TS Photoline 72 F/6 - lemon?

[alexbb]

I ordered Monday a TS Photoline 72 F/6 from Teleskop Express. https://www.teleskop-express.de/shop/product_info.php/language/en/info/p8866_TS-Optics-Doublet-SD-APO-72mm-f-6---FPL-53---Lanthan-Objektiv.html

Yesterday I received it and today I got it under the stars. I also ordered a Baader filter to mod my 550D which I managed yesterday evening.

So, under the stars, both scopes (the TS and a SW72ED) this evening. I didn't pay too much attention when focusing, but now I verified the stars and they seem horrible.

I also recall hearing some crackle sounds which were resembling ice cracking.

The target I shot was Rosette with both scopes.

Below are some crops from the stacked and stretched luminance, single 120 subs stretched L, Ha and OIII + a quick combination of RGB with the SW72ED and Canon 550D mod + L and Ha and OIII.

Is it probably a sign of pinched optics?

Does anyone has first hand experience with these scopes? What should I do? Ask for a replacement or a refund? I'm afraid that another example can behave the same way.

PS. I used a TS flattener/reducer 0.79x which worked well with the SW72ED.

[vlaiv]

I would personally do couple of things prior to sending the scope back - just to eliminate any possibility of it not being the optics.

1. Do some test shots without x0.79 FF/FR, to be sure that nothing else except the telescope is at fault

2. Do couple of shots of single star with in / out focus image - to check collimation. Maybe check collimation with high power eyepiece if you have one? It is best to have it in images.

Just use a bright star, defocus star a bit in steps (small defocus, larger defocus, and a bit larger defocus) - in both directions - in and out focus and take some shots to see what pattern looks like. Make sure you don't use any other piece of optics - remove any filters, or use good narrowband filters you know are working - like Ha or OIII. Make sure star is in the center of the field.

I know this is waste of imaging night, but if you have artificial star - you can do it when it's cloudy and you can't image otherwise - just place artificial star far away enough.

[alexbb]

Thank you, Vlaiv! I will try a star test, not sure when it's possible again.

However, the flattener worked well in combination with the SW72ED.

 

These images were shot with the SW72ED, the Orion is luminance and the elephant trunk is hydrogen.

I used the same reducer/flattener and filters and the same backfocus. The focal length of the SW72ED is ~430mm, measured with astrometry.

Edited February 28, 2020 by alexbb
SW72ED images uploaded

[alexbb]

I did a pseudo star test this morning. The star hole was a bit large (~1mm) to asses more optical defects, however, the out of focus disks appeared round with concentric circles.

[vlaiv]

2 minutes ago, alexbb said:

I did a pseudo star test this morning. The star hole was a bit large (~1mm) to asses more optical defects, however, the out of focus disks appeared round with concentric circles.

I'm not sure you can use 1mm hole. I mean, you can - but you need to calculate distance so that image of a hole is smaller than let's say half or third of airy disk diameter.

For 72mm aperture Airy disk diameter is about 3.5", so you want your hole to be about 1" or less. This gives about 206.26 meters of distance. Anything closer to that and you will impact shape of star with shape of aperture producing a star (1mm hole).

How close did you do it?

If you want to try DIY artificial star - maybe look into metal ball from ball bearing - you need something that is very round and very reflective - so any sort of very round metallic ball with small diameter will be good. It also needs to be placed far away from telescope.

[alexbb]

3 minutes ago, vlaiv said:

I'm not sure you can use 1mm hole. I mean, you can - but you need to calculate distance so that image of a hole is smaller than let's say half or third of airy disk diameter.

For 72mm aperture Airy disk diameter is about 3.5", so you want your hole to be about 1" or less. This gives about 206.26 meters of distance. Anything closer to that and you will impact shape of star with shape of aperture producing a star (1mm hole).

How close did you do it?

If you want to try DIY artificial star - maybe look into metal ball from ball bearing - you need something that is very round and very reflective - so any sort of very round metallic ball with small diameter will be good. It also needs to be placed far away from telescope.

It was ~10m away. But shouldn't it be good enough to verify the colimation at least?

[vlaiv]

Just now, alexbb said:

It was ~10m away. But shouldn't it be good enough to verify the colimation at least?

I don't really know. There are two things that you need to pay attention to when doing star testing with artificial star.

1. You should not resolve your artificial star

2. You should account for spherical aberration introduced by source that is close

Above we did first criteria - how not to resolve artificial star - by making it smaller than fraction of airy disk.

We can also do second criteria - we need to express difference in distance from artificial star to center of aperture and distance of artificial star to edge of aperture - in wavelengths of light and make that less than a fraction - let's say less than 1/8 or /10 waves of spherical in order to test telescope for correction.

I think that for collimation you need to satisfy first point and second is not important. For star testing you need to satisfy both.

Artificial stars are made with apertures in microns - like 20um. That is about x50 smaller aperture than one you used. This of course means that you could do collimation test at only 4-5 meters away with such device. Spherical would be very pronounced at that distance, so you still need to do something like 50m if you want to eliminate that.

 

[JamesF]

I have one of these and have used it both with and without the TS 0.79x reducer.  I've not seen stars looking anything like the ones in your images.  I have found it tricky to get the reducer spacing just right, partly because there have been so few clear nights available to just play around with it, but even then the stars are still the shape I'd expect.

The colour image you posted is from the Skywatcher OTA, is that correct?

James

[alexbb]

4 minutes ago, vlaiv said:

I don't really know. There are two things that you need to pay attention to when doing star testing with artificial star.

1. You should not resolve your artificial star

2. You should account for spherical aberration introduced by source that is close

Above we did first criteria - how not to resolve artificial star - by making it smaller than fraction of airy disk.

We can also do second criteria - we need to express difference in distance from artificial star to center of aperture and distance of artificial star to edge of aperture - in wavelengths of light and make that less than a fraction - let's say less than 1/8 or /10 waves of spherical in order to test telescope for correction.

I think that for collimation you need to satisfy first point and second is not important. For star testing you need to satisfy both.

Artificial stars are made with apertures in microns - like 20um. That is about x50 smaller aperture than one you used. This of course means that you could do collimation test at only 4-5 meters away with such device. Spherical would be very pronounced at that distance, so you still need to do something like 50m if you want to eliminate that.

 

Thank you! It might be possible to do a star test tonight, otherwise I will make a proper artificial star test. Nice tip with the pen ball!

3 minutes ago, JamesF said:

I have one of these and have used it both with and without the TS 0.79x reducer.  I've not seen stars looking anything like the ones in your images.  I have found it tricky to get the reducer spacing just right, partly because there have been so few clear nights available to just play around with it, but even then the stars are still the shape I'd expect.

The colour image you posted is from the Skywatcher OTA, is that correct?

James

I found the backfocus to work well with the SW72ED somewhere at ~68mm. Focal length is ~430mm so I assume it should work fine with the TS scope too at the advertised ~432mm focal length.

The colour image is made by all the data combined, both SW scope and TS scope. I did not post the result from the SW scope only because it seems I did a poor job when I put the Canon sensor back and I have some tilt.

[Adam J]

Could be astigmatism could be caused by pinched optics. Worth doing a star test but I am not optimistic. 

[vlaiv]

1 minute ago, Adam J said:

Could be astigmatism could be caused by pinched optics. Worth doing a star test but I am not optimistic. 

If it is, and it is very possible that it is either pinched optics or maybe collimation issue, it will show in star test.

I would personally feel better sending back the scope together with image to confirm my finding of issue (not saying that one should not be able to return faulty scope without image - it just better to include evidence of scope's poor performance in my opinion).

[Adam J]

Well I did say "worth doing a star test". I say astigmatism as there is a hint of a Maltese cross going on in the center stars. 

 

[Skyline]

I don't think the reducer spacing is st fault here since the op says the reducer performs well with the ed72. 

Was you using a dew strip around the lens cell?

Send the scope back and get a refund.

Edited February 29, 2020 by Skyline

[alexbb]

Indeed, it will take the road back next week. On the left is Regulus through the SkyWatcher, on the right is Regulus through the TS. Both without reducer, 5s exposure.

Thank you all for your support!

I will discuss with them if they can send me a tested example or I will follow another route.

[david_taurus83]

The star on the right looks like the stars Adam was getting on his Esprit.

@Adam J did you get sorted in the end?

[Adam J]

7 hours ago, david_taurus83 said:

The star on the right looks like the stars Adam was getting on his Esprit.

@Adam J did you get sorted in the end?

I got a replacement Esprit 100 from FLO, it looking much better than the first one, but still not managed to take an full image with it.