F2 lens = short exposures ???

[kirkster501]

Going to have a play with connecting my Samyang 135mm  F2 to my 8300 chip Moravian CCD camera with 31mm Astrodons.

Just sanity checking myself; I'm correct thinking that keeping the lens wider (at F2 or F2.8 say) doubles the data collected for each stop wider?  Of course, coma , distortion and aberration has to permit that wide aperture but in principle what I said is true???

[Gina]

Yes, that's right.  Each stop doubles (or halves) the amount of light coming through.

[Alien 13]

12 minutes ago, kirkster501 said:

  Of course, coma , distortion and aberration has to permit that wide aperture but in principle what I said is true???

I didnt think the Samyang had any of those issues even wide open.....

Alan

[Xplode]

The Samyang 135mm is pretty much perfect wide open, but note that there's often problems with sensor tilt or sometimes lens not perfectly adjusted. 
Focus is also extremely critical, critical focus zone is just 0.01mm at f2.

Edited July 7, 2019 by Xplode

[Alien 13]

The trick with these types of lenses is to focus away from the center at a point a third the distance along an imaginary diagonal...

Alan

[vlaiv]

Doubles the data collected? You'll have to be more precise.

If you keep your sensor and your exposure length - amount of data collected per set total imaging time will be the same - that is number of exposures times amount of data per each exposure.

It is not the lens that has impact on amount of data. Swap the sensor for one with more pixels (even smaller chip, but more "megapixels") - you will collect more data. Use shorter exposure regardless of F/stop - more data collected.

If you were thinking about signal per single exposure - then yes, keep the exposure length and sensor same and change F/stop of lens - level of signal per exposure will change accordingly (you are keeping everything the same just changing light collecting surface - greater surface more light collected and vice verse).

[kirkster501]

Many thanks folks for you input.

Yes Ole, aware of the focus criticality.  I am thinking of a way to autofocus the lens with a SW motor and a pulley belt of some sort. 

[ollypenrice]

5 minutes ago, kirkster501 said:

Many thanks folks for you input.

Yes Ole, aware of the focus criticality.  I am thinking of a way to autofocus the lens with a SW motor and a pulley belt of some sort. 

TS do these devices which I found helpful.

If you do want to stop down you are not obliged to use the diaphragm. A front aperture mask will reduce the F ratio without introducing diffraction effects. Graphics outlets sell the compass-cutters.

Focusing at the intersection of the 1/3 lines is brilliant advice.

Olly

 

[kirkster501]

Thanks Olly, the focuser thingy is very ingenious.  I might try motor focus first though.

[ollypenrice]

On 08/07/2019 at 09:09, kirkster501 said:

Thanks Olly, the focuser thingy is very ingenious.  I might try motor focus first though.

Probably better still and less infernally fiddly. 

[kirkster501]

8 hours ago, ollypenrice said:

Probably better still and less infernally fiddly. 

My thoughts too.  I might even try autofocus too but one step at a time.  I need to get the EoS to Moravian adapter before I start worrying about focus.

[Mark1489]

On 07/07/2019 at 20:25, Alien 13 said:

The trick with these types of lenses is to focus away from the center at a point a third the distance along an imaginary diagonal...

Alan

I have the samyang and a few issues with coma that seem to vary night to night. I always stop down to f2.8 but one corner is pretty bad, and what has me at a loss is tonight the worst corner was the opposite corner of the image! Bearing in mind the rotation of the lens/camera was not changed from previous nights. I think I may need to try out your idea, I normally just focus at the centre of live view with a bahtinov mask....

[vlaiv]

11 hours ago, Mark1489 said:

I have the samyang and a few issues with coma that seem to vary night to night. I always stop down to f2.8 but one corner is pretty bad, and what has me at a loss is tonight the worst corner was the opposite corner of the image! Bearing in mind the rotation of the lens/camera was not changed from previous nights. I think I may need to try out your idea, I normally just focus at the centre of live view with a bahtinov mask....

Could be that you have mounting issues. See if gravity has something to do with it - examine part of sky you imaged in each session and see if there could be a lens sag or something similar. Maybe stronger mounting (supporting lens / camera assembly on multiple points) could resolve this.

[davew]

On 08/07/2019 at 07:43, kirkster501 said:

Many thanks folks for you input.

Yes Ole, aware of the focus criticality.  I am thinking of a way to autofocus the lens with a SW motor and a pulley belt of some sort. 

I've been down the route of connecting camera lenses to the Moravian and I can say it's very easy to do and can be quite successful.

If you can make out what's going on from the photo all becomes fairly self explanatory. The camera body is bolted down quite tightly so it remains solid no matter what. The scope ring centres the lens and stops any sag *. The skywatcher motor is bolted to a garage door mount adaptor ( Like you do ) and I bought in the belt and pulley wheel ( Can't remember where from now ! )

* The lens is roughly centred by pointing the camera and lens at zenith with the scope ring adjusters slightly loose. Waggle the lens about a bit to settle it and VERY carefully tighten up the adjuster bolts. If it turns out a corner is a touch out the scope ring is easy enough to adjust and get the lot dead straight.

I didn't automate the focusing as using a Bahtinov mask and the hand controller on a long cable was accurate enough. One thing I did discover that helped no end was to use the focus motor at 5v instead of 9v.

Dave.

[Mark1489]

6 hours ago, vlaiv said:

Could be that you have mounting issues. See if gravity has something to do with it - examine part of sky you imaged in each session and see if there could be a lens sag or something similar. Maybe stronger mounting (supporting lens / camera assembly on multiple points) could resolve this.

I managed to go out again and try the focusing away from centre...it made a huge difference and much better! I will look into this though since I still have a little bit of elongation of stars at the very bottom, I usually crop them out. What would you recommend for a D5300 and Samyang? I’m guessing all mounted to a single dovetail? 

[vlaiv]

1 hour ago, Mark1489 said:

I managed to go out again and try the focusing away from centre...it made a huge difference and much better! I will look into this though since I still have a little bit of elongation of stars at the very bottom, I usually crop them out. What would you recommend for a D5300 and Samyang? I’m guessing all mounted to a single dovetail? 

Look at above post by @davew, that setup looks quite sturdy. You should have at least two bracing points - one for camera and one for lens. Setup where there is only one point holding camera / lens assembly - usually where two join can possibly make lens or camera sag under their weight.

If you are using DSLR - then you can use standard 1/4 thread (or whatever standard may be) to secure camera body to dove tail bar, but use a suitable ring to hold lens firm as well.

[Alien 13]

1 hour ago, Mark1489 said:

I managed to go out again and try the focusing away from centre...it made a huge difference and much better! I will look into this though since I still have a little bit of elongation of stars at the very bottom, I usually crop them out. What would you recommend for a D5300 and Samyang? I’m guessing all mounted to a single dovetail? 

I cant find the post at the moment but a rigid cage type structure (3D printed) was discussed in one of the Samyang 135 topics and also other tips for making the assemble more rigid.

I did note that you are using a bat mask on a DSLR with live view, in my opinion this is not good and focusing on the very faintest star in the FOV with perhaps X5 or X10 zoom activated will give better results, the focus action should effectively be on/off with the star invisible or blinking into view.

Alan

[kirkster501]

On 14/07/2019 at 16:50, davew said:

I've been down the route of connecting camera lenses to the Moravian and I can say it's very easy to do and can be quite successful.

If you can make out what's going on from the photo all becomes fairly self explanatory. The camera body is bolted down quite tightly so it remains solid no matter what. The scope ring centres the lens and stops any sag *. The skywatcher motor is bolted to a garage door mount adaptor ( Like you do ) and I bought in the belt and pulley wheel ( Can't remember where from now ! )

* The lens is roughly centred by pointing the camera and lens at zenith with the scope ring adjusters slightly loose. Waggle the lens about a bit to settle it and VERY carefully tighten up the adjuster bolts. If it turns out a corner is a touch out the scope ring is easy enough to adjust and get the lot dead straight.

I didn't automate the focusing as using a Bahtinov mask and the hand controller on a long cable was accurate enough. One thing I did discover that helped no end was to use the focus motor at 5v instead of 9v.

Dave.

Thanks for that Dave.

I take it that if the lens is solely supported by the Moravian adapter it will sag due to the lens' weight?

 

[davew]

16 hours ago, kirkster501 said:

Thanks for that Dave.

I take it that if the lens is solely supported by the Moravian adapter it will sag due to the lens' weight?

 

In my case I'm certain it would. There's another aspect I forgot to mention about the pull of the belt towards the focus motor. There has to be some tension on the belt, especially for damp winter nights, and therefor the lens needs holding steady. I didn't go wild on the tension but if I slackened off the scope ring for any reason the lens pulled over.

I did find that it was easy enough to centralise the lens but if I didn't take some care in the initial setting up the bad focus on one side or corner would show. 

Modern fast camera lenses are now built with very large and heavy front elements so I would support the lens even without the focus motor.

Using the scope ring also gets you round another slight problem in so far as you can use adapted lenses and have no movement. The lens you see here  has a Moravian / Nikon adaptor and then a Nikon / Hasselblad adaptor. Think of the sag that would introduce !

Dave.