AP gear (for the future)?

[turboscrew]

I played a bit with the FOVs and image scales with different barlows/reducers and different targets, and I started to see the sense of @SkyHiker's suggestion for ASI2600.

[turboscrew]

I'd rather operate via computer, so I think, astrocam is my way to go.
Maybe I should start with cheaper planetary camera?
And is there a minimum useful pixel size? How does stacking affect that?
Also, what would be the maximum useful chip size? That is, how big is the primary focus image?

And it's not that cold all the time, not even during the winter - at least not lately.
And in winter, I think, I'd have plenty of time for setting up. Around Christmas you can polar-align around 4 PM.

[SkyHiker]

I'd rather operate via computer, so I think, astrocam is my way to go.
Maybe I should start with cheaper planetary camera?

A planetary camera with a small FOV will be a disaster for plate solving.

And is there a minimum useful pixel size? How does stacking affect that?

The pixel size of most cameras is about 3.25 pixel, so no choices there.

Also, what would be the maximum useful chip size? That is, how big is the primary focus image?

Isn't this high school math? Joe did a bit of math for you but should be able to figure it out.

About the ASI2600, it's not cheap and there are others that will work too. With mine I can see strong vignetting in the corners, so a ASI1600 would be fi e too. I doubt if you have to worry about darks in your climate. Be aware that a 1200 mm focal length makes things harder, alternatively you could get an 8" astrograph which is cheap, or a small refractor. I worked months before I could get a total RMS error of 0.8", which is what you need, or less of course. The G11S adds has its own quirks, and add OnStep and Ekos to it, and you have a lot of work. That's why I suggested to start working on software before the season.

[OzEclipse]

turboscrew wrote:
I'd rather operate via computer, so I think, astrocam is my way to go.
Maybe I should start with cheaper planetary camera?

A planetary or guide cam will have a small sensor. Most are between 5x3mm to 7x6mm. This will be ok for measuring PE which was my initial suggestion but will leave you with a tiny field for photography. All the planets are < 1 arc minute. The cameras have small sensors.

The field of view in an optical system (degrees) = sensor size(mm) ÷ focal length(mm) ÷ (180/pi)

And is there a minimum useful pixel size?
No point way over sampling. Maybe 2 x 2 pixels per seeing element.

How does stacking affect that?
It doesn't affect it.


Also, what would be the maximum useful chip size? That is, how big is the primary focus image?
Henk alluded to vignetting. None of us can tell you this unless we have an identical scope. The vignetting is a function of focal length, f ratio, OTA diameter, how far forward of the focusser the OTA tube extends, secondary size, focus height above the outside of the tube, coma corrector properties, and the ID and height of your focusser.

There is a ray tracing program called "Newt for the web" on the Stellafane website. Measure all the quantities carefully and plug them into the specifications page. It will draw a ray trace and calculate the 100% and 75% illumination field diameters.

And it's not that cold all the time, not even during the winter - at least not lately. And in winter, I think, I'd have plenty of time for setting up. Around Christmas you can polar-align around 4 PM.

It is that cold, I've seen it on cartoons! Everything is frozen solid in Finland.

I read the comment you are referring to in another post and I understood it to mean keep it simple so that you don't get too cold while taking a long time to set up, not that there is a lack of darkness hours.

[turboscrew]

I'd rather operate via computer, so I think, astrocam is my way to go.
Maybe I should start with cheaper planetary camera?

A planetary camera with a small FOV will be a disaster for plate solving.

And is there a minimum useful pixel size? How does stacking affect that?

The pixel size of most cameras is about 3.25 pixel, so no choices there.

Also, what would be the maximum useful chip size? That is, how big is the primary focus image?

Isn't this high school math? Joe did a bit of math for you but should be able to figure it out.

About the ASI2600, it's not cheap and there are others that will work too. With mine I can see strong vignetting in the corners, so a ASI1600 would be fi e too. I doubt if you have to worry about darks in your climate. Be aware that a 1200 mm focal length makes things harder, alternatively you could get an 8" astrograph which is cheap, or a small refractor. I worked months before I could get a total RMS error of 0.8", which is what you need, or less of course. The G11S adds has its own quirks, and add OnStep and Ekos to it, and you have a lot of work. That's why I suggested to start working on software before the season.

The formuli give the field coverage of a sensor, but if I want to know the image size for selecting a sensor?

About planetary cameras: This has sensor size of 6.4 mm x 4.6 mm, but pixel size of 1.67 um.
https://www.astroshop.eu/astronomical-c ... r_1_select

What I wonder is, that maybe it requires a very good seeing? Or maybe stacking could sort out the image errors due to atmospheric fluctuations - at least to some extent?
Maybe I could get good pictures of planets and smaller DS targets? For most nebuli that's too small, though.

[turboscrew]

turboscrew wrote:
I'd rather operate via computer, so I think, astrocam is my way to go.
Maybe I should start with cheaper planetary camera?

A planetary or guide cam will have a small sensor. Most are between 5x3mm to 7x6mm. This will be ok for measuring PE which was my initial suggestion but will leave you with a tiny field for photography. All the planets are < 1 arc minute. The cameras have small sensors.

I was thinking of getting 2 cameras - due to the restrictions of the telescope. One for small targets, and another for big targets. The planetary camera first, since it's a lot cheaper. The "real" camera later.

The field of view in an optical system (degrees) = sensor size(mm) ÷ focal length(mm) ÷ (180/pi)

The problem here is finding out the maximum usable sensor size. For reference, when trying to select a camera.

And is there a minimum useful pixel size?
No point way over sampling. Maybe 2 x 2 pixels per seeing element.

I was thinking of pixel size in um, On the second thought, it's actually the image scale that I'm interested in.
Or maybe seeing (in arc seconds) vs. image scale.

How does stacking affect that?
It doesn't affect it.


Also, what would be the maximum useful chip size? That is, how big is the primary focus image?
Henk alluded to vignetting. None of us can tell you this unless we have an identical scope. The vignetting is a function of focal length, f ratio, OTA diameter, how far forward of the focusser the OTA tube extends, secondary size, focus height above the outside of the tube, coma corrector properties, and the ID and height of your focusser.

There is a ray tracing program called "Newt for the web" on the Stellafane website. Measure all the quantities carefully and plug them into the specifications page. It will draw a ray trace and calculate the 100% and 75% illumination field diameters.

Thanks, I'll check it out.

And it's not that cold all the time, not even during the winter - at least not lately. And in winter, I think, I'd have plenty of time for setting up. Around Christmas you can polar-align around 4 PM.

It is that cold, I've seen it on cartoons! Everything is frozen solid in Finland.

I read the comment you are referring to in another post and I understood it to mean keep it simple so that you don't get too cold while taking a long time to set up, not that there is a lack of darkness hours.

Well, I usually need gloves when it's below -5°C. I guess my limit is about 0°C when handling metal stuff.

[SkyHiker]

The formuli give the field coverage of a sensor, but if I want to know the image size for selecting a sensor?

The telescope maps angular features of the sky on the sensor by a simple projection based on the focal length. It is near trivial to calculate that. If you don't want to or can't, Stellarium has a plugin that shows what the camera sees, for any sensor you specify. Maybe you have something specific in kind with your terms field of coverage and image size. If you can define what you mean by that, that would be helpful.

The issue of a CC is difficult if you want the exact answer but easy jf you accept that it will be about 15% different from the normal view and that such an error is acceptable to get an idea.

I'm on a road trip now going from motel to motel so I can't elaborate much, sorry gotta go.

[OzEclipse]

turboscrew wrote:


Also, what would be the maximum useful chip size? That is, how big is the primary focus image?
Henk alluded to vignetting. None of us can tell you this unless we have an identical scope. The vignetting is a function of focal length, f ratio, OTA diameter, how far forward of the focusser the OTA tube extends, secondary size, focus height above the outside of the tube, coma corrector properties, and the ID and height of your focusser.

There is a ray tracing program called "Newt for the web" on the Stellafane website. Measure all the quantities carefully and plug them into the specifications page. It will draw a ray trace and calculate the 100% and 75% illumination field diameters.

Thanks, I'll check it out.

There are repeated questions about the maximum sensor size after I posted this. The 75% illuminated field diameter is probably the maximum sensor size. The area will probably be a full frame sensor unless the telescope has vignetting problems. You may not like the price.

Remember that there are only so many large objects. These can be photographed using a mosaic technique with a smaller sensor or you can buy a shorter focal length OTA or a good 300mm telephoto lens to give you a wide field option. A small photo refractor or telephoto will cost a lot less than the difference between a maximum sized sensor for your instrument and a more modest sensor.

I'm just throwing up possibilities because you haven't said much about your budget. Large sensors are expensive

[sdbodin]

Getting a "full-frame-sensor" is a common error made when starting out in AP. Most scopes do not cover that size without sever vignetting and a focal reducer only makes that worse. see cartoon pic:

I experimented with my 2400mm efl and my 1600MC (22mm diag.) this spring and found that only TWO common galaxy object in the northern hemisphere sky do not fit, M31 and M33. Spending a boat load more money to get only these two additional targets is an awful waste.

I still think that the ASI294MC-cool is a very good choice for first camera. If you use SharpCap for taking pics, it has the option of using a sub-frame, like 800x600 pixels which is very good for planetary images with this camera.

Lots of research still needed, I like the ray-trace option of understanding your fov.
Steve

[turboscrew]

turboscrew wrote:


Also, what would be the maximum useful chip size? That is, how big is the primary focus image?
Henk alluded to vignetting. None of us can tell you this unless we have an identical scope. The vignetting is a function of focal length, f ratio, OTA diameter, how far forward of the focusser the OTA tube extends, secondary size, focus height above the outside of the tube, coma corrector properties, and the ID and height of your focusser.

There is a ray tracing program called "Newt for the web" on the Stellafane website. Measure all the quantities carefully and plug them into the specifications page. It will draw a ray trace and calculate the 100% and 75% illumination field diameters.

Thanks, I'll check it out.

There are repeated questions about the maximum sensor size after I posted this. The 75% illuminated field diameter is probably the maximum sensor size. The area will probably be a full frame sensor unless the telescope has vignetting problems. You may not like the price.

Remember that there are only so many large objects. These can be photographed using a mosaic technique with a smaller sensor or you can buy a shorter focal length OTA or a good 300mm telephoto lens to give you a wide field option. A small photo refractor or telephoto will cost a lot less than the difference between a maximum sized sensor for your instrument and a more modest sensor.

I'm just throwing up possibilities because you haven't said much about your budget. Large sensors are expensive

I think, what you mean by repeated questions were rather tries to clarify the earlier questions.
And like I mentioned, I was interested about the maximum sensor size as reference.

What I calculated: (Barlow < 1 is reducer)

Code: Select all

ASI2600: pix sz [um]	3.7	Sensor sz [mm]	17.5 pixels 6248 x 4176
(sensor size = shorter edge)

Barlow	TFOV [‘]	TFOV [“]	scale [“/pix]
0.5	    100.22	    6013.00	    1.27
0.8	    62.64	    3758.13	    0.79
1	    50.11	    3006.50	    0.64
2	    25.05	    1503.25	    0.32
3	    16.70	    1002.17	    0.21
4	    12.53	    751.63	    0.16
5	    10.02	    601.30	    0.13

Sizes in pix								
                Size [“]    0.5         0.8     1       2       3       4	    5
Neptune 	2.3         1.8         2.9     3.6     7.2     10.9	14.5	18.1
Jupiter	        40          31.5	50.4	63.0	126.0	188.9	251.9	314.9
M31	        7200	    5667.8	9068.5	11335.6	22671.2	34006.8	45342.4	56678.0
M42	        4500	    3542.4	5667.8	7084.8	14169.5	21254.3	28339.0	35423.8
M57	        150         118.1	188.9	236.2	472.3	708.5	944.6	1180.8

Oh, and my budget is not carved in stone. I've been thinking of buying the ASI 2600 MC pro (2136 €).

[turboscrew]

The Newt Web program gave:
Angular field of view for the 100% illuminated area is 0.2457°. The diameter is 0.2048 inch
Angular field of view for the 75% illuminated area is 2.1122°. The diameter is 1.7602 inch

But I have to use 50 mm extension tube for eyepieces - added that to the focuser height.

The 75% ray is vignetted by the inside of the tube at the front.

The tube inside diameter should be a little larger, or the diagonal minor axis could be made smaller to shrink the illuminated field.

The baffles can't be calculated until the vignetting at the front is corrected.

[Bigzmey]

Would starting with 12" scope make harder for Juha to learn AP?

Would it will be better to start with something like 80mm APO? Which is a nice scope to have anyway.

[OzEclipse]

The Newt Web program gave:
Angular field of view for the 100% illuminated area is 0.2457°. The diameter is 0.2048 inch
Angular field of view for the 75% illuminated area is 2.1122°. The diameter is 1.7602 inch

But I have to use 50 mm extension tube for eyepieces - added that to the focuser height.

The 75% ray is vignetted by the inside of the tube at the front.

The tube inside diameter should be a little larger, or the diagonal minor axis could be made smaller to shrink the illuminated field.

The baffles can't be calculated until the vignetting at the front is corrected.

You did the right thing. The focusser height is the height of the focal plane above the edge of the tube.

Yes I mentioned this vignetting was a possibility in the post where I described NEWT FOR THE WEB.

You can address this via several choices. I am not familiar with your OTA so some of these choices may not apply.

1. Shift the primary mirror back in the tube to lower the focal plane height (only if possible)
2. Shift the spider further up the tube to lower the focal plane height. Turn the secondary mirror 180 deg then cut a
new hole on the other side of the OTA and mount the focusser there. Cover the current mounting hole.
3. Purchase a larger diameter 2.5 inch or 3 inch focuser and enlarge the mounting hole.

I note that the 75% illuminated area is 1.76 inch. That's basically a full frame sensor. The 75% fall off will have to be addressed with flat field exposures. You could use one of the smaller size sensors as is without doing anything APS or micro 4/3.

You can't in general use reducers with a newt. Most 0.8X reducers are designed as field flatteners for refractors. The 0.5X reducers give a tiny usable field of perhaps a few mm diameter. They are designed to be used with tiny sensor video cameras. The only reducing coma corrector I am aware of that's suitable for use with an f4 newt is the ASA 2 Korr giving a 0.73x reduction.

https://www.teleskop-express.de/shop/pr ... raphy.html

This would turn your newt into an f2.9 Boren-Simon astrograph. However, it can only be used with smaller sensors. It's fully corrected field is 22mm eg micro 4/3 (18x13mm) sensors. They do note that by focussing part way out rather than on the centre, you can get coverage to 30mm (APS-C). You are going to have to have very precise collimation. Focussing accurately at f2.9 is going to be difficult.

Joe

[turboscrew]

Would starting with 12" scope make harder for Juha to learn AP?

Would it will be better to start with something like 80mm APO? Which is a nice scope to have anyway.

I think I've chosen my way - regardless of the anticipated frustrations. :smile:

[turboscrew]

The Newt Web program gave:
Angular field of view for the 100% illuminated area is 0.2457°. The diameter is 0.2048 inch
Angular field of view for the 75% illuminated area is 2.1122°. The diameter is 1.7602 inch

But I have to use 50 mm extension tube for eyepieces - added that to the focuser height.

The 75% ray is vignetted by the inside of the tube at the front.

The tube inside diameter should be a little larger, or the diagonal minor axis could be made smaller to shrink the illuminated field.

The baffles can't be calculated until the vignetting at the front is corrected.

You did the right thing. The focusser height is the height of the focal plane above the edge of the tube.

Yes I mentioned this vignetting was a possibility in the post where I described NEWT FOR THE WEB.

You can address this via several choices. I am not familiar with your OTA so some of these choices may not apply.

1. Shift the primary mirror back in the tube to lower the focal plane height (only if possible)
2. Shift the spider further up the tube to lower the focal plane height. Turn the secondary mirror 180 deg then cut a
new hole on the other side of the OTA and mount the focusser there. Cover the current mounting hole.
3. Purchase a larger diameter 2.5 inch or 3 inch focuser and enlarge the mounting hole.

I note that the 75% illuminated area is 1.76 inch. That's basically a full frame sensor. The 75% fall off will have to be addressed with flat field exposures. You could use one of the smaller size sensors as is without doing anything APS or micro 4/3.

You can't in general use reducers with a newt. Most 0.8X reducers are designed as field flatteners for refractors. The 0.5X reducers give a tiny usable field of perhaps a few mm diameter. They are designed to be used with tiny sensor video cameras. The only reducing coma corrector I am aware of that's suitable for use with an f4 newt is the ASA 2 Korr giving a 0.73x reduction.

https://www.teleskop-express.de/shop/pr ... raphy.html

This would turn your newt into an f2.9 Boren-Simon astrograph. However, it can only be used with smaller sensors. It's fully corrected field is 22mm eg micro 4/3 (18x13mm) sensors. They do note that by focussing part way out rather than on the centre, you can get coverage to 30mm (APS-C). You are going to have to have very precise collimation. Focussing accurately at f2.9 is going to be difficult.

Joe

Well, it also said:

Diagonal too small to admit 100% ray: No
Vignetting of 75% ray at front aperture: Yes
Vignetting at focuser of 100% ray: None
Vignetting at focuser of 75% ray: Yes

And this is the thing: I was looking into the tables (generated using your formuli), and it looked like with ASI2600 I could get nice pictures of (somewhat) big objects, but most planets tend to be just a couple of tens of pixels in size. And it's still impossible to fit very big objects (like M42) onto the sensor.
I could get bigger pictures of planets and small targets using a planetary camera (~500 €) with pixel size of 1.6 um and chip size of 4.6 mm, but then I couldn't fit even pinwheel galaxy onto the sensor.

That's why I wondered if it's a good idea to start with a (cheap) planetary camera and get a bigger and "shinier" DS camera later.

I wonder if stacking could "average out" atmospheric fluctuations. I'd guess that a single shot with image scale of 0.8"/pix won't work if the seeing is about 2".

[turboscrew]

The full "dimensions" list from Newt for the web:

Unit of Measure inch
Primary Mirror Diameter 12.000
Focal Length 48.000
Focal Ratio 4.000
Tube Inside Diameter 12.500
Tube Thickness 0.125
Focuser Minimum Height 4.000
Focuser Inside Diameter 2.000
Focuser Extra Travel 1.000
Focuser Camera Travel 0.000
Diagonal Minor Axis 3.000
Diagonal Offset 0.188
100% Illumination Diameter 0.205
75% Illumination Diameter 1.760
Front Aperture Diameter 13.532
Mirror Face to Focuser Hole 36.625
Focuser to Front End of Tube 6.000
Mirror Face to Back of Tube 4.000
Tube Length 46.625

I just didn't know how to account for the coma corrector. It'd make the focal length 6% longer, and the F-number becomes 4.24.

[OzEclipse]

I had a play with design parameters until I eliminated the vignetting. It's the tube diameter as well as the focuser diameter.
Increasing the tube size to 15" is a major rebuild. New cell, new spider or at least new vanes on the spider.
A 3" focuser will also cost a lot. Orion UK make the AG12 designed for this purpose.
Looks like they designed the VX for visual.

Joe

[SkyHiker]

That's why I wondered if it's a good idea to start with a (cheap) planetary camera and get a bigger and "shinier" DS camera later.

You are right, planetary cameras have smaller pixels. The 3.25 mu I mentioned is for DSO cameras.

A DSO camera like the 2600 or 1600 or 294 would work for plate solving, a planetary won't. However if you get an autoguider with a decent FOV you can use that. My SSAG works very well for plate solving. And you can get a DSO camera later.

Plate solving is nice to have, it makes everything so much easier (polar alignment and goto alignment).

I wonder if stacking could "average out" atmospheric fluctuations. I'd guess that a single shot with image scale of 0.8"/pix won't work if the seeing is about 2".

Yes for planetary if you sample at say 120 Hz, Autostakkert selects the best frames to stack, so it will eliminate the bad ones. Make sure that your camera can do at least 120 Hz. That worked for me. DSO cameras may not be able to do that. I have not checked mine yet. You can definitely use a Barlow, I think optical elements are the best way to boost the resolution. I did Saturn once with a ELPH 100 HS at 120 Hz zoomed in to the maximum optical, it filled a good deal of the sensor.

Keep in mind that there are only 2 interesting planets, well Mars sometimes, and that their positioming has been miserable lately so nonody has been able to do much planetary work. It could take a few years, I haven't checked.

[turboscrew]

I've been playing with the SW most of the evening, and I came to the same conclusion: "Replace everything, but the main mirror. Well, what the heck, replace that too."

I wonder... the 75% diameter is supposed to be 44.7 mm.
Would ~20 mm sensor still work?

[OleCuss]

I'd probably first do something which might seem a bit different (maybe it's actually silly). I'd call Orion and simply ask them about the maximum sensor size they recommend. They should have a number of owners who've been using that OTA for imaging and even if they don't, their designers and testers should have a good idea what those optics will do with a camera.

You can use large sensors and you can use small sensors. Much depends on your preferences and other equipment and software.

You can get a sensor which has a much larger sensor than is the image circle of your OTA. You'll be missing virtually none of the light your system is collecting for you. Do remember that if you get a big sensor with lots of pixels your computer is going to have to stack them all and with a bunch of sub-frames you can end up with long stacking sessions. With modern sensors, however, you can define a region of interest and utilize the sensor as if it is much smaller than its physical size and that will give you flexibility in terms of image size and computing demands. Defining a much smaller ROI can also give you the option of using a big-sensor camera as a planetary camera in that you can often get a passably faster frame rate when the sensor is acting as if it is a tiny sensor and doesn't need that much bandwidth.

I would calculate the sampling and would tend a little toward over-sampling. I've seen some images which suggest over-sampling for even regular wide-field AP can get you more detail than should be expected but the idea is controversial at the least.

Another thing to think about is whether sampling for the greatest detail will actually do you any good. If you are going to be viewing the final image on a tiny low-resolution monitor at long distance - collecting a huge amount of data in order to get your sampling right where your calculations suggest it should be might be a waste.

One more suggestion? If you've not already done so, read a couple of books on astrophotography. I thought "The Deep-sky Imaging Primer" was overall the best although I liked "Astrophotography" by Thierry Legault almost as well (and it was prettier). I'll put in a gratuitous shout for the out-dated but never obsolete "A Complete Manual of Amateur Astronomy" by P. Clay Sherrod which is not exactly AP oriented but is still a treasure for understanding the heavens and the gear.

But remember to be skeptical about what I say. I spent quite a bit of time and money figuring out that I don't like AP - and others love it. The eyepiece was what worked for me. I figured out that I was spending a lot of time and energy trying to duplicate what others had typically done better - and I wanted to bring peace and relaxation to my soul rather than a technical exercise. I still find the tech fascinating but I don't want to deal with it. OK, OK, I actually do think quite a bit of AP can be done better but I don't want to be the one doing it. The key is that the person who doesn't like doing AP might not be the one you want to rely upon for advice on how to do it.