Skywatcher ESPRIT 80 PRO with ASI183MM PRO : better resolution with OIII over Ha?

[HAlfie]

Hello there 😀,

I'm still trying to improve my imaging Workflow and did some tests on NGC 6992.
My SETUP is : Skywatcher ESPRIT 80ED 80/400mm + Flattener + ASI 183MM PRO + Baader LRGB-SHO filters.

I use SGP (Sequence Generator Pro) for many things and among them autofocus routine.
But, it's like my OIII channel is always sharper with tighter stars than with my Halpha/SII channels which seem more blurry and with stars less "pinpoint".

See for yourself. Do I have a slight out of focus issue in Halpha/SII or is it normal that the OIII seems sharper and more detailed? Maybe it's my truiplet lense which is better corrected in green light than red light?

OIII channel : 

 

Halpha channel: 

 

SII Channel : (don't bother the change of angle of the ampglow, it's just after a meridian flip with of course another autofocus just after the flip).

 

Thanks in advance for your answer and clear skies !!

Edited October 15, 2019 by HAlfie

[Laurin Dave]

I did this recently and to my surprise the Oiii seemed somewhat stronger and more detailed than the Ha..  so maybe that’s all you are seeing. Is it the same on other targets? If you haven’t then maybe try another target before concluding something’s wrong 

[vlaiv]

Could be many reasons, but let's list a few:

1. changing seeing conditions between OIII, Ha/SII - you could analyze this maybe by examining what time of the night did you do each set, was it on same or successive nights and what was guiding like for each session. If OIII was first or last in single night - it is more likely that seeing had something to do with it, if it is in the middle of the night - it is still possible but less likely. If each was taken on a separate night - highly likely that it was the seeing. Poorer guide RMS often means that it was either the seeing or maybe wind?

2.  Different signal levels will cause different level of "auto stretch" / "auto develop". If you want to see if stars are similar in size (or different), you should check actual FWHM of respective stars to see change in "bloat".

3. It might be that optics + flattener is better corrected in green than it is in red. I doubt that you would see this from scope alone, but with flattener - it might be an issue.

4. In principle it could also be due to filters, but I don't think Baader filters are to blame here. Many people use them and no one (including me) reported this. In general it is other way around - Ha/SII tends to be sharper than OIII because longer wavelengths are less impacted by seeing than shorter ones.

[Adam J]

1 hour ago, HAlfie said:

Hello there 😀,

I'm still trying to improve my imaging Workflow and did some tests on NGC 6992.
My SETUP is : Skywatcher ESPRIT 80ED 80/400mm + Flattener + ASI 183MM PRO + Baader LRGB-SHO filters.

I use SGP (Sequence Generator Pro) for many things and among them autofocus routine.
But, it's like my OIII channel is always sharper with tighter stars than with my Halpha/SII channels which seem more blurry and with stars less "pinpoint".

See for yourself. Do I have a slight out of focus issue in Halpha/SII or is it normal that the OIII seems sharper and more detailed? Maybe it's my truiplet lense which is better corrected in green light than red light?

OIII channel : 

 

Halpha channel: 

 

SII Channel : (don't bother the change of angle of the ampglow, it's just after a meridian flip with of course another autofocus just after the flip).

 

Thanks in advance for your answer and clear skies !!

I can see that you mean but it looks like seeing differences to me, did you take the images at different times?

[HAlfie]

Hello everyone and thanks for your answer 😉!

Laurin Dave &  Adam J : I'm afraid I'm not sure the reason would be the seeing (I wish I would) as the effect is the same on several nights on differents objects 😞 .

vlaiv :
2. With SGP, the HFR (which is a bit different from FWHM) values follow my empirical observation. I mean, when I can reach HFR around 2.5 in OIII, I usually not get below 2.9/3.0 with Halpha and SII. The HFE is very stable during the night (range of 0,3 because I refocus every 0,6°C temperature drop and every 30min).
3. Regarding your point N°3, my ESPRIT 80ED comes from Teleskop austria and was "perfectly" collimated by Tommy :

Quote

I am ready with Esprit 80/400 Nr. 1801057 - it reads 95% Strehl now and has a smooth star halo. The colors are very well balanced and no lateral color shift exists. The residual errors aste a trace of undercorrection and astigmatism, both in small digestable dose (95% Strehl without any subtractions)". The lens buffer ring has been exchanged, and lenses collimated one by one. The lens cell has been centered to the focuser (to have the optical axis at center of your picture)

Ususally I was expecting to have a cleaner Halpha channel than OIII, but it's the opposite. I also know that asiderom the optical color correction,  in green, you get a better resolution and even better in blue than in red. 

 4. Yes I agree, if one filter was faulty, I would have some issue with both Halpha AND SII (which share the red light) but they both look quite similar regarding this effect.

 

Maybe the N°3 explanation is the right one as I looked into autofocus routine and analyzed more precisely pictures.

Here is an example of my autofocus routine with RED filter. I have exactly the same results with Green filter, my triplet seems to have the exact same focus position with R & G (and almost same with blue).

If you look at the circle with the 2 small stars. You'll probably find that the best focuser position seems to be 32434 or 32407.
Now if you look at the "big stars" with HFR value, the best focuser position seems to be 32353 which was chosen by SGP but the small stars seems slightly out of focus.
It's like I can"t get a perfect focus with both big and small stars. How is that even possible 🙄? Do you see the same pattern as me?

 

Here is another example taken the same night on another object, the effect remains the same. Autofocus seems perfect with tight pinpoint stars but it's kind of slightly blurry/out focused with Halpha & SII .

On pixinsight, I applied exactly the same process. Autohistogram, Joint RGB/K channel : 0.18  /  histogramtranformation with exactly same values for the 3 images.

OIII Channel : 

Halpha channel : 

 

SII : channel

 

Thanks for your answer ansd best regards!!

Edited September 7, 2019 by HAlfie

[vlaiv]

32 minutes ago, HAlfie said:

3. Regarding your point N°3, my ESPRIT 80ED comes from Teleskop austria and was "perfectly" collimated by Tommy :

Quote

I am ready with Esprit 80/400 Nr. 1801057 - it reads 95% Strehl now and has a smooth star halo. The colors are very well balanced and no lateral color shift exists. The residual errors aste a trace of undercorrection and astigmatism, both in small digestable dose (95% Strehl without any subtractions)". The lens buffer ring has been exchanged, and lenses collimated one by one. The lens cell has been centered to the focuser (to have the optical axis at center of your picture)

Ususally I was expecting to have a cleaner Halpha channel than OIII, but it's the opposite. I also know that asiderom the optical color correction,  in green, you get a better resolution and even better in blue than in red. 

I'm not going to question how good collimation is on this particular scope, but there might be an issue with correction. Although from your quote we can see that "The colors are very well balanced" - I'm not sure what it means.

Triplets are tricky beasts with regards to collimation and proper lens spacing - and I for sure, have no clue about how it is properly done. What I do know is that there is no single strehl ratio for such a scope. If single strehl is quoted - then it is probably in single wavelength - usually around 550nm (or similar - peak of visual sensitivity in scotopic vision I think). Corrections in other wavelengths might and usually are different. Here is an example of strehl vs wavelength graph:

This is result for a doublet scope, and while around 580nm it approaches respectable 98% strehl, at Ha this particular scope is going to have strehl around 65% or so.
 

I "tested" my own TS80mm F/6 triplet with Roddier analysis - nothing fancy, RGB image split, and I for example got following results per "band": Blue was around 80% (let's say average), Green around 94% while Red was at around 98%. Such scope would have opposite characteristics - very sharp Ha, while OIII would be softer.

Have a look at this post to see results of test:

However, not sure if this is the cause of your issues with star bloat in Ha.

1 hour ago, HAlfie said:

If you look at the circle with the 2 small stars. You'll probably find that the best focuser position seems to be 32434 or 32407.
Now if you look at the "big stars" with HFR value, the best focuser position seems to be 32353 which was chosen by SGP but the small stars seems slightly out of focus.
It's like I can"t get a perfect focus with both big and small stars. How is that even possible 🙄? Do you see the same pattern as me?

No, actually, I'm not seeing it. I'm seeing what you are seeing and thinking that best focus position for small stars is as you said somewhere around 32407, but this is just a visual thing. You have no HFR for these stars, and because they are so dim - when not in perfect focus they will be even more dim and their "wings" (wings of star profile) will be below read noise. If you look carefully, you will actually be able to tell that it is spread around. it is very faint, but can be seen. I'll make a screen shot to point it out. Here it is - focus position 32407:

Now the same image - ridiculous level of stretch - just look at noise distribution:

Noise is more dense in rather large circle around central "tight" points - and this is also light from those stars but very low in intensity due to less than perfect focus, and being at the level of surrounding noise or below it, so it can't be easily spotted.

Also compare how much the size of disk of large star increased (it is already stretched) and how much of those two tiny stars.

[HAlfie]

Hello and thanks a lot for your answer!

you're totally right, a Roddier on 3 channels would give me the answer :)! I'll look into it!! That's very interesting especially to see which channel is the strongest suit and which isn't.

Yes the Strehl of 0,95 was on 550nm...! The fact that I'm à F/5 (which is quite fast) could be a part of the explanation.

The difference of focus quality between colors channel isn't really catastrophic but I may be flirting with my optics's limits :). Cannot trying to be a Takahashi when you're not ;).

Have a nice day and clear skies!!

 

[vlaiv]

9 minutes ago, HAlfie said:

Hello and thanks a lot for your answer!

you're totally right, a Roddier on 3 channels would give me the answer :)! I'll look into it!! That's very interesting especially to see which channel is the strongest suit and which isn't.

Yes the Strehl of 0,95 was on 550nm...! The fact that I'm à F/5 (which is quite fast) could be a part of the explanation.

The difference of focus quality between colors channel isn't really catastrophic but I may be flirting with my optics's limits :). Cannot trying to be a Takahashi when you're not ;).

Have a nice day and clear skies!!

 

There is a simpler test to perform rather than doing Roddier test - not that it is complicated, but will require a bit more processing.

Try imaging without field flattener. Maybe optics of the scope is fine, and field flattener is poorly corrected at red part of the spectrum. You will have smaller usable field for your test, but stars close to optical axis should be a good indication - they will be sharp.

 

[Ikonnikov]

As you mention above, shorter wavelengths will intrinsically give higher resolution images (with a smaller airy disk and therefore star size) than longer wavelengths. The theoretical airy disk size for Ha emission wavelength is 31% larger than for for OIII primary emission wavelength so could this not account for the differences in star sizes? I appreciate that bad seeing affects longer wavelengths less but i don't know at what point this would outweigh the intrinsically larger airy disk size. From experience I've  consistently seen smaller star fwhm measurements for OIII over Ha with reflectors and refractors.

 

Paul 

[vlaiv]

51 minutes ago, Ikonnikov said:

As you mention above, shorter wavelengths will intrinsically give higher resolution images (with a smaller airy disk and therefore star size) than longer wavelengths. The theoretical airy disk size for Ha emission wavelength is 31% larger than for for OIII primary emission wavelength so could this not account for the differences in star sizes? I appreciate that bad seeing affects longer wavelengths less but i don't know at what point this would outweigh the intrinsically larger airy disk size. From experience I've  consistently seen smaller star fwhm measurements for OIII over Ha with reflectors and refractors.

 

Paul 

Not likely that it would cause difference. Here is a little comparison of effects:

Let's suppose that we have rather decent seeing at 1.5" FWMH and it affects both wavelengths the same (500nm and 650nm). Both are on the same mount that is decent in performance for such a focal length, and guides with 0.5" RMS error.

What are expected FWHM of stars for each?

500nm: ~2.3"

650nm: ~2.52"

Although there is something like 30% or more difference in the size of airy disk, given that convolution of PSFs (seeing, airy and guide error) adds FWHM as square root of sum of squares, and airy disk being the smallest of the three, result will be much less in total FWHM - in this case, there will be increase of about 9.5%.

Once we take into account seeing impact vs wavelength, we should end up with roughly the same FWHM, or even small Ha having a small edge.

This is of course for perfect aperture. Any aberrations will have blurring effect that will raise FWHM.

[Ikonnikov]

3 hours ago, vlaiv said:

Not likely that it would cause difference. Here is a little comparison of effects:

Let's suppose that we have rather decent seeing at 1.5" FWMH and it affects both wavelengths the same (500nm and 650nm). Both are on the same mount that is decent in performance for such a focal length, and guides with 0.5" RMS error.

What are expected FWHM of stars for each?

500nm: ~2.3"

650nm: ~2.52"

Although there is something like 30% or more difference in the size of airy disk, given that convolution of PSFs (seeing, airy and guide error) adds FWHM as square root of sum of squares, and airy disk being the smallest of the three, result will be much less in total FWHM - in this case, there will be increase of about 9.5%.

Once we take into account seeing impact vs wavelength, we should end up with roughly the same FWHM, or even small Ha having a small edge.

This is of course for perfect aperture. Any aberrations will have blurring effect that will raise FWHM.

Using various online calculators (e.g. the Wilmslow Astro website) the airy disk size for green light (510nm) at 80mm aperture comes out at 3.2 arcsec and for red (650nm) as 4.1 arcsec i.e much larger than typical guiding error and larger than typical uk seeing. This being the case then wavelength would have more of an effect on the final image fwhm between oiii and Ha filters than you suggest above. I still typically see smaller star sizes with shorter wavelength filters when imaging with a mirrors only 250mm RC scope (where the airy disc size is in the order of 1 arcsecond for green and 1.3 for red) suggesting some effect of wavelength on fwhm even at larger apertures.

 

Paul 

 

[vlaiv]

On 09/09/2019 at 16:02, Ikonnikov said:

Using various online calculators (e.g. the Wilmslow Astro website) the airy disk size for green light (510nm) at 80mm aperture comes out at 3.2 arcsec and for red (650nm) as 4.1 arcsec i.e much larger than typical guiding error and larger than typical uk seeing. This being the case then wavelength would have more of an effect on the final image fwhm between oiii and Ha filters than you suggest above. I still typically see smaller star sizes with shorter wavelength filters when imaging with a mirrors only 250mm RC scope (where the airy disc size is in the order of 1 arcsecond for green and 1.3 for red) suggesting some effect of wavelength on fwhm even at larger apertures.

 

Paul 

 

You are comparing airy disk diameter vs FWHM (seeing) or Sigma (guide RMS) values.

If we do gaussian approximation to airy pattern you will find that sigma of that approximation is about one third of disk radius. Value that you quote - about 3.2" is airy disk diameter - which means that sigma is about six times less than that.

This would mean that comparable values (all converted to sigma of gaussian) are like:

Airy disk - 0.53"

Guiding RMS (that one stays the same) - you can put it at 0.5" RMS or what ever you are getting.

Seeing of let's say 2" (this is FWHM value, and to get sigma you need to divide it by ~2.355) so we have ~ 0.85"

In this case you can see that Airy disk size expressed in the same units is not dominant component and is not much larger than either seeing or guiding error.

[HAlfie]

Hello,

I check with Tommy and his explanations were crystal clear :

Quote

Esprit80 is working at f/5 and that means the spherical aberration is different at different wavelengths (spherochromasy is the term).  When it is balanced at green, there is undercorrection in red and overcorrection in blue.

Undercorrection in red can affect your autofocus procedure, as the psf function can be a modified Gauss curve, so the autofocus may be a bit confused. In your case it seem better to  me to choose 32380 than 32353.

Too much defocus intrafocal shows the ring around small stars, while the bright stars are overstretched and do not show it on your screen.

 

The other thing is:

Your pictures in red (and the deeper red the more) will be a somewhat more blurred than in green, due to lower spherical correction status in red. Usually people target this problem using a psf trained sharpening method like deconvolution, to restore best sharpness.

Everything was explained here and it is the same for ESPRIT 80 and ESPRIT 100 (in German but we can translate): http://interferometrie.blogspot.com/2013/09/skywatcher-esprit-80400-und-100550.html?view=magazine

 

The solution is either and/or use deconvolution process when processing the image, or apply a little offset when focusing in SequenceGeneratorPro (I apply +12 for instance but I may try +20 or +25) to find the "sweet spot" and deal with that.

For those who would be worrying about the results in RED or HA light, and the level of sharpness we can achieve with Esprit Triplet, using both of the solutions (optimized sharpness in Photoshop & offset to the Ha focusing), here is a first attempt on the Pelican nebula, 62x600S Baader Ha 7nm, some part under full moon, Esprit 80 PRO 80/400 with flattener, ASI183MM PRO, please click on the image to see the full version :

I think the level of detail/sharpness/tiny stars is pretty good especially considering it's taken with "only" 80mm of aperture. I'm still confident that the Esprit line are a bargain in the price/quality range.

Hope this topic helps some future owner of Esprit refractor 😉

Edited October 15, 2019 by HAlfie

[alexbb]

I found the same with my Esprit 80. I focus manually using a Bahtinov mask. The stars in narrowband through the Esprit are A LOT softer/larger than through a recently acquired AstroProfessional 102/714 ED with FPL51 + 0.8x reducer.

In fact, I didn't really check, but I believe the stars are softer in narrowband through the Esprit 80 compared to those I get through the SkyWatcher 72ED + the 0.85x reducer for 80ED.

[HAlfie]

Hello Alex, 

and congrats for your very nice images! I'm following you on Astrobin 😉!

Yes a triplet at F6 or more combined with a very good quality reducer/flattener (such as Riccardi 0.75x which is a beast or the TS 0.79x which is good) may give better stars in the red Channel. However, everything has a price to pay. How are corners exactly with your 102/714 and the reducer/flattener? This solution is often more critical regarding TILT and right distance with sensor than with only flattener that doesn't change the focal ratio.

In my case I want perfect stars on the whole image, including corners because I want to be able to perform mosaics.

I'm wondering, if your stars with the Esprit 80 are that big and soft, are you sure the lenses are properly collimated? I know that Ed Reis check them if bought on FLO and Tomy as well if bought on Teleskop Austria. I think it's a nice "insurance" just in case.

Have a nice day and clear skies to you!

 

Edited October 16, 2019 by HAlfie

[alexbb]

1 hour ago, HAlfie said:

Hello Alex, 

and congrats for your very nice images! I'm following you on Astrobin 😉!

Yes a triplet at F6 or more combined with a very good quality reducer/flattener (such as Riccardi 0.75x which is a beast or the TS 0.79x which is good) may give better stars in the red Channel. However, everything has a price to pay. How are corners exactly with your 102/714 and the reducer/flattener? This solution is often more critical regarding TILT and right distance with sensor than with only flattener that doesn't change the focal ratio.

In my case I want perfect stars on the whole image, including corners because I want to be able to perform mosaics.

I'm wondering, if your stars with the Esprit 80 are that big and soft, are you sure the lenses are properly collimated? I know that Ed Reis check them if bought on FLO and Tomy as well if bought on Teleskop Austria. I think it's a nice "insurance" just in case.

Have a nice day and clear skies to you!

 

Hello and thank you! Not sure about your user on astrobin, but I've more than a thousand recent notifications that I wasn't been able to follow yet due to many other activities. Will check them all one day.

I've one example of narrowband image at hand, but I believe the data was blended with some Esprit data too. https://drive.google.com/open?id=1VR3CCD_hTJYEsWAHHXXL7HbYl8CMphHO

Narrowband stars are good. Broadband? Not so good, the following image was taken solely with the 102/714 ED with its dedicated 0.8x flattener. You can make an idea about the stars on the whole image. https://drive.google.com/open?id=1xIM4ZSBuBLhNw9omr11uFYH8krmqXw4u

I bought my Esprit 80 from FLO and it was checked by Es Reid. My copy displays some dark gap in the stars halos, but I don't see a symmetrical opposite gap. If I remember well, in the lightpath I saw a gap in the O ring holding a lens so that might be the cause. You can see at full resolution here https://www.astrobin.com/362549/ or here https://www.astrobin.com/389908/C/

Overall I'm happy with the small Esprit with regards to its broadband performance, but I'll probably use the larger ED for narrowband.

Also probably, I will sell the 80 Esprit and buy the larger 100 with a good matching flattener that can deliver a corrected flat field for a 43mm sensor.

Have a lovely day too and clear skies as well!

[HAlfie]

Hello Alex!

Thanks for your answer!

Aah, you're picture of NGC6992 remind me of the one I took 1 month ago that I don't totally yet finished in post processing but I'll show you anyway !! What camera and time integration per filter did you use?

In my case ASI183MM PRO, 57x600s Baader Ha 7nm and 48x600S with Baader OIII 8.5nm filter.  And you?

This is impressive how the 2 pictures are similar. Your stars seem smaller though!

 

Regarding dark gap in the stars halo, yes some Esprit can be "affected" by that. That's why Tommy from Teleskop Austria guarantee a smooth star halo. In my case,  the lens buffer ring has been exchanged (to avoid the dark gap in stars halo), and lenses has been collimated one by one. It's a step above "just" check the collimation but it's more expensive. But if Es Reid has chdk the collimation, no need to worry 😀! 

Regarding the picture with AstroProfessional 102/714 ED with FPL51 + 0.8x reducer, yes corners aren't perfect as I presumed, especially at the top left corner. It's not catastrophic but not as "flat as with the Esprit.

Gosh, why is it so hard to find the perfect refractor 😜! Except the FSQ lines which cost an arm and a leg, the ideal fast focal ratio < 5, with excellent color correction in red & green channel, with perfect corners and a stiff rotating focuser is hard to find. Maybe an "old" TMB 80/500 or 80/480 with Feathertouch and Riccardi reducer could be a solution?

On another hand, there is the Takahashi TSA 102 (still expensive) but it's only F6 with the reducer. As the ASI183MM is very demanding with its the tiny pixels, it pushes you to increase your time exposure to smooth noise. I would not dare to use it with a focal ratio > 5.5. The esprit 120 with Riccardi Reducer F5.25 could be a good match but it's heavy... Near 12kg/26lbs 😖!

Clears skies to you and best regards!

Edited October 16, 2019 by HAlfie

[ollypenrice]

On 07/09/2019 at 18:52, HAlfie said:

Cannot trying to be a Takahashi when you're not ;).

Have a nice day and clear skies!!

 

I wouldn't assume Takahashi would be better. I like Taks, having had the FSQ85 and now using an older FSQ106N Fluorite, but they are not perfect. Whatever the spot diagrams say, in practice the TEC140 with TEC flattener beats the Taks in the challenging blue channel.

I've never had tighter stars in OIII than Ha in any scope. Always the opposite. Then again I've never had a good OIII filter. My Baader is so-so and my Astronommik is poor.

Surely your issue is troubling you more out of curiosity than because it impacts on your final images?

Olly

[alexbb]

12 hours ago, HAlfie said:

Aah, you're picture of NGC6992 remind me of the one I took 1 month ago that I don't totally yet finished in post processing but I'll show you anyway !! What camera and time integration per filter did you use?

In my case ASI183MM PRO, 57x600s Baader Ha 7nm and 48x600S with Baader OIII 8.5nm filter.  And you?

This is impressive how the 2 pictures are similar. Your stars seem smaller though

I didn't save any notes on this one, it was only to test the performance of the scope/flattener combo. But, as I blended all the data I had available, I don't really believe it's relevant. Perhaps the old data was shot through the Esprit and filtered by Optolong narrowband filters (which I sold as the OIII filter was really reflective), the new data was through the 102ED and Astronomik 6nm filters.

12 hours ago, HAlfie said:

Regarding the picture with AstroProfessional 102/714 ED with FPL51 + 0.8x reducer, yes corners aren't perfect as I presumed, especially at the top left corner. It's not catastrophic but not as "flat as with the Esprit.

The flattener on the 102ED is a push-fit one. It might have been tilted a bit as other times I didn't notice any deformation on stars. OTOH, all the connections with the Esprit are threaded, but I noticed elongated stars with that too. Easily visible in this one: https://www.astrobin.com/362549/. Note: it is made of 2 panels in portrait orientation.

[HAlfie]

On 17/10/2019 at 10:37, ollypenrice said:

I wouldn't assume Takahashi would be better. I like Taks, having had the FSQ85 and now using an older FSQ106N Fluorite, but they are not perfect. Whatever the spot diagrams say, in practice the TEC140 with TEC flattener beats the Taks in the challenging blue channel.

I've never had tighter stars in OIII than Ha in any scope. Always the opposite. Then again I've never had a good OIII filter. My Baader is so-so and my Astronommik is poor.

Surely your issue is troubling you more out of curiosity than because it impacts on your final images?

Olly

 

@ollypenrice :  thanks for your answer!

How is your experience with both FSQ? What are their weaknesses?
I may presume the spherochromatism should be better in the red (HA & SII) light? Yes the TEC 140 is a very nice piece of optic :).I would love to get my handson the TEC 110 lenses in the WO optical tubes.
Yes the issue is a little bit troubling but can be compensated with a slight offset applied with autofocus some nothing dramatic indeed!

@Alexbb: thanks for your answer! Yes you're right, I'm curious to see your next images with your new refractor :).

 

Have a nice day and clear skies

 

Edited October 18, 2019 by HAlfie

[ollypenrice]

22 minutes ago, HAlfie said:

 

@ollypenrice :  thanks for your answer!

How is your experience with both FSQ? What are their weaknesses?
I may presume the spherochromatism should be better in the red (HA & SII) light? Yes the TEC 140 is a very nice piece of optic :).I would love to get my handson the TEC 110 lenses in the WO optical tubes.
Yes the issue is a little bit troubling but can be compensated with a slight offset applied with autofocus some nothing dramatic indeed!

@Alexbb: thanks for your answer! Yes you're right, I'm curious to see your next images with your new refractor :).

 

Have a nice day and clear skies

 

My FSQ85 was optically stunning though it wouldn't cover a full frame chip. I had no quibbles with it at all and it wasn't too sensitive to focus/temp variation.

I also very much like my fluorite FSQ106N. It gives good star shapes on full frame but it does have quite a fall off in illumination at the corners, so flats are obligatory. They sort it out fine, though. It is much less temperature sensitive than the newer ED106 which is why I prefer it. I don't use robotic focus. (I don't like computers/softaware or extra USB leads and I don't fancy paying for three motor focus installations! One of setups is a dual rig, hence the three focusers.)

Both the FSQs create the familiar Tak 'inverted lighthouse beam' effect on some bright stars but this has never bothered me. It does upset some people.

Olly