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Post by davy on Nov 25, 2019 10:49:54 GMT
Sorry Paul. I'm getting old-timers! Thought it was davy who had the AZ GTe. Kim does nothing but AP, so he is always in EQ mode. 30 secs in alt az would be quite possible with it in Alt Az mode in many areas of the sky. Not all areas as some areas of the sky you can barely go 2 secs before field rotation leaves streaks in the image. Other areas up to 50 seconds. But .... it all depends on the sensor size and pixel size and focal length and number of pixels acceptable to have streakiness in before you notice it. Sound too complex? Just use the tables below. The colored one is my DSLR with 600mm FL at lat 30 (close to my lat). NOTE that the blue areas are really only 50 secs max before field rotation. You cannot actually get 100+ secs is it indicates. It's just the maths yields that result. I put the table for 40 degrees for you to likewise color in like I did my table. Becomes easier to spot the sweet parts of the sky where you can get decent exp times. BUT, it is going to be starting points ONLY for you. IE Point your camera at some target where the table might say you can go 30 seconds and try 30 seconds in that area. If it looks streaky then cut exposure time back until it isn't. If you found 15 seconds is nice and sharp, then as its 50% of the value in the table, then cut all the seconds in the table by 50%. Away you go ... you have a working alt az max exp time for that scope and camera combination. Put a different camera and/or different OTA on and you will have to redo the table due to the different sensor, pixel size, FL, etc ... but still pretty easy to produce your own based on that test in a 30 sec area of the sky. And of course, if you find 30 secs is sharp, then go longer and maybe your combination of ED72 and whichever camera can get to 45 seconds. IE a 50% increase in exp time! Then your table would have every value increase by 50% and you'd have heaps of options to shoot alt az. My colored table - use just to see the colors to more easily pick where you can shoot longer - you dont have to do this it is just how i use it. Here's your starting exp times for latitude 40 (think thats close enough for your site)
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Post by Deleted on Nov 25, 2019 11:04:32 GMT
Back on the topic of restrictions to posting , there is quite a struggle going on at the moment between a few of the very influential members and the administrators on CN. If administrators continue to ignore the will of the masses,it will eventually blow up in their face. cheers Paul
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Post by howie1 on Nov 25, 2019 12:29:08 GMT
Thanks Howie. You have provided that table to me a few times before. I had it in my mind that smaller scopes somehow increased the allowed exposure times, but it is related to FL .Anyway, I need to try out both the EQ and the alt/az with this little system. I will add that to the list of things I have to do. cheers Paul Sorry Paul I forgot ... darn my old-timers! You are correct re FL increasing the exp times (ala the 500 rule and 300 rule for astro wide field on normal tripod for instance). FL definitely affect alt az field rotation exposure times too, along with sensor size and pixel size and latitude and mount build quality (!). The formulae is tough to figure and I understand the gist but not the detail. When I read how in the end you have to insert into equations the "streak" you are prepared to "see" as the object rotates (and so signal moves across N number of pixels on your sensor) - and do that by trial and error for each camera/FL combination - I figured what the heck! You might as well start with some published tables and like I suggested to you, use the method I described. IE picking the alt and az of supposedly 30 secs and see how long you can actually get on such and object with camera X and OTA Y and then just adjust the whole thing once you take a few test shots. A bit of a shortcut leap of faith but it worked ok for me and hopefully you'll find it works out ok for your rig. It is that streak as field rotation moves data across 2, 3, 4 or more pixels which is key in both tripod astro wide field (300 and 500 rule), as well as field rotation in alt az. Both are related similarly. The wider the FOV the slower the stuff moves across multiple pixels and so you can take longer shots. The narrower the FOV the faster the field rotation causes stuff to move across multiple pixels. If we tested and found with a particular camera whose pixel size was X, that we didn't like it when stuff crossed 20 pixels, but 10 pixels was acceptable or hard to notice, then on a particular FL scope that would mean shorter exposures to limit the field rotation to crossing only 10 pixels. Going longer at that FL and camera combination would mean field rotation would cross more than 10 pixels and we'd notice the streakiness. So as well as OTA FL the sensor size is involved too as combined they determine magnification and thus FOV. The problem is if you look at those lovely long exposure star trails around the poles, you can see all the streaks have different lengths as you move away from the pole. Yet the exposure time was one set exposure time. IE stuff further away moved across more pixels in that set time, than stuff near the pole. But that fact is then complicated by the fact that an Alt Az mount will easily track stuff going straight up from the horizon - not much rotation going on - yet in a set exposure time, it moves a long way upwards. Entry level alt az have worse build quality in the motors and backlash etc so many cannot actually track well trying to scream straight up and at angle depending on latitude / equator angle. But high quality alt az have better gear and motors and motor pulses when tracking that area and dont have a problem. So mount also comes into it. Egad! But while stuff near horizon is flying in pretty much straight lines at some latitude angle near the equator, stuff near the meridian near zenith may be still going in a straight line but the AZ motor of alt az starts to turn around very quickly to track down to the west (just ends up making 180 degree turn! Kind of like the AltAz equivalent of a meridian flip) ... so any exposure time is pretty well impossible at that point with any mount. And near the pole where stuff is not moving particularly fast across multiple pixels the movement appears to alt az very circular and hence tough to not get field rotation. Etc etc!!!! .... like I said, I get the problems I've seen various sites say there are in calculating longest alt az exposure times, but trying to figure the many variables to plug into equations .... well I gave up and decided Heck just give me the tables for different latitude (which again makes difference! Argh!) and I'll use them as starting point. cheers
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Post by janetalvarez on Dec 18, 2019 17:57:56 GMT
This is an interesting topic, I'm also looking for something like that.
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Post by davy on Dec 19, 2019 8:37:23 GMT
Welcome to the forum Janet.
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Post by davy on Dec 19, 2019 8:47:06 GMT
The scope package is good,, going by the video.. But the price and adaptability is the concern. It comes across as high end optics and camera.. I have a 178 zwo with the starvis sensor so I know its a good camera,, but I also know on an 80mm scope it has its limitation. For the price I personally would build a rig,, more adaptable.
What I would like to see is a goto skywatcher adventurer goto,, Yes I know there is the Sam mini but I would by a goto adventurer in a minute if it came out,, bang an ed72 on it control via WiFi link a zwo asair pro on it.. Now that's a portable kit I would go for. Electric focuser and filter wheel.. Yip dream set up for me for portability
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Post by howie1 on Dec 20, 2019 3:06:20 GMT
What I would like to see is a goto skywatcher adventurer goto,, I have an accurate push-to star adventurer. It will place objects near dead centre of the FOV everytime - 250mm lens on an APS/C sensor. Given that's 5 degree's FOV I'd say it always lands the target within a degree of dead centre. No RDF or star hopping necessary. I use a highly accurate digital level (+/-0.1 degrees accuracy) combined with a DIY DEC circle which has half degree graduations on it and which is placed on the SA rotating DEC head. If you set the DEC to your latitude, and release the RA clutch and point the OTA opening up towards Zenith ... it will be pointing straight up! Very easy to see and so very easy to set up. Key for accuracy is to then put the digital level across the aperture of the tube .... and set it level. The OTA will be pointing straight up at Zenith with an accuracy of +/- 0.1 degree. What use is this? Well, if you lookup an object in SkySafari, it not only shows RA but also shows the "Hour Angle". The Hour Angle is the RA expressed as the HMS East or West of the meridian. A simple calc on the iPads calculator converts that to degrees East or West of the Meridian. IE I use the highly accurate digital level across the OTA aperture to 'lean it over' in RA to that degree's E or W .... and that's the RA of the object taken care of within a half degree! DEC is then easy-peezy .... SkySafari shows the DEC so just wind the SA DIY DEC circle to that DEC value and ... you are within a degree - often even closer - to the object. If none of this makes sense, tell me to make a video. LOL happy to help out with that if anyone interested. Having said all that, I've seen astro mate Tony's SA wedged AzGti + ED72 many times ..... and that is the way I'll be going! Well ... once the upstairs renovation is complete, and when Christmas celebrations are over, and when visiting relatives leave to go home, and the bigone - when the rotten red smoke haze skies and strong winds we've had here for months finally goes away. I'll be buying said AzGti and ED72 as soon as all those conditions are met Merry Christmas everyone
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