Better shaped stars with the Star Adventurer

Hi all,

I have just started astro-photography a few month ago and I use now a Canon DSLR/Lens with a Star Adventurer on a good photo tripod without guiding.

I am quite happy with the result I get with my 70-200mm L @ 200mm up to 60sec or even 120sec exposure, but I am not happy with the images I get when I use the 100-400mm L mk I @ 400mm and 60 sec exposure : the stars are awful and often show a triangle-like shape as seen below. This is a crop near the center of the image.

Now, I would like to get better result with nice rounded stars, but I am not sure of the cause of this problem and I need your input

My question: is this star shape caused by a tracking issue (no auto-guiding) and/or vibration?

I would expect the shape to be more egg-shaped if it were linked to the limited tracking accuracy of my setup, but I am not sure.

As the 100-400 mk I is quite long when extended at 400mm, I was thinking it may also be a stability issue which would imply a different solution.

Now, I am considering purchasing a Lacerta Mgen III (or maybe ASIAIR based) guiding solution or possibly go with a far better EQ mount such as the iOptron GEM28EC which may not require auto guiding for the focal length I use (400mm). But as those items are not cheap, I would like to be sure that it would actually give me far better photos with my DSLR and lenses. As the Star Adventurer can only be guided on RA, I have some doubt auto-guiding would solve the problem.

TIA for your help!

i think this comes from your lens.
you can try with higher f value…

Hello Didier,

My first guess would be, that this is an optics problem of the lens. Stars are one of the things where you will notice COMA (which I assume this is) from a lens. Many times it looks like on your photo. But the center is usually the less affected area and it is getting worse towards the edges. Try to stop down the aperture of the lens a bit. It usually helps.
I have also started my astrophotography path with an 200 an 400 mm L lens but both were fixed EF 200 f/2.8 L and EF 400 f/5.6 L and had not issue with coma.
For zoom lenses where more tradeoffs are done in design and construction optical issues are more common.

I have used the SA at 5 minutes and 400 mm on an APS-C sensor but with guiding (Orion Mini Auto Guider Kit). I suggest you buy a guide scope 50 mm with a small camera which you could latter use on a bigger mount or scope. Normally all new mounts CEM, GEM, EQ... need to be guided. And those who do not need guiding usually have installed encoders, like the GEM28EC you mention and makes them very expensive! I have heard lot of good things about Lacerta Mgen. Try to ask Star-Watcher .... https://www.star-watcher.ch/ He had an SA before and used it with Lacerta Mgen and now he owns an GEM28 but not the EC version. CS

It's your lens. That sort of aberration happen with zoom lenses and it is no surprise you see them there. You might try to stop down the lens but do not put your hopes too high. Besides it gets less and less rewarding as you need longer time to reach the same image brightness. Zoom lenses are notoriously fickly and best not used unless very recent ones. Or tested beforehand. Mount and tracking has nothing to do with it.

Hey, that's useful feedback: thanks!

Apparently, my old 100-400 is even worst than I thought for astrophotography

The good thing is that I will consider now a better optical solution first without spending too much money on something which would not help solve the problem.

I will try my Canon 135mm L f/2 + 2x TC first to see if it behave better (the 135L is supposed to be optically very good), but not sure about the TC effect and 270mm is a little short of what I want. Otherwise I will have a look at a new astronomical telescope and I will then probably comes again to ask for some advice here.

Thanks again!

I have found that my best use for a zoom of that type is for various quick images of the moon. No tracking mount is required, stars are not an issue, and it doesn’t take but a moment to set it up on a tripod. BTW, the Rokinon fixed 135mm is a very good lens for wide fields. This was taken with the Nikon AF VR-NIKKOR 80-400 f4.5-5.6. With this image, the clouds were an issue until it was about ready to set over my house. I got some quick images in just before it was gone.

Thanks @Jim Matzger for your answer.

I am more a nebula guy than a Moon guy, but I have indeed done a few Moon shots with this lens without noticeable issue.

Concerning the 135mm prime lens, I already have the Canon version as well as a the 35L and 85L, so I think I am OK for wide field. And the 70-200 seems OK: I am only in need for a better optic around 400mm currently.

As others have said, absolutely the lens. I had the exact same results when starting out using a stock Canon 55-250 EFS zoom lens, then purchased the WO RedCat 51 quadruplet and the difference is astonishing; superb quality. Avoid generic zoom's; prime focus telephotos are better but you have to research which ones are geared for astrophotos; stopping down the lens is another option but the quality is what counts.

CS

Thanks @Nicholas Gialiris for confirming the optical issue.

I will look for an astro telescope around 400mm suitable for the Star Adventurer, but I am new to astro-photo and it is quite difficult to know exactly what extra accessories you need for now

The WO RedCat 51 focal is too short for me and the WO stuff quite expensive compared to other options me think.

Currently I have listed a few options but I need to investigate a little bit more. Your feedback is however welcome on this list if you think one of them is not a good choice.

Sky-Watcher EVOSTAR-72ED 420
Tecnosky APO 60/360
Tecnosky APO 70/420
William Optics 61/360

These two images were taken with the Rokinon 135mm. The first is an 18 panel mosaic that involved a smaller chip camera with filters, the second one was taken with a star adventurer mount and a DSLR (short 30 second exposures). If you want to avoid guiding with the SA mount, it is probably best to stick with a maximum of 135mm or so. I recollect that I did not have to stop down the lens. So you can really use it at F2. If you crop the DSLR image, you actually get a respectable image of Orion.

The first one is quite impressive: I have already tried a few months ago to get a nice photo go the Cygnus wide field but all those tiny stars make it quite challenging

I am not against using auto-guiding, but I need to progress step by step and the optic seems to be my problem now beyond 200mm which is a must for me because I am mostly interested in nebulae right now. I have just posted my first photo of the Orion constellation @ 200mm and I like the result, but I need more focal length to get some close-up shots.

Here is a cropped version of the second image.

I am a photographer interested in many subject and I don't think I will ever become a real astro-photographer because it requires too much time for my taste

But I would like to be able to have a few very nice and detailed shots of a some well known DSO to put on my wall and I need high resolution pictures for that. The Orion Nebula is obviously one of my target as are the Pleiades or M31 and a few other, but I need 400mm I think to get enough resolution to make a print at some point.

But some wide fields images may also be very nice such as the Cygnus or even Orion @ 50mm if I am able to make all the Ha and dust clouds visible with details… which I am not able to do yet.

A better optic might help, but the SA mount is a tough mount to use at higher magnification. I have to discard quite a few images even at 135mm.

Well, after the optic, I will consider auto-guiding for the SA first and then a better EQ mount

But at this point, I don't know how much money I want to spend on astro-photography gears especially considering I am making photo from my balcony which puts some constraints as well.

Yes, careful with over-loading the SA with too heavy a camera/lens/scope/accessory setup. It is rated at 11lbs and recommendation with astrophotography in general on any mount is to try and stay within 50% of the weight capacity for optimal performance. If you put too much on it, tough to keep stars round as sagging will occur.

Depending on the weight of the lens, 400mm is starting to push it for SA; doable of course, but exposures will have to be kept short.

If you want a simple all in one device solution for shooting nebulae, you might just want to take a look at the Vaonis Vespera.

https://vaonis.com/vespera

This is my first stab at a mosaic with the Vespera.

Others have discussed the possible issues with the lens. But there are also limits to your star adventurer.
The star adventurer is limited to how much focal length you can reasonably throw on there, especially unguided.
Most don't use it for more than the focal lengths that are already mentioned, 135mm or the 250mm of a redcat (or similar) for that reason.

One reason is the weight capacity of the mount of 5kg. (if you have the counterweight)

The scopes you mention are pushing 2kg already. Such a 360-420mm dedicated scope + flattener + camera + guide scope + guide camera + cables, and you're getting pretty close to the limits of that weight, or at least closer than you might want to.
And a sidenote on the scopes; yes, the Redcat is on the expensive side, but don't forget that most other scopes you mentioned will require a field flattener or reducer. Most adding 200-300+ euro to the final price. The Redcat is a different design, where a flattener isn't required.

the weight of the mount isn't helping either, only 1.2kg. Adding close to 4-5kg on top of that creates quite the centre of gravity for a tripod to deal with. Making it more sensitive to wind and vibrations, which might affect your imaging.

Guiding will of course help, but might not be enough to really increase exposure times with 400mm.

Folks usually cut the advertised weight capacity in half to arrive at a real world weight capacity maximum for a mount for Astrophotography. At a 2.5 kg maximum, the SA mount would be a challenge for just about any payload. Those who utilize the mount are stretching the envelope for what is possible with small mounts, and I admire them for their perseverance and results!

Yes, I know the weight is an issue I may have to consider very quickly with the SA.

I should hopefully be able to find a small telescope to keep the load below the 5kg limit, but definitely above the 50% capacity.

Thanks @Jim Matzger for the link, but I am definitely into more high-end gears for photography in general. The problem here is that I am not sure I want to spend thousands of dollars on new gears just to take 10 very good photos to put on my wall and then forget about astro-photography. Just not sure at this point how deep I want to go into this kind of photography.
I guess with astro-photography, the more you buy gears, the more you want/need more gear

Thanks @Steven de Vet for your input regarding the RedCat... all those extra accessories needed or not is something I don't master at this point for sure.

Yes, with Astro gear one thing leads to another. Don’t ask about filters! 😉. But then you have a lifetime ahead of you to explore the possibilities and the night sky.

Jim Matzger:
I guess with astro-photography, the more you buy gears, the more you want/need more gear

Sounds like you already learned the basics of astrophotography!

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