TS Photon 10" f/5 (or similar telescopes) experiences wanted - Forums

Topic starter

Mar 16, 2021

Hi folks,

I am quite new to (real) astro photography. About a year ago, I started with a Bresser Exos-1 mount and my DSLR. I was quite happy with my results and soon wanted a real mans telescope. Last autumn I bought an EQ6-R Pro mount and was really excited about it. In my excitement, I also invested in a used TS Photon 10" f/5 telescope (1270mm focal length) and also bought a ZWO ASI183MM Pro Mono camera, a 60mm guide scope, an Altair GPCam3 178M and some other things. Of course, I did some research before I spent that much money on astro gear. As you all might know, it was absolutely worth it. But that was also, when my trouble began.

With some experience with AstroBackyard, a lot of theoretical knowledge from the web and the will to do it right, I put all the equipment together and spent some nights out to do THE ONE image that justifies the money I lost. I aimed to my most interesting subjects: Galaxies. But that one image never happened. To be exact, I was not able to get nearly in the ballpark. The images were very blurry (used also an ZWO EAF and an Explore Scientific HR Coma Corrector) but my guiding seemed to be right. Compared to the DSLR, I had to expose much longer. So I began to analyze my problems and tried to find some solutions.

What I found was, that there are some things that could cause the issues:

1.) The weight of the telescope and its attachments was too high for the EQ6-R Pro (about 18 kg in total). Maybe, the system began to swing in some directions. Maybe that caused the blurry stars. That would be no surprise with that long focal length. But the blurry stars were blurry in every direction. Strange...

2.) The camera resolution was way too high for that focal length. I didn't know about oversampling and seeing conditions. But did that really effect the exposure times so drastically? I used a gain of 110 (unity). Maybe that was too low? I tried binning 2x2, but it seems to be the same situation as with binning 1x1.

3.) My Coma Corrector seemed to be really useful. I saw sample images that compare this thing with some other correctors and it seemed to be quite impressive. I also made sure, to be exactly in that 55mm working distancef to the sensor. I am quite sure, that this was not the problem, or was it?

4.) The mirror of that telescope may not be the greatest one on earth. But did that effect the image quality that much? Why are my stars so big? Why do they have that blurry edge?

5.) Is there some other issue I didn't realize yet?

Examples:
https://www.astrobin.com/pjl95r/?nc=user
https://www.astrobin.com/y47n1y/?nc=user

Some weeks ago, I risked to invest in an Explore Scientific MN-152 Comet Hunter. I put the same equipment together, got out in the field and did some amazing images (at least to me). So my theoretical knowledge could't be that bad. Obviously, some of the issues could be solved. The weight of that scope is about 13 kg with all the additional stuff. The resolution of the camera fits the optics better and has less oversampling (it still does, I know). Because the scope is a Maksutov-Newton, I don't need a coma corrector anymore. The mirror and the lens of that scope may have a better quality. So now I am really happy with my results. Only the weather is problematic ;-)

So... very long introduction, but here are the main questions...

Has someone out there some expierience with that 10" f/5 TS Photon scope (or similar ones)? How satisfied are you with your images? Did you see some of the same problems? Do you have some suggestions what I did wrong? Is it maybe the mirror or the scope? Is that scope size hard to handle (of course it is as a beginner as I am)? Is it too much for the mount?

I ask, because I think about giving that scope a second chance. Because I put most of my equipment on the Comet hunter, I have to invest some money (about 350€ for a Losmandy plate and a second EAF focuser) to make the scope really usable again. But is it worth it? Is it possible to get some quality images out of that optics?

Please share some of your experiences with me and help me or other beginners once more to make the right choice...

Thank you very much...

Christian

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Torben van Hees

Mar 16, 2021

Looking at the images I see that you are not perfectly in focus - the stars are little donuts if you adjust the stretch „just right“ - This might be curvature because the sensor-corrector distance needs fine-tuning. Also, collimation is a bit off (central stars are oval).

The long needed exposure times in comparison with the DSLR is because of the small pixels,
this is no problem, you do not loose quality by that (you can resample in post).

I have the 12“ f/5 Photon, purely for visual. The 10“ is certainly a bit oversized for the Eq-6R. My mirror is good - the rest is good for visual but not for DSO imaging: My 12“ is quite sloppy, not only the tube, but also the primary (and secondary) mirror holder and especially the focuser. This leads to the scope losing collimation while slewing (ever so slightly, not a big problem for visual, but certainly with imaging with those small pixels). I am amazed your images are as they are with the 10“. I feel the focus might cover some guiding/collimation errors.

I feel that you should concentrate your efforts on the comet hunter. When you have learned that scope and it’s quirks, it is time to step up, maybe to the 10“ Newton.

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Jérémie

Mar 16, 2021 · Edited: Mar 16, 2021

@Christian Großmann Well, I am a newbee as well, so take my comments with care :-)

Your stars are quite round, so in imo, no polar alignment nor tracking problems, though I would definitely use an Off Axis Guider with longer focal length. By the way, the small sensor of the ASI183MM Pro avoid the OAG prism to be visible.

I think your subs maybe too long (600 sec ?) and maybe the pixels saturate with bright stars, making them quite large ? You should try with shorter exposure time on your subs.

On the focus side : do you use a Bahtinov mask to help or check your focus ? Maybe you should double check with that at least at the beginning ?

Last but not least, have you tried to perform a deconvolution on your images, using a manual PSF based the stars on your images ?

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Andrea Alessandrelli

Mar 16, 2021 · Edited: Mar 16, 2021

I don't think there's a focus problem in the pictures. I think you used too long exposures considering the type of mount, the weight of the scope, the guiding FL compared to the imaging FL (maybe .. i'll get to that later). The "donuts" are caused by an high stretch of the stars, probably helped by the overexposure.

If it was out of focus we would have seen double spikes and the smaller stars would have been donut shaped.

Also, the galaxies seem detailed enough maybe a bit blurry, still in my opinion caused by too long exposures. Let's take drift as example: using a guidescope, even if well pointed and firm, even with the best polar align you can get, you'll always experience some drift even if mininal. An then there's seeing, a gust of wind, backlash, a satellite bombing.. all the good reasons to shorten the exposure time.

I suggest to start looking at the ADU of the images when choosing the exposure time. I own an ASI1600MM-Cool and I've found the exposure tables for the camera very useful. Just look for the "sub exposure tables for the ASI183MM".

As I wrote before also you are guiding with quite a load for the mount, 300sec max (I underline MAX) would be more reasonable.

About the guidescope FL compared to the FL of the scope, it's usually best to guide with at least 1/3 of your scope Focal Lenght, but to be honest I don't know if this statement is valid anymore since PHD2 implemented the multistar guiding. A relatively small guide scope (50-60mm) plus a guiding camera with a large chip may grant an amount of stars to use as reference that an OAG or a long FL guidescope wouldn't be able to grant.

Summing up: look at the exposure tables for you camera, look at the ADU you get before choosing the exposure time, never go over 300sec (MAX!! - at least with that setup) and give your scope a second chance, a third, a forth…. and when you are bored sell it to me since I was looking for something similar. (Joking.. of course I would like the same scope but I'm sure you'll be happy with it once you get a handle).

Clear Skies,

Andrea

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Christian Großmann

Topic starter

Mar 17, 2021

Thank you all for your help. It seems, that there is some hope…

If I remember those nights out, there are several things that keeps me saying, that Andreas post could possibly be the right answer. At first, I did the same things as with the Comet Hunter, where the results are really pleasing. Then I also recognized, that light wind breezes effects the telescope. But I was quite surprised, that the high weight on the EQ6 was handled relatively stable compared to the EXOS-mount with much less weight on. I also thought about reducing the exposure times. I'm sure I did during other nights from which I didn't post pictures. But I am also sure that I never got under 300s per frame. Maybe thats what I should try.

In case of the focal length of the guide scope, I did some research before I decided to go for the 60/240mm one. I was told that the ratio of 3:1 is not needed anymore with modern cameras. That might be right. Coming from "normal" photography, I am always thinking about the crop factor. The main scope has a focal length of 1270mm. The camera has a sensor size that leads to a crop factor of about 2.7 compared to full frame. That means, that the real focal length (with the additional 1,06x from the corrector) is somewhere in the range of 3600mm!!!. That might not be usable 1:1 with my astro gear, but it is obvious, that different sensor sizes lead to different requirements for the gear. There has to be some calculations with the guide scopes focal length as well. So it might be true, that the ratio is in my case not optimal. By the way - that might be the case, why nowadays a ratio of 1:10 is recommended. The sensors of modern guide cams are quite small compared to the sensors of the main cameras. That could lead back to the old ratio. It also might be, that my guide scope was not adjusted that well.

Using a longer focal length for the guiding will surely add some additional weight on the mount, didn't it? To be honest, I am not sure if this is a helpful solution. It may be the right one, but by instinct I will give this try a lower priority.

In case of image editing, I will also have a look at deconvolution and PSF. Those are new terms for me. I was always more on the technical side and image editing is something I will have to learn. But I also know, that if the basic exposures aren't right, image editing could (or should) not be the helpful wizard.

As a long term project, I will reactivate the scope and do some research with it. As I know myself, there will be a time when I need the new investments at least for other scopes. The next weeks I will keep shooting with the Comet Hunter. The reason is, that I am in need of some positive experiences to keep the fire burning. And this scope has enflamed the fire massively.

So thank you guys for the kind help so far.

Clear skies!

Christian

Jérémie

Mar 17, 2021

Christian Großmann:
Using a longer focal length for the guiding will surely add some additional weight on the mount, didn't it? To be honest, I am not sure if this is a helpful solution. It may be the right one, but by instinct I will give this try a lower priority.

The solution to gain weight on your system AND have a better guiding is « off axis guiding » : no need for the guidescope as you use the main imaging scope, so less weight on the mount, plus you are more precise as the focal length correspond to your imaging resolution (arcsec/pixel, modulo the different pixel size between the guide camera and imaging camera)

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Christian Großmann

Topic starter

Mar 17, 2021

Thanks for the reminder, Jeremie. Of course, this is the best solution. I never thought seriously about it. So it was out of my head…

Eduardo Oliveira

Mar 17, 2021

Are you sure your telescope was well collimated? In an f/5 reflector, collimation would be very critical. Your image seems to lack resolution. But otherwise you have round stars. This could be poor focus or a badly collimated telescope.

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Christian Großmann

Topic starter

Mar 17, 2021

Hello Eduardo,

I am quite sure. I collimate my scopes usually every time I am out in the field before I go to bussiness. I use a laser collimator and the light beam could be seen in the little mark on the main mirror and also pointed to the center mark of the collimator itself. But I will check it twice, the next time I use it. The lack of resolution might result from cropping the image at some point. I have to check the original files but I can't currently do it. But maybe thats another thing to keep in mind.

Today I ordered the focuser and the losmandy plate. Over the next two weeks, I might put everything on the scope together to do some additional tests. But the forecast is bad. Maybe there are some holes in the clouds to do some research with the scope. However, the really promising conditions are for the comet hunter…

dkamen

Mar 17, 2021 · Edited: Mar 17, 2021

Hi Christian,

To my eyes at least, the stars in the second image are not exactly round. They are slightly oval shaped, I would say in fact almost shaped like a D but with the straight edge approximately at 3 o' clock. Not sure what could be causing this, if I had to venture a guess it is a combination of some asymmetry in the optical train + trailing from the 600 second sub.

I actually think the galaxies are not bad at all on either pic, it's the stars that are (relatively) problematic. Also, if you scale either image at the same resolution as your images with the DSLR you will see they both beat it in terms of quality, hands tied behind their back. What you are dealing with are issues that simply would be invisible with your previous gear.

That said, the overall look is probably explained by the 600 second exposure. f/5 is relatively fast and the 183 has a small full well depth, have you checked if the stars are simply blown out (let alone the fuzziness which will be introduced during 600 seconds of exposure, from seeing variations if nothing else)? I see your image with the MN used 300 second subs, this must have played it's part in the overall higher quality (alongside the superior optics and the smaller focal length, much better suited to the average seeing conditions experienced by us mortals). I would say try 180 second subs (obviously take about 3x as many). Don't worry if you don't see much on each sub, you will after integrating them. I expect the result tk be significantly better than with 600 second subs, even if you don't change anything else.

There are objective ways to measure your star quality and shape on Pixinsight. Simplest one is to load a sub and hover the stars, see if K seems stuck at 1.000 even at smaller ones. You can also use DynamicPSF, start picking stars that are barely visible in the unstressed image and see what PSFs it is chosing. If they are gaussian you are almost certainly overexposed. If the generated PSF is asymmetric you have tracking or optical issues.

Cheers,

Dimitris

PS what you are saying about the crop factor is not accurate. What matters with sensors is pixel pitch. A smaller sensor doesn't magnify more compared to a larger sensor with same size pixels. It simply displays a smaller (cropped) part of an otherwise identically resolved field of view. Focal length is unchanged and there is no difference for purposes of guiding, seeing, detail etc. Smaller sensors are actually better if you are a perfectionist because you are working near the center of the image projected by the scope, where all optical aberrations are minimal or negligible. So don't worry that the OAG will result in an unworkable ratio of guider optical train vs imaging optical train

Actually the two chips have identically sized pixels so their ratio will be 1:1. But I agree with you that an OAG will add complexity at this point.

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Christian Großmann

Topic starter

Mar 18, 2021

Hi Dimitris,

thanks for your comments on the image quality. After all these conversations, I have a lot of stuff to try and your opinion opens a whole new aspect of the whole "problem".

About the crop factor thing, I only partly agree. With a cropped sensor, you only pick a smaller field of view from the whole circle the scope can deliver. But choosing a smaller FOV magnifies not only the final image (basically as if you see a cropped part of a full image magnified on the same sized computer screen), but also increases the problems you get with the same optics. If a larger pixel is collecting light from a small star and handles also the (lets say) abberations etc. surrounding it, a sensor with smaller pixels may collect those light and the surrounding problems with more pixels. That said, you get a much sharper view of all those problems, that in the end does not increase the image quality. You only increase the sharpness of the fuzziness (i.e. the seeing if you will) and do not get more detail. Of course, the focal length of the scope stays the same. If you compare the work to handle those magnified problems, it feels like you are using a scope with the focal length multiplied by the crop factor. To handle a scope with a feeled focal length of 3500mm may be nearly the same as handling a real scope of that size (just a guess ;-) ).

CS

Christian

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Torben van Hees

Mar 18, 2021

No, sorry, you mix things up here. „Crop factor“ is useless in astrophotography (I feel it is useless in terrestrial photography, too).

A smaller sensor does not magnify problems, quite contrary, as you are only dealing with the best-corrected part of the field. Small pixels can magnify problems, of course, but pixel size is independent of sensor size. There are 8x6 mm sensors with 9my pixels and there are 45x36mm sensors with 3.75my pixels. A smaller FOV does not magnify anything. It is just that: A smaller part of the sky. My QHY294M has smaller pixels than an ASI533MC but a larger FOV.

Long focal length imaging does lead to problems that short-FL imaging does not have, irrespective of sampling, caused by the construction of a long-FL image train (flex, weight, etc.).

Also, there are no small and large stars. If your optics are diffraction limited and well-corrected, all stars have the same size airy disc. You want to pick an image scale where the convolution of that airy disc with seeing is well matched. This is picking the right pixel size for your scope and seeing conditions. The sensor size plays only a part with respect to which objects you can fit on the sensor.

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Christian Großmann

Topic starter

Mar 18, 2021

OK, you are right. But I used to watch the final image on a screen with a size of 24 inch (lets say Fullscreen). I use this screen nearly all the time. And even if not, I print all my images on paper with the size of 15x20cm. And if you scale all your images in the end to that size(s), the crop factor does somehow matter. Then it becomes really a crop as the photographers would have done it in history. You don't see the pixel size in the final image. You just see the image itself. That is when I compare the quality.

But I agree with your explanation and I admit mixing things up.

My ,oh my! There seems to be a lot of knowledge out there to take into account

This is getting very interesting…

andrea tasselli

Frequent contributor

Mar 18, 2021

Yet is all in the name: Crop Factor. You don't crop the focal length, you crop the view (or more specifically the frame angle of view). Yet people keep repeating this magnification thing. I assume it is because it is easy in practical terms and resolution limit doesn't really come into the equation for terrestrial pictures. Most of the times anyway.