# A A telescope for asteroids

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1. Mar 27, 2016

### pioneerboy

Would such a telescope be useful for discovering new main belt asteroids:
http://www.officinastellare.com/products_scheda.php?idProd=29 [Broken]

Last edited by a moderator: May 7, 2017
2. Mar 27, 2016

### davenn

Sorry, cannot get that page to load

Last edited by a moderator: May 7, 2017
3. Mar 27, 2016

### Staff: Mentor

It loads for me. What do you get?
http://www.officinastellare.com/image/resize/id/1435/width/610/height/410 [Broken]
http://www.officinastellare.com/image/resize/id/1435/width/610/height/410 [Broken]

Last edited by a moderator: May 7, 2017
4. Mar 27, 2016

### andrew s 1905

This is a very capable scope and on a dark site with a high quantum efficiency CCD of suitable size and a matching mount it should be able to do many research projects. You need to research what is involved in such an asteroid search and match the requirements that with the full systems capabilities. Many professional systems now tend to sweep up asteroids etc.

Regards Andrew

5. Mar 28, 2016

### Chronos

An good asteroid hunting scope has one essential feature - it is very fast [short f ratio]. This is due to the practical need for a wide field of view and short exposure times to enable an efficient survey of a large portion of the sky.

6. Mar 29, 2016

### andrew s 1905

In their book "Remote Observatories for Amateur Astronomers" ISBN973-3-319-21905-9 Hubbell, Williams and Billard, describes a search for Asteroids by Rob Matson. He hired telescope time on the Sierra Stars Observatory Network (SSON) and discovered or rediscovered 57 asteroids between 2009 and 2015. He used 3 telescopes in the network : 0.37m F/15 classical Cassegrain with a Kodak KAF-16803 CCD, 0.61m F/10 classical Cassegrain with a Kodak KAF- 09000 CCD and a 0.81m F/7 Richey-Chretien with a Kodak KAF-16803 CCD.

Clearly fast focal ratio's are not that essential as these are all relatively slow telescopes. Rob Matson studied what was needed to optimise his chances ensuring the asteroid would be at their brightest, clear of crowded star fields and not in areas just searched by professional scopes.

Regards Andrew

7. Mar 29, 2016

### Chronos

That is a great approach for discovering previously unknown asteroids, but, how many amateur hunters are inclined to such extremes? The scope the OP inquired about is f/5, obviously pretty fast. A fast scope does lose some faint detection capability, it's the price you must pay to view a larger area of the sky.

8. Mar 29, 2016

### andrew s 1905

Not quite clear on what you find extreme in Rob Maston's approach. He hired telescope time at approximately $100 per hour compared to$60,000 just for the scope so maybe \$120,000 for a full system based on the telescope proposed by the OP?

The real point I am trying to make is that there is not one factor that dominates but it is a balance of factors optimised to the science task you wish to undertake. For example as well as speed you need a large enough image scale and limiting magnitude coupled to a good search strategy.

Regards Andrew

9. Mar 29, 2016

### andrew s 1905

PS I just realised you might have thought he used the three at once, which would be extravagant. I think he used them individually as appropriate for the target area.

10. Apr 3, 2016

### pioneerboy

Can you go a little bit more into detail what you mean exactly with your last sentence, please? Do you have Modulation Transfer Function and Contrast in mind or under which keywords can I find the explanation to the phenomenon you mention here?

11. Apr 3, 2016

### Chronos

A fast scope typically suffers some light loss due to a larger secondary or extra corrector elements in the optical path.

12. Apr 4, 2016

### andrew s 1905

In addition for a scope with equivalent aperture and light loss but with a longer focal length (slower F/ number) has more contrast between "point" sources e.g stars and asteroids and the background sky.

Regards Andrew

13. Apr 4, 2016

### pioneerboy

There are two aspects: 1. the larger obstruction makes the light gathering area smaller for a faster scope, 2. the faster scope reaches a specific magnitude in a shorter exposure time. What you say is that aspect 1 weights more than aspect 2. Intuitively, I'd agree for insanely large obstructions like 60, 70, 80 %. But I doubt that aspect 2 is beaten up to 50% or so of obstruction. But I'd like to check it more. Can you provide some equation or link please?
These three equations are very interesting in this regard "EXPOSURE COMPARISON FOR EXTENDED OBJECTS", "EXPOSURE COMPARISON FOR POINT SOURCES", and "LIGHT-RECORDING POWER OF A SYSTEM" : http://www.saguaroastro.org/content/formulas.htm#FORMULAEFORASTROPHOTOGRAPHY .
Let's take equation 3 as it takes into account the light collection and fastness of a telescope (so the obstruction). Let's assume f/5 and f/3 for a 50cm telescope. Following the equation this means that the f/3 telescope collects 2.778 times more photons than the f/5 telescope with equal aperture. This means that it reaches a certain magnitude faster than the f/5 telescope. I do not yet see how this affects the detection capability. But I'm curious and hope you can explain some more.

14. Apr 5, 2016

### andrew s 1905

You are mistaken. The number of photons collected by a telescope is not a function of it's Focal Ratio F. It is a function of it's effective aperture and any losses in refection or refraction. What differs for scopes with the same effective aperture and different focal ratio is the image scale and hence the density of photons per unit area per second on the detector. With a CCD detector they can have similar photons per second per pixel by changing the effective the pixel size.

Also while for a given reflecting telescope design the faster the scope the more central obstruction it is not true across designs. I have two Newtonian reflectors( F5 and F5.4) that have central obstructions of 13% and 6 % respectively while OP's proposed scope F5 has a 52% obstruction and their F7 model 48%.

Regards Andrew

15. Apr 5, 2016

### pioneerboy

But you just confirm my general statement that faster optics have larger central obstructions: F5 => 52%, and F7 => 48% obstruction. The faster F5 has a 4% larger obstruction. I've never come across any telescope design that from Newton to RC where the faster scope has a smaller obstruction. If such an optical design exists please let me know! I wanna have such a telescope!!!

Concerning your first section. Now it becomes a problem with the CCD and not solely a telescope problem anymore. I was thinking about how this detection sensitivity could be solved only telescope-wise, but it obviously can't. So let's look at the pixels.
Generally faster optics have a larger "/pixel number; for a given pixel number, e.g. 2048 pixels, that increases the field of view FOV, therefore making fast corrected optics, wide-field optics. But, in principle, having a pixel scale of 2"/pixel instead of 0.5"/pixel means that more light falls into the same pixel. Take a look at page 39 of this link http://www.fcaglp.unlp.edu.ar/~observacional/papers/PDFs/phot_psf_merline-howell1995.pdf where two pixel scales are compared. You see that albeit much fewer pixels characterize the star, the light falling into the central pixel increases its ADU counts. Thus increasing the stars detectability for the telescope with larger pixel scale...meaning the telescope with shorter focal length, or in other words, the faster telescope (IF we assume a given unchanged pixel size without binning!). So despite the larger obstruction the detectability is not reduced in my opinion.
But where you would be absolutely right (not saying you're wrong above, but I still don't see why even a larger pixel scale would turn against a better detection sensitivity of a faster telescope) is, that the fewer pixel numbers characterizing the star, significantly reduces the precision in astrometry because the program has increased difficulty finding the correct centroid.

16. Apr 5, 2016

### Chronos

Generally speaking the light gathering capability of a telescope is sensitive to both effective aperture and light transmission efficiency. Effective aperature is pretty simple being the surface area of the primary optical element less the area of any secondary systems [seconary surface along with its support structure]. Stops also serve to reduce effective aperture. The number of optical surfaces in an optical path includes the front and rear suface of all lenses as well as the first surface of all refelective elements. You have a [hopelfully] small loss in transmission efficiency for each optical surface due to reflection, absorption and scattering. These losses are cumulative and can add up in a hurry in multi element configurations. Note that coatings, while clearly beneficial, constitue another surface for each element. No optical surface is 100% efficient. The apparent sky background brightness also plays a role in limiting magnitude detection capability, which means a fast scope, which sees more of the sky suffers a small loss in contrast compared to an identical, but, slower scope. This limiting effect is, however, trivial compared to the afforementioned considerations. For further discussion, see http://www.telescope-optics.net/functions.htm.

17. Apr 5, 2016

### andrew s 1905

As I said before "The real point I am trying to make is that there is not one factor that dominates but it is a balance of factors optimised to the science task you wish to undertake." This is best done by modelling all important factors both technical, practical and financial as the link to Merline and Howell demonstrates.

Also I was not clear before I was just pointing out that say an F7 RC can have a larger central obstruction than an F5 Newt. Although I can easily design a F5 Newt with a smaller obstruction than an F7 it will just have a smaller fully illuminated field and where the focal point will fall with respect to the tube side will be different. Again it depends on the task at hand.

Regards Andrew

18. Apr 5, 2016

### pioneerboy

Sorry for the misunderstanding. I thought we unwrittenly agree to the same optical design instead of comparing between different designs. Still, for a given design I'd first like to see...