Imaging Scope Decision - Forums

Topic starter

Feb 22, 2021

Dear Astrobin Community

I am fairly new to the deepsky imaging hobby and just started recently with tracking. I bought the MSM Star Tracker for wide field stuff and I hope that soon the Skywatcher Star Adventurer will arrive (already ordered it one month ago).

For imaging cameras I have the Sony A7 iii (which I also use for daytime fotografie) and I sent in my A6000 for an astromodification. For the lenses I have the Sony 16-35 GM F2.8, the Sony 24-105 F4 and the Sony 100-400 GM F4.5 - 5.6. A few days ago I ordered the Samyang 135mm but unfortunately it could not focus to infinity (just stopped a pinch before that), so I sent it back. I also read in the internet that a few people have the same problem, and I did not want to "mod" a new lens to counter that problem.

One problem I have with the Sony 100-400GM is the focussing. Whenever I want fo focus manually it is quite tricky to find the right spot, because there is no infinity point and (my guess) it uses the autofocus motor to focus. Also for the flats (which I usually would take on the other day at home) I would have to keep the focus which is impossible with this lens.

Since I watch a lot of youtube videos about astrophotography and I am not yet into autoguiding (I want to start "small"

, I came across a few people that use the William Optics Redcat 51. That scope would allow me to focus better (at least I hope so) and also has a nice wide field of view (no autoguiding needed). The scope is also fairly light for my portable skywatcher setup.

But, the scope is mostly sold out, and since I live in Switzerland (not easy to get specific astro stuff here) and with the Covid situation, I have to find a shop that delivers into Switzerland. So I looked for other scopes that are at the moment on stock. I found the Omegon 61/274 which has a similar field of view as the Redcat 51 and the TS Optics 61/274. The nice thing about these three scopes (Omegon, TS Optics, William Optics) is, that they all have a flattener already built in (or are shipped with one).

So, is anybody here that has already used the TS Optics or the Omegon Scope for imaging? I cant find any pictures here for the Omegon Scope and just a few for the TS Optics. Or should I just wait until the Redcat is back in stock?

Sorry for the long post!

Björn Arnold

Perfect post

Feb 22, 2021

Hi!

First of all, let me say that I haven't used any of the three scopes you were asking for. So if these are definitely your choices, you might skip the rest of this message. Otherwise, let me report on my scope: William Optics ZenithStar 61II (for some images, you can check my (small) gallery or simply find other images taken with the same scope from other people that have way more experience than I do).

First of all: a flattener is a must for the ZenithStar. Otherwise you would be restricted to smaller objects located in the field center (check my M31. It was shot before I received the flattener). The ZS61II has a focal length of 360mm. Don't know if this is too much for your taste but still ok to go unguided, given a good mount and polar alignment. It's an f/6, so slower than the optics you have in mind.

Regarding focusing: the ZenithStar (and the RedCat) come with a Bahtinov mask that will allow *perfect* focusing. With other scopes, I tried things like live-view focusing on the DSLR but in my opinion, a Bahtinov mask beats everything.
If any of the other scopes come without such a mask, you should guy one together with the scope.

From my personal experience, I fell in love with the William Optics. They are very good optics, and their design is also nicely done.

I also know that right now the market is scarce and sometimes one buys something because it's available. For me it's ok for some accessories (adapters and even guide cameras and guide scopes) but not for any main optical components or mounts. Here, you should carefully chose and wait if necessary. But you're obviously taking the right approach and ask for opinions.

Hope that helps a bit!

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dkamen

Feb 22, 2021

Hi,

I don't think any telescope in that class is spectacularly better than your 100-400 lens optically. Neither will they be easier to focus.

Why don't you just get or DIY a Bahtinov mask for it?

Cheers,
D.

the_moonman92

Topic starter

Feb 23, 2021

Björn Arnold:
Hi!

First of all, let me say that I haven't used any of the three scopes you were asking for. So if these are definitely your choices, you might skip the rest of this message. Otherwise, let me report on my scope: William Optics ZenithStar 61II (for some images, you can check my (small) gallery or simply find other images taken with the same scope from other people that have way more experience than I do).

First of all: a flattener is a must for the ZenithStar. Otherwise you would be restricted to smaller objects located in the field center (check my M31. It was shot before I received the flattener). The ZS61II has a focal length of 360mm. Don't know if this is too much for your taste but still ok to go unguided, given a good mount and polar alignment. It's an f/6, so slower than the optics you have in mind.

Regarding focusing: the ZenithStar (and the RedCat) come with a Bahtinov mask that will allow *perfect* focusing. With other scopes, I tried things like live-view focusing on the DSLR but in my opinion, a Bahtinov mask beats everything.
If any of the other scopes come without such a mask, you should guy one together with the scope.

From my personal experience, I fell in love with the William Optics. They are very good optics, and their design is also nicely done.

I also know that right now the market is scarce and sometimes one buys something because it's available. For me it's ok for some accessories (adapters and even guide cameras and guide scopes) but not for any main optical components or mounts. Here, you should carefully chose and wait if necessary. But you're obviously taking the right approach and ask for opinions.

Hope that helps a bit!

I already checked out the Zenithstar 61 but I think for the astromodified a6000, which is an APS-C sensor, the focal lenght will be too long without any guiding. Also the Zenithstar with the flattener might add too much weight for the star adventurer. But thanks for the reply!

the_moonman92

Topic starter

Feb 23, 2021

Hi,

I don't think any telescope in that class is spectacularly better than your 100-400 lens optically. Neither will they be easier to focus.

Why don't you just get or DIY a Bahtinov mask for it?

Cheers,
D.

Yeah I might stick with the 100-400 for the start and see how it will go. I will think about the Bahtinov mask!
Maybe if I am not that happy, I will buy the Redcat (when its back on stock). Thanks!

Björn Arnold

Perfect post

Feb 23, 2021

I already checked out the Zenithstar 61 but I think for the astromodified a6000, which is an APS-C sensor, the focal lenght will be too long without any guiding. Also the Zenithstar with the flattener might add too much weight for the star adventurer. But thanks for the reply!

I'm sorry, I should have read more carefully and notice the type of mount you are planning to use.
In this case, I agree with you. While you still might shoot unguided with the ZenithStar (given an appropriate mount) for some exposure lengths, you won't be happy with the ZS on your mounts.
In that case, I join the other poster: use the optics you have. When you're going for a dedicated scope, you'll also buy an appropriate mount.

Clear skies!

Björn

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dkamen

Feb 23, 2021

I must say focusing -which I understand is your main issue- can be a tricky business. Perhaps the trickiest. You would expect to be able to nail it and just leave it there but this is not the case: addition/removal of filters and thermal expansion change the critical focus point. Sometimes even gravity can play a part, pushing or pulling the telescope (or perhaps a mirror) and altering the focus point.

Having an infinity mark gives you a good starting point but astrophotography has much stricter requirements than daylight photography. Being right on the mark is fine when you are taking a photo of a mountain, but for actual astronomical infinity you might need a couple microns to the left or to the right, especially when the temperature changes.

A Bahtinov mask helps a lot but it is not always possible to find a good star (not too low on the horizon, not too bright otherwise you cannot tell you are in focus, not too faint otherwise there are no diffraction spikes). And when seeing is suboptimal, it is just like having an ultra bright star: the difraction spikes are too wide to be able to tell if the middle one is, well in the middle. Or the mask could have a slight tilt so the pattern is not exactly symmetric.

I use computer-aided focusing: I center an average sized star, and keep taking exposures while Ekos is measuring the HFR. When the HFR is minimum, I have reached critical focus. The problem is "minimum HFR" can vary dramatically with seeing conditions. One night I reach a HFR of 0.7 and am very happy with it, the next night it's 1.3 and I tell myself "okay I need a little more" but because of seeing (or because I picked a different star) 1.3 is actually the minimum and moving the focuser starts a 10 minute torture until I am able to reach it again

The morale of the story is if you want to get better focusing, the only thing the Red Cat (or any other WO telescope) will help you with is that it includes a Bahtinov mask. This is very convenient but not a good enough reason if you already have a 400mm f/5.6 lens. If shape of the stars you get from your lens is not good enough, especially within the inner 3/4ths of the frame or so, then by all means go for the Red Cat. Also if you detect a sensor tilt (stars having a different shape at one side of the frame). The RedCat includes a corrector for that.

Generally speaking, most telescopes tend to deliver a better image than most photographic lenses once focused at infinity. They are not easier to reach that focus.

Cheers,
Dimitris

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the_moonman92

Topic starter

Feb 23, 2021

I must say focusing -which I understand is your main issue- can be a tricky business. Perhaps the trickiest. You would expect to be able to nail it and just leave it there but this is not the case: addition/removal of filters and thermal expansion change the critical focus point. Sometimes even gravity can play a part, pushing or pulling the telescope (or perhaps a mirror) and altering the focus point.

Having an infinity mark gives you a good starting point but astrophotography has much stricter requirements than daylight photography. Being right on the mark is fine when you are taking a photo of a mountain, but for actual astronomical infinity you might need a couple microns to the left or to the right, especially when the temperature changes.

A Bahtinov mask helps a lot but it is not always possible to find a good star (not too low on the horizon, not too bright otherwise you cannot tell you are in focus, not too faint otherwise there are no diffraction spikes). And when seeing is suboptimal, it is just like having an ultra bright star: the difraction spikes are too wide to be able to tell if the middle one is, well in the middle. Or the mask could have a slight tilt so the pattern is not exactly symmetric.

I use computer-aided focusing: I center an average sized star, and keep taking exposures while Ekos is measuring the HFR. When the HFR is minimum, I have reached critical focus. The problem is "minimum HFR" can vary dramatically with seeing conditions. One night I reach a HFR of 0.7 and am very happy with it, the next night it's 1.3 and I tell myself "okay I need a little more" but because of seeing (or because I picked a different star) 1.3 is actually the minimum and moving the focuser starts a 10 minute torture until I am able to reach it again

The morale of the story is if you want to get better focusing, the only thing the Red Cat (or any other WO telescope) will help you with is that it includes a Bahtinov mask. This is very convenient but not a good enough reason if you already have a 400mm f/5.6 lens. If shape of the stars you get from your lens is not good enough, especially within the inner 3/4ths of the frame or so, then by all means go for the Red Cat. Also if you detect a sensor tilt (stars having a different shape at one side of the frame). The RedCat includes a corrector for that.

Generally speaking, most telescopes tend to deliver a better image than most photographic lenses once focused at infinity. They are not easier to reach that focus.

Cheers,
Dimitris

Its not per se that focusing is a problem, but the lens I have does not have an infinity focus indicator nor does it have a focus stop (or end). The focusring just turns and as soon as you come close to infinity it "jumps" and thats what makes focusing with the lenses I have hard. The image quality they produce is very good, and I also did some milky way shots with the wide angle. They are perfect for autofocusing, but hard to focus manually (especially at infinity). Thats why I thought, that the WO or the other two choices might be better for focusing, since they are designed to focus manually.

But thanks for the explanation with the Bahtinov mask (and the computer aided focusing!), I have to look more into that way of focusing

Die Launische Diva

Feb 23, 2021

@the_moonman92 look for a software which allows you to manually focus from a computer. Something like this: https://support.d-imaging.sony.co.jp/app/imagingedge/en/instruction/4_3_panels.php#cnt5

(Disclaimer

: I don't own a Sony camera and I don't know if this is a good piece software and if it works with your cameras! If your cameras have wifi maybe there is an official app for your smartphone!)

In that way the lens focusing motor will probably stay engaged and if you don't touch anything on the lens probably you will be safe.

the_moonman92

Topic starter

Feb 24, 2021

Die Launische Diva:
@the_moonman92 look for a software which allows you to manually focus from a computer. Something like this: https://support.d-imaging.sony.co.jp/app/imagingedge/en/instruction/4_3_panels.php#cnt5

(Disclaimer : I don't own a Sony camera and I don't know if this is a good piece software and if it works with your cameras! If your cameras have wifi maybe there is an official app for your smartphone!)

In that way the lens focusing motor will probably stay engaged and if you don't touch anything on the lens probably you will be safe.

Thanks! I will definitely look into that!

Doug Summers

Feb 24, 2021

I use computer-aided focusing: I center an average sized star, and keep taking exposures while Ekos is measuring the HFR. When the HFR is minimum, I have reached critical focus. The problem is "minimum HFR" can vary dramatically with seeing conditions.

Hi Dimitris, Just a note to let you know that temperature and target elevation based focus updates (autofocus start seed position and focus updates between Light exposures) have been developed in Ekos and are being tested now. That should help reduce some issues with bad HFRs during seeing instability. In the near future, assuming a reasonably well behaved focuser, only 1 good initial focus setpoint will be needed (manual or autofocus) to hold focus while temperature and target altitude changes. Stay tuned.....hopefully this will be released soon.

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Rich Sky

Feb 25, 2021

I recommend a RASA or sct with starizona hyperstar. The sct is Good for planets and hyperstar for nebulae.