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Last night I learned a lesson and saved some money....


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Last night I had some clear skies and decided to go out to observe Jupiter for a while. I've mostly been using a 102mm Mak for planetary observing for some time, in preference to my 8 inch dob (easier to set the tube up on an equatorial mount) and have been impressed with the views. I'd been considering splashing for a 127 mm (maybe even a 150), tempted by the promise of decent aperture and the long focal ratio that makes it so much better for planetary detail than a Newt. Then, on a whim, I decided to dig out my old Heritage 130P and stick it on the mount, just to compare the performance. I figured the extra aperture would make for a close contest, even though the Heritage is F5. How wrong I was.

The 130P blew the 102 Mak out of the water, even on a Jupiter where contrast is at a premium. It's noticeably better at comparable magnifications, and handles being pushed just as well as the Mak, which means it can go higher (crystal clear images though a 6mm ortho and 2x barlow for x217 given decent seeing). Even as a pure planetary scope, a 130mm F5 Newt beats my 102mm F13 Mak hands down. Add in the portable truss tube design, good wide field performance and price (you can pick the tube up for £45 on Astroboot) and I've just (re)discovered my perfect grab and go!

More to the point, and to the great benefit of my wallet, it reminded me of the importance of concentrating on the differences that matter. A quality (99%) star diagonal compared to a bog-standard gets you up to 0.04 of a magnitude, a high F ratio is nice. But aperture kills.

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Very interesting report,from my point of view,as I own a 130p solid tube model on an EQ2,but have moved somewhat to the Frac camp,due to a growing interest in double stars.

Strangely enough,I've never had the opportunity to try the F10 Tal 100rs and the F5 130p side by side.

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Hahaha thanks for the tip - I've just bought the 130P with the 2" focuser that was on astroboot, the other two for £35 looked a bit old to me - I was saving up for a 130PDS new, but for £45 and £1.50 for delivery it had to be worth the punt :)

Now I guess I need to figure out how to collimate...

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Don't worry about collimation - it's straightforward. FLO sell a basic collimatioin cap for about £5, and it's worth getting. A Cheshire is better, but the cap will do fine. Then:

1) Position the tube horizontally and get a couple of pieces of paper. Push one into the tube so that it blocks the primary mirror and place one behind the secondary. Be careful when doing this.

2) Looking through the collimation cap, check that the secondary is centred in the focuser and orientated towards it (the mirror should appear circular).

  - If it isn't, adjust it with the screw in the top of the secondary mirror housing.

3) Take the paper out and look at the reflection of the primary mirror in the secondary. Is it centred? Can you see all three mirror supports? If yes then leave it.

  - If not, adjust the secondary mirror using the three allan bolts so that the primary appears in the centre of the secondary.

4) Usually you don't need to do the first two steps. The next one is quite common, but the easiest. Look into the collimation cap and make sure the circular ring in the centre of the primary mirror is reflected in the center of the cap (i.e. with eye hole right in the middle). If not, loosen the retaining bolts and move the adjusting bolts on the base of the tube to tilt the mirror so that the ring is centred. Tighten the retaining bolts.

5) Go out and observe!

 

 

Edited by billyharris72
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Here's the link to Astro_Baby's collimation-paper:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

One of the very best!

The 130p is always full of surprises - and each time it serves to show us why it's likely the most popular telescope in this forum.

Congratulations on your re-discovery!

Dave

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On 08/03/2016 at 14:41, John78 said:

Hahaha thanks for the tip - I've just bought the 130P with the 2" focuser that was on astroboot, the other two for £35 looked a bit old to me - I was saving up for a 130PDS new, but for £45 and £1.50 for delivery it had to be worth the punt :)

Now I guess I need to figure out how to collimate...

I was looking at these as well. They will have some 130 PDS tubes in the next few days so might wait and see.

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Aperture wins :) You may have different contrast and image quality dependant upon type of scope and quality of optics but, unless the primary is pants then a larger size will give better results.

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Hi Michael. What do U think is the max size

of scope, to use in the UK, to get good results. 

Sometimes bigger, is not always better.

 

Steve.

22 minutes ago, Mr Spock said:

Aperture wins :) You may have different contrast and image quality dependant upon type of scope and quality of optics but, unless the primary is pants then a larger size will give better results.

 

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I had another experience last night: dark skies. With my 127mm I looked up some of the familiar objects that I have seen at home. Here at our cabin in dark country the galaxies are almost too easy to spot. At home I have seen M65 and M66 with averted vision. Here I saw the whole Leo triplet at once, very clear. M13 is usually a light grey blob without detail. Here it started to resolve. I could see some of the individual stars in the outer regions. Dark skies rule!

I am now wondering if there is any point in continuing astronomy at home.

I also decided last night that if I buy a bigger scope, that it will stay at the cabin.

Edited by Linda
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Hi Michael. What do U think is the max size

of scope, to use in the UK, to get good results. 

Sometimes bigger, is not always better.

On most nights 10" is about the perfect size. Larger is better, but only on nights of excellent seeing - something we don't see too often in the UK.

I suspect this is where the C9.25 gets it's reputation from. Not because it is optically better than other scopes, but because it's aperture sits in the 'zone' of good seeing.

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Quote

 

 

2 hours ago, Mr Spock said:

 

 

On most nights 10" is about the perfect size. Larger is better, but only on nights of excellent seeing - something we don't see too often in the UK.

I suspect this is where the C9.25 gets it's reputation from. Not because it is optically better than other scopes, but because it's aperture sits in the 'zone' of good seeing.

Come to my house and view Jupiter on a good or bad night at around x180 am sure you would change your mind 

pat

Edited by todd8137
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Come to my house and view Jupiter on a good or bad night at around x180 am sure you would change your mind

I find at such low magnifications Jupiter's finer detail is not visible ;)

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7 minutes ago, Mr Spock said:

 

 

I find at such low magnifications Jupiter's finer detail is not visible ;)

Ok then y fl is around 1710 mm x240 is easy on most nights on planets I do have a dark location as well and I do not view over the tops of houses ect 

Edited by todd8137
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An extra inch of aperture can make a big difference. It is quite deceptive. I believe it has something to do with the inverse square law, or the law of inverted squares, or something like that. You normally calculate the extra light gathering ability by multiplying the square root of the length of the cat's left back leg with the sum of the focal lengths of all the eyepieces you always wanted, but never actually bought, divided by the average RPM of the Pink Fairies' début album. OK, I admit I made all that up lol, but an extra inch of aperture can seriously reveal more detail. I noticed this comparing a 130mm Newtonian virtually side by side with a 102mm Mak one time during a dusk setting of Venus and Jupiter. The 130mm scope showed some cloud detail/colouration difference on Venus that wasn't easy to see with the 4" Mak and the EZ, NEB, SEB and SSTB on Jupiter were definitely more detailed on the bigger scope. The magnification varied between 180x and 162.5x on the 130mm and 102mm scopes respectively (15mm TV Plossl with a 3x TV Barlow on the 130mm and a 20mm TV Plossl and 2.5x Powermate on the 102mm). An extra inch of aperture can make a big difference.

Edited by Mak the Night
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21 minutes ago, Mak the Night said:

An extra inch of aperture can make a big difference. It is quite deceptive. I believe it has something to do with the inverse square law, or the law of inverted squares, or something like that. You normally calculate the extra light gathering ability by multiplying the square root of the length of the cat's left back leg with the sum of the focal lengths of all the eyepieces you always wanted, but never actually bought, divided by the average RPM of the Pink Fairies' début album. OK, I admit I made all that up lol, but an extra inch of aperture can seriously reveal more detail. I noticed this comparing a 130mm Newtonian virtually side by side with a 102mm Mak one time during a dusk setting of Venus and Jupiter. The 130mm scope showed some cloud detail/colouration difference on Venus that wasn't easy to see with the 4" Mak and the EZ, NEB, SEB and SSTB on Jupiter were definitely more detailed on the bigger scope. The magnification varied between 180x and 162.5x on the 130mm and 102mm scopes respectively (15mm TV Plossl with a 3x TV Barlow on the 130mm and a 20mm TV Plossl and 2.5x Powermate on the 102mm). An extra inch of aperture can make a big difference.

Just a square law - area = pi x r x r.  If you go from 5" to 8" that's 8/5 or 1.6 times the the diameter/radius.

So the area is multiplied by 1.6 squared, i.e. 2.56 times the light-gathering.  Quite an increase.  (Ignoring obstructions and so on.)

4" to 5" - that's 1.56 times the light gathering - or a 56% increase.  So yes, an inch can make a lot of difference!

Doug.

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27 minutes ago, cloudsweeper said:

Just a square law - area = pi x r x r.  If you go from 5" to 8" that's 8/5 or 1.6 times the the diameter/radius.

So the area is multiplied by 1.6 squared, i.e. 2.56 times the light-gathering.  Quite an increase.  (Ignoring obstructions and so on.)

4" to 5" - that's 1.56 times the light gathering - or a 56% increase.  So yes, an inch can make a lot of difference!

Doug.

Thanks Doug. As you can possibly guess, maths isn't my forte, but I'm slowly figuring out a lot with optics. It's comparing Imperial with Metric measures that can be the most confusing. Calculating exit pupils for particular telescopes was a bit of a revelation for me. This is useful if you want to figure the maximum realistic magnification for small aperture scopes. I've recently acquired a 90mm f/11.3 Mak and I'm reasonably sure I can get a maximum magnification of 181x with it for just about a 0.5mm exit pupil with a 5.5mm eyepiece (I actually Barlow an 11mm TV Plossl to achieve this).

 

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24 minutes ago, Mak the Night said:

Thanks Doug. As you can possibly guess, maths isn't my forte, but I'm slowly figuring out a lot with optics. It's comparing Imperial with Metric measures that can be the most confusing. Calculating exit pupils for particular telescopes was a bit of a revelation for me. This is useful if you want to figure the maximum realistic magnification for small aperture scopes. I've recently acquired a 90mm f/11.3 Mak and I'm reasonably sure I can get a maximum magnification of 181x with it for just about a 0.5mm exit pupil with a 5.5mm eyepiece (I actually Barlow an 11mm TV Plossl to achieve this).

 

Hi Mak - I'm OK with maths, but have only recently started applying it to telescopes and astronomy!  From the equation I have I agree with the figure you have for the exit pupil - 0.487mm - but that degree of accuracy is meaningless, so call it 0.5mm!  I understand this is about the minimum usable value, which as you say ties in with maximum mag..  

Doug.

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6 hours ago, Linda said:

I had another experience last night: dark skies. With my 127mm I looked up some of the familiar objects that I have seen at home. Here at our cabin in dark country the galaxies are almost too easy to spot. At home I havexseen M65 and M66 with averted vision. Here I saw the whole Leo triplet at once, very clear. M13 is usually a light grey blob without detail. Here it started to resolve. I could see some of the individual stars in the outer regions. Dark skies rule!

I am now wondering if there is any point in continuing astronomy at home.

That's exactly the message some of site members (me included) have trying to convey. For faint galaxies and nebulas, dark sky is the key, not aperture. If you read around in the site, you're find some backyard observers never see the Leo tripet, despite their 8" scopes. 

Large aperture will let you see a larger image scale of a these faint fuzzies with EXACTLY the same brightness at the same exit pupil size. The larger image scale gives better resolution of fine details, makes it easier to see. Therefore large apertures are good at seen those small galaxies and planetary nebulas with good surface brightness, and point source objects(stars, open cluster, globular clusters). While for large galaxies (say 5' upwards), a 80mm  scopes in mean magnification can get large enough image scale for our eyes to see, for these targets, a 80mm in dark site will beat 200mm or 300mm scope in light-polluted backyard soundly.

The sticky post in this section "What is a dark site" is a must read for all gazers, IMHO.:smiley:

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10 hours ago, cloudsweeper said:

Hi Mak - I'm OK with maths, but have only recently started applying it to telescopes and astronomy!  From the equation I have I agree with the figure you have for the exit pupil - 0.487mm - but that degree of accuracy is meaningless, so call it 0.5mm!  I understand this is about the minimum usable value, which as you say ties in with maximum mag..  

Doug.

It can also often get confusing when taking manufacturers' claims into account. I originally calculated to f/11.1 (1000/90) but I discovered that it was actually f/11.3. This stumped me for a while until I realised I hadn't taken the meniscus obstruction into account. But, like you say, it's splitting hairs with essentially meaningless decimal points. Another point is the often varied claims of absolute magnifications by manufacturers. I've seen 213x touted as an absolute maximum for a 90mm scope. I'm guessing this is something like converting the Metric exactly into the Imperial equivalent and then multiplying each converted inch of aperture by 60x. Which is the 'per inch' magnification TeleVue give as a rule of thumb if you have perfect conditions and high quality optics (although it only applies to 6" and under). I'm pretty certain that's why the supposed highest magnification of a 102mm aperture is often quoted as being 241x. In reality, with my 102mm Mak, the best magnification I can get for a 0.5mm exit pupil is 208x. I can achieve this perfectly with a 2x Barlow and a 12.5mm Astro Hutech orthoscopic eyepiece. Still, 208x or 181x for a 102mm and 90mm aperture is nothing to be ashamed of lol. As long as you can get around the 150x mark you should see quite a lot of planetary detail considering the aperture size. 

Edited by Mak the Night
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3 hours ago, YKSE said:

That's exactly the message some of site members (me included) have trying to convey. For faint galaxies and nebulas, dark sky is the key, not aperture. If you read around in the site, you're find some backyard observers never see the Leo tripet, despite their 8" scopes.

Yep! That sounds like me. My 8 x 56 bins show me more detail on holiday in Cornwall than my 8 inch dob does in my Bedford back garden. I don't spend that much time at home looking for DSOs any more, as they are either invisible or disappointing. The occasional trip to a (slightly) dark(er) site for DSOs and that's about it. At least I've got Jupiter to keep me company.

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