46 replies to this topic

[MikeMiller]

Aperture matters some in imaging, but a lot less than it does for visual.

That's a great point. Op should not really worry about f number. Just decide what focal length they are interested in, and find a scope/lens that fills the bill.

 

Since it is for imaging, just take a longer exposure if you aren't getting enough photons. A bigger aperture means shorter exposures and more detail. 

 

And if you do decide you need 400mm at f/2... well, see my profile picture.

Edited by MikeMiller, 12 January 2021 - 12:28 PM.

[213Cobra]

You can get to f/3 with a Takahashi FSQ-106ED + #TKA38581 0.6X reducer. The reducer also works with the FSQ-130ED. The reducer alone is almost $1800 in the US, but between the FSQ flat field quad and it, you won't have an image full of aberrations.

 

Phil

[Borodog]

I think you guys have missed the part about using this for EAA. I am not trying to take 24 Mpxl astrophotos for printing in a magazine. I am literally using a 2 Mpxl planetary camera that you can buy brand new for $125. I’m currently using a $23 C-mount lens which at its slowest is F1.6. Is the lens crappy? Yes. Is there some aberration in the stars? Yes. But because of the small sensor size (5.5x3.1 mm) it’s actually a lot less than you’d think. And it is unbelievably fast. It achieves the same pixel intensity as an F/4, for example, 6.25 times faster. That is an enormous difference for EAA for a hobbyist with limited time. Ten minutes is like an hour. Seeing is completely irrelevant. Guiding is totally unnecessary. Hell, you don’t even need to track. At its zoomiest 60mm you can take 3s exposures at the celestial equator completely untracked and SharpCap will stack them just fine. The screen crossing time is 20 minutes minimum untracked.

 

The only thing I wanted to know from this thread is, can I keep all of these advantages but get better star shapes with a better hunk of glass in front? If I can’t, I will happily stick with my $23 telescope, because frankly, it is flipping amazing for what I am using it for. Don’t knock till you try it!

Edited by Borodog, 12 January 2021 - 01:59 PM.

[garret]

How wide is the 'sensor' of the EAA unit? if it is typical C-mount lens size the field with a 135mm lens is very narrow (about 3 degree)

[Borodog]

How wide is the 'sensor' of the EAA unit? if it is typical C-mount lens size the field with a 135mm lens is very narrow (about 3 degree)

3 degrees at 135mm would be a much larger sensor. I am 5.3x3 degrees at 60mm.

 

Which brings up another question. Can I get a better quality C-mount lens that would produce better star shapes? Perhaps a fixed 60mm instead of a zoom? Where would I search for those?

[rk2k2]

Hmmm,

 

it's been a few years but before I bought a mount I had a a mere tripod, DSLR's (1.5X and 2X crop), and numerous lenses.  During that time I 'experimented' with all kinds of stuff, one being a cheap CCTV camera and lens.  It had a 1/3" sensor which puts it's crop factor somewhere above 6X, which is why I bought it- the micro 4/3 (2x) was providing me an effective FL of 400mm with may 200mm f/4 lens .. wonder what 6X, or 1200mm would give me?   Never found out (but if size wise you have a similar sensor you have an effective 360mm- 500 rule, you have 500/360 or 1.7 second exposure time without noticeable trailing.  Are you  shooting beyond 1.7 secs? What leads you to believe it's the lens causing poor star shapes?   Just wondering because I've experimented some really 'cheap' stuff that I first blamed for poor performance.  I even had I've had  really good tripod (a Manfrotto) that turned out to have slightly noticeable vibrations in a light breeze.  That was effecting even short exposures, particularly the sun (1/80th sec).

 

As for high quality c-mount lenses, when I was tinkering around with C-Mount, I was on the 'cheap' ($20), and as I recall there were some expensive ones.  Whether they are any good or not, can't say, I wasn't going to spend $50 - $100 to find out.

[CHASLX200]

Field Curvature.

CS

Jonathan

FC drives me nuts as bad as coma does in fast Newts.  At F/6.5 in fracts i can live with some FC.   With wide field eyepieces and sweeping the bending of the FOV makes me go nuts.
 

[Sarkikos]

FC drives me nuts as bad as coma does in fast Newts.  At F/6.5 in fracts i can live with some FC.   With wide field eyepieces and sweeping the bending of the FOV makes me go nuts.
 

Have you tried the TSFLAT2?  I believe you were in at least one of the threads that mentioned this field flattener, but I don't recall if you ever got one for your refractors.

 

Mike

[CHASLX200]

Have you tried the TSFLAT2?  I believe you were in at least one of the threads that mentioned this field flattener, but I don't recall if you ever got one for your refractors.

 

Mike

Not yet. Seems shipping and things being in stock are a mess now. I will try one out one day.

[slavicek]

I have Canon 200mm f/1.8 lens which I converted for use with the Night vision devices. NVD loves fast lens. While I was at it I also got myself a eyepiece adapter and diagonal for it. So camera lens can be used with eyepieces. However, for different reasons, I would not recommend it for eyepiece use. Basically, I just wanted to prove that it could be done.

[Borodog]

Hmmm,

 

it's been a few years but before I bought a mount I had a a mere tripod, DSLR's (1.5X and 2X crop), and numerous lenses.  During that time I 'experimented' with all kinds of stuff, one being a cheap CCTV camera and lens.  It had a 1/3" sensor which puts it's crop factor somewhere above 6X, which is why I bought it- the micro 4/3 (2x) was providing me an effective FL of 400mm with may 200mm f/4 lens .. wonder what 6X, or 1200mm would give me?   Never found out (but if size wise you have a similar sensor you have an effective 360mm- 500 rule, you have 500/360 or 1.7 second exposure time without noticeable trailing.  Are you  shooting beyond 1.7 secs? What leads you to believe it's the lens causing poor star shapes?   Just wondering because I've experimented some really 'cheap' stuff that I first blamed for poor performance.  I even had I've had  really good tripod (a Manfrotto) that turned out to have slightly noticeable vibrations in a light breeze.  That was effecting even short exposures, particularly the sun (1/80th sec).

 

As for high quality c-mount lenses, when I was tinkering around with C-Mount, I was on the 'cheap' ($20), and as I recall there were some expensive ones.  Whether they are any good or not, can't say, I wasn't going to spend $50 - $100 to find out.

It’s definitely the lens. And I am tracking with it; I was just illustrating the point that you don’t need to.

Edited by Borodog, 13 January 2021 - 01:53 AM.

[deonb]

I think you guys have missed the part about using this for EAA. I am not trying to take 24 Mpxl astrophotos for printing in a magazine. I am literally using a 2 Mpxl planetary camera that you can buy brand new for $125. I’m currently using a $23 C-mount lens which at its slowest is F1.6. Is the lens crappy? Yes. Is there some aberration in the stars? Yes. But because of the small sensor size (5.5x3.1 mm) it’s actually a lot less than you’d think. And it is unbelievably fast. It achieves the same pixel intensity as an F/4, for example, 6.25 times faster. That is an enormous difference for EAA for a hobbyist with limited time. Ten minutes is like an hour. Seeing is completely irrelevant. Guiding is totally unnecessary. Hell, you don’t even need to track. At its zoomiest 60mm you can take 3s exposures at the celestial equator completely untracked and SharpCap will stack them just fine. The screen crossing time is 20 minutes minimum untracked.

 

The only thing I wanted to know from this thread is, can I keep all of these advantages but get better star shapes with a better hunk of glass in front? If I can’t, I will happily stick with my $23 telescope, because frankly, it is flipping amazing for what I am using it for. Don’t knock till you try it!

 

Does it really need to be 240mm F/2? 

A RASA 8 will get you pretty close to that. You can probably gap any difference with a wider camera and/or better QE.
 

[MikeMiller]

Can I get a better quality C-mount lens that would produce better star shapes? Perhaps a fixed 60mm instead of a zoom? Where would I search for those?

Possibly. The $10 lenses that come with cheap cameras such as the one that came with my ASI224 are pretty terrible. I replaced it with a better one for all-sky purposes and am happier with it.

 

But a small and fast lens is always going to have field curvature. Even the highly regarded Rokinon 135.

 

However, there is another idea you can use. Match your small sensor camera with a lens that is designed for a larger sensor. For instance, the Rokinon 135mm on a C or CS-mount sensor will only be looking at the very middle of the lens's field. This means that the outer edges where the curvature is the worse. 

 

For me, I pair the full frame sized Rokinon 135mm with a m4/3 sized sensor camera and get better FC. It isn't perfect, it can't really ever be, but it is improved. This does come at a cost of size and weight and money, so you will have to decide if it is worth it for you.

[noisejammer]

With a camera that tiny, you should be able to get away with an oversized spherical mirror and an entrance pupil that stops it down to f/2 or so.

 

IIRC, Celestron offered a 5" and 8" f/1.5 version of this in the 70's.

 

The primary from a small Maksutov might be a good place to look for a mirror.

[Borodog]

Does it really need to be 240mm F/2? 

A RASA 8 will get you pretty close to that. You can probably gap any difference with a wider camera and/or better QE.
 

No, it's doesn't need to be exactly that. And yes, I am aware of the RASA. Some day.

 

Possibly. The $10 lenses that come with cheap cameras such as the one that came with my ASI224 are pretty terrible. I replaced it with a better one for all-sky purposes and am happier with it.

 

But a small and fast lens is always going to have field curvature. Even the highly regarded Rokinon 135.

 

However, there is another idea you can use. Match your small sensor camera with a lens that is designed for a larger sensor. For instance, the Rokinon 135mm on a C or CS-mount sensor will only be looking at the very middle of the lens's field. This means that the outer edges where the curvature is the worse. 

 

For me, I pair the full frame sized Rokinon 135mm with a m4/3 sized sensor camera and get better FC. It isn't perfect, it can't really ever be, but it is improved. This does come at a cost of size and weight and money, so you will have to decide if it is worth it for you.

I am liking this option more and more. A camera lens will have much better quality. Even if it has field curvature, I can probably largely correct that in post, and as you say, it is a very small sensor. I have an array of E-mount and EF-mount lenses available to experiment with. I need to see what adapters are necessary to mount these on the camera, which is C-mount.

 

With a camera that tiny, you should be able to get away with an oversized spherical mirror and an entrance pupil that stops it down to f/2 or so.

 

IIRC, Celestron offered a 5" and 8" f/1.5 version of this in the 70's.

 

The primary from a small Maksutov might be a good place to look for a mirror.

This intrigues me. Can you elaborate on this? How does an oversize mirror help me? Are you talking about using the mirror from say an SCT or Mak and putting the camera where the secondary is? Would that focus?

[noisejammer]

... This intrigues me. Can you elaborate on this? How does an oversize mirror help me? Are you talking about using the mirror from say an SCT or Mak and putting the camera where the secondary is? Would that focus?

This really belongs in the ATM forum, so I'll outline the idea but further discussion needs to go to a different thread.

 

Let's say the primary of an SCT is f/2 (most are). This means they have a focal length that's twice the aperture. Being spherical, they have a fair bit of spherical aberration and some field curvature. So far, so good. You can make a simple camera by placing the sensor at the focus on the mirror. This will be some distance beyond the corrector of an SCT but not that far. Focus will be fiddly but it's not impossible.

 

If you were to stop the mirror down so that it was ~ f/3, you'll increase the usable area in which focus is reasonable. The Celestron cameras essentially did this.
 

In a crude sense, you're creating a Fastar-wannabe without the corrector lenses. It won't be perfect but it may suffice for EAA.

[Borodog]

Sounds amazing, actually. Thanks.

[bobzeq25]

Here's a slightly larger telescope with that design.  Note the lack of anything on the back.

 

https://www.eso.org/...illa/1mschmidt/

 

This design is the basis for the RASA.  There's a 4 element lens corrector that flattens the field, and corrects other aberrations.

 

https://www.365astro...scope-ota-d.jpg

 

Bottom line - just buy a RASA.  <smile>  Much easier/better.

Edited by bobzeq25, 14 January 2021 - 09:52 PM.

[Mitrovarr]

The only practical way I can think of to get a telescope at F/2 is a SCT with a Hyperstar system, or a Schmidt Camera.

[Borodog]

Here's a slightly larger telescope with that design.  Note the lack of anything on the back.

 

https://www.eso.org/...illa/1mschmidt/

 

This design is the basis for the RASA.  There's a 4 element lens corrector that flattens the field, and corrects other aberrations.

 

https://www.365astro...scope-ota-d.jpg

 

Bottom line - just buy a RASA.  <smile>  Much easier/better.

Oh I will. Some day.

[MikeMiller]

How does an oversize mirror help me? Are you talking about using the mirror from say an SCT or Mak and putting the camera where the secondary is? Would that focus?

 

Yeah, long before Hyperstar/Fastar was the "Schmidt Camera" which is just a SCT with some film instead of a secondary holder.  It works on the same principle that using a bigger-than-necessary camera lens. It reduces curvature by only imaging the middle of the field.

 

I have never seen one, but I think the Schmidt Camera only worked so well because you could mount the film in a curved holder. Imaging sensors are not curved, so this won't work as well.

[Borodog]

Yeah, long before Hyperstar/Fastar was the "Schmidt Camera" which is just a SCT with some film instead of a secondary holder.  It works on the same principle that using a bigger-than-necessary camera lens. It reduces curvature by only imaging the middle of the field.

 

I have never seen one, but I think the Schmidt Camera only worked so well because you could mount the film in a curved holder. Imaging sensors are not curved, so this won't work as well.

Actually, this is in the works right now. There is a lot of incentive to produce curved sensors because you can reduce the glass. They would be concave sensors, and small, for phone camera applications at first, but who knows where it may go.