after spending over a grand on an OTA. Of course, these scopes are not designed to be user-adjustable like a Newtonian. 

[rob1986]

I'm hoping there will be an easy fix for my issue, but it is disappointing when you realize what a crapshoot it is after spending over a grand on an OTA. Of course, these scopes are not designed to be user-adjustable like a Newtonian.

One of their central advantages

[Bob2023CL]

I have the Gear60 and I have never seen anything like that.  I was looking for bigger scope so I might want to stay away from those older models.  Might have to look at the Askar SQA series.

[Megawatt]

ED elements shim spacing.( foil sometimes) 0.01 mil

If the hold down rings on the front of the glass are to tight, you will get strange star shapes...

 

Whats this?

 

 

this.jpg

Hey Starhunter, 

 

I had a closer look and you might be onto something; good eye! Here is an extreme crop of the area you pointed out. Could this be the culprit? I guess I have to figure out my image orientation and if that lines up with this apparent flaw in the lens edge.

Attached Thumbnails

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