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PlanetGazer

Is this a good collimation for a f/4.7?

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Good Afternoon

 

I have been trying to get my SW 250P (f/4.7) to the best possible collimation as I couldn't manage to focus on any of the planets so far. I just collimated the scope to ready it up for tonight's session. Your feedback is much appreciated!

IMG_20190824_150728.thumb.jpg.69eb9ddd2f0f53ad9324121f98dd4eca.jpgIMG_20190824_150735.thumb.jpg.b5440094e913520c4075ca51231bf711.jpg

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It is quite difficult to take a photo through a Cheshire but everything except for the shadow of the secondary should be centred under the crosshairs. In your photos it appears that this is not the case. You also don't have the focuser and/or Cheshire far enough out to see if the secondary mirror is centred under the focuser. However, none of this will prevent you from focusing the telescope. Focusing is simply a matter of having the eyepiece the correct distance out from the telescope. With respect to the planets, the low altitude will mean the atmosphere will often defocus the image, especially if you are observing over houses etc. I suggest you try focusing a high star. If you can focus the star then you have the correct adapters and extensions in your focuser and will also be able to focus the planets if the atmosphere allows. If you can focus a star but not a planet then consider more carefully what you are observing over and if changing the positioning of your telescope or the time that you observe will give you more favourable conditions. 

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Still looks slightly off... the Cheshire reflection needs to be in the middle too.

 

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Posted (edited)
On 24/08/2019 at 16:25, Ricochet said:

It is quite difficult to take a photo through a Cheshire but everything except for the shadow of the secondary should be centred under the crosshairs. In your photos it appears that this is not the case. You also don't have the focuser and/or Cheshire far enough out to see if the secondary mirror is centred under the focuser. However, none of this will prevent you from focusing the telescope. Focusing is simply a matter of having the eyepiece the correct distance out from the telescope. With respect to the planets, the low altitude will mean the atmosphere will often defocus the image, especially if you are observing over houses etc. I suggest you try focusing a high star. If you can focus the star then you have the correct adapters and extensions in your focuser and will also be able to focus the planets if the atmosphere allows. If you can focus a star but not a planet then consider more carefully what you are observing over and if changing the positioning of your telescope or the time that you observe will give you more favourable conditions. 

 

Yeah I wasn't satisfied with the photos uploaded as well, the doughnut wasn't as clear as I wanted it to be. I knew the cross-hairs did not coincide, but I was focused to get the hole centered in the doughnut. Do I have to keep adjusting the primary for the cross-hairs to align? or do I need to adjust the secondary. I will post another picture with the focuser extended.

 

I had a problem before when trying to focus with my 250p, planets were high, I tried using my old SW 130 f/6.9 and the planets looked crisp clear, could have been the collimation or mirror temperature. I was also worried it could be something wrong with the optics. But tonight, I managed to get acceptable views of Jupiter and Saturn, I left the scope outside for an hour to avoid temperature difference, and with today's collimation, it may not be perfect, but it was a noticeable improvement from what it was before, (could have collimated it before with tubes not fully extended). Though I'm eager to perfect the collimation.

 

I have done the star test on AlTair, and it seemed ok to me.

 

Another question comes to mind, is atmospheric turbulence more noticeable in low focal ratio scopes? or is it related of how well collimated the scope is? As I noticed that my slower scope show better views of Jupiter when low in the sky, than my fast scope (250p).

 

edit: Typo

 

 

Edited by PlanetGazer

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8 hours ago, MarsG76 said:

Still looks slightly off... the Cheshire reflection needs to be in the middle too.

 

I'm guessing the secondary needs to be adjusted?

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Posted (edited)
10 hours ago, PlanetGazer said:

Yeah I wasn't satisfied with the photos uploaded as well, the doughnut wasn't as clear as I wanted it to be. I knew the cross-hairs did not coincide, but I was focused to get the hole centered in the doughnut. Do I have to keep adjusting the primary for the cross-hairs to align? or do I need to adjust the secondary. I will post another picture with the focuser extended.

Adjust the secondary to get the doughnut under the crosshairs. Once this is done adjust the primary to put the centre spot in the middle of the doughnut.

10 hours ago, PlanetGazer said:

I had a problem before when trying to focus with my 250p, planets were high, I tried using my old SW 130 f/6.9 and the planets looked crisp clear, could have been the collimation or mirror temperature. I was also worried it could be something wrong with the optics. But tonight, I managed to get acceptable views of Jupiter and Saturn, I left the scope outside for an hour to avoid temperature difference, and with today's collimation, it may not be perfect, but it was a noticeable improvement from what it was before, (could have collimated it before with tubes not fully extended). Though I'm eager to perfect the collimation.

I've just seen you list your location as Arabia. Given this, planet altitude will be less of an issue but I believe that cooling will be more of an issue. A telescope with a larger, thicker mirror takes longer to cool and your location may be prone to such rapid temperature drop that the mirror can never catch up. I would be inclined to fit a fan behind the primary to help speed up cooling. You should experiment to determine whether you get better performance with the fan still running or turned off once observing, but having it running while the mirror is out to cool should help. Having the telescope outside in the shade for as long as possible before an observing session will also help as you have found.

10 hours ago, PlanetGazer said:

Another question comes to mind, is atmospheric turbulence more noticeable in low focal ratio scopes? or is it related of how well collimated the scope is? As I noticed that my slower scope show better view of Jupiter when low in the sky, than my fast scope (250p).

It is related to the aperture of the telescope. A 250p is almost 2X the diameter of a 130p so the tube of air you observe through is almost 4x as large. You need this larger area of air to be stable in order to not see turbulence. The 250p also has a resolution almost twice that of the 130p so you can also see turbulence that is half the size of the turbulence that the 130p can see. 

Edited by Ricochet
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If it seems OK then that's good enough for now. Start using the scope and build up a further opinion over the next few nights of observation.

Don't worry about it to much.

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10 hours ago, Ricochet said:

Adjust the secondary to get the doughnut under the crosshairs. Once this is done adjust the primary to put the centre spot in the middle of the doughnut.

My next mission before the end of the week, Thanks!

Quote

I've just seen you list your location as Arabia. Given this, planet altitude will be less of an issue but I believe that cooling will be more of an issue. A telescope with a larger, thicker mirror takes longer to cool and your location may be prone to such rapid temperature drop that the mirror can never catch up. I would be inclined to fit a fan behind the primary to help speed up cooling. You should experiment to determine whether you get better performance with the fan still running or turned off once observing, but having it running while the mirror is out to cool should help. Having the telescope outside in the shade for as long as possible before an observing session will also help as you have found.

Indeed, the ecliptic plane is well positioned most seasons, in my location. I took a group of people, not interested in astronomy, and they were impressed with the views of Jupiter and Saturn (SW 130), now they'll bring their kids to observe!

As for the temperature difference, I was thinking of getting a fan for the 10 inch, but then I thought it's the temperature difference between (and correct me if I'm wrong) the mirror and the the surrounding air that cause the tube currents. The scope is stored in a cool and dry room (my office), and just before packing the mirror should be very cool, and AC is on in the car until reaching the observing site. Then I'll have to wait for aperture to reach equilibrium with the surrounding warmer temperature. It's the opposite over here, outside temperature is higher than home temperature.

Quote

It is related to the aperture of the telescope. A 250p is almost 2X the diameter of a 130p so the tube of air you observe through is almost 4x as large. You need this larger area of air to be stable in order to not see turbulence. The 250p also has a resolution almost twice that of the 130p so you can also see turbulence that is half the size of the turbulence that the 130p can see. 

It makes logical sense, thanks for the help!

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1 hour ago, PlanetGazer said:

As for the temperature difference, I was thinking of getting a fan for the 10 inch, but then I thought it's the temperature difference between (and correct me if I'm wrong) the mirror and the the surrounding air that cause the tube currents. The scope is stored in a cool and dry room (my office), and just before packing the mirror should be very cool, and AC is on in the car until reaching the observing site. Then I'll have to wait for aperture to reach equilibrium with the surrounding warmer temperature. It's the opposite over here, outside temperature is higher than home temperature.

Yes, it is the difference between the mirror temperature and the air temperature that causes the problems. I think a fan should work both ways, in that when the air is warmer it will help warm the mirror and when the air is colder it will help cool the mirror. Perhaps you can get better results simply by not using the ac in the car on your drive to the observing site (if that is bearable for any people in the car).

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22 hours ago, Ricochet said:

Yes, it is the difference between the mirror temperature and the air temperature that causes the problems. I think a fan should work both ways, in that when the air is warmer it will help warm the mirror and when the air is colder it will help cool the mirror. Perhaps you can get better results simply by not using the ac in the car on your drive to the observing site (if that is bearable for any people in the car).

Thank you very much for the help, Ricochet 🙂

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On 25/08/2019 at 00:02, PlanetGazer said:

I had a problem before when trying to focus with my 250p, planets were high, I tried using my old SW 130 f/6.9 and the planets looked crisp clear, could have been the collimation or mirror temperature. I was also worried it could be something wrong with the optics. But tonight, I managed to get acceptable views of Jupiter and Saturn, I left the scope outside for an hour to avoid temperature difference, and with today's collimation, it may not be perfect, but it was a noticeable improvement from what it was before, (could have collimated it before with tubes not fully extended). Though I'm eager to perfect the collimatio

 

Funny, I just made the same move from SW 130 to 250p. I had the same problem with collimation but someone at a club observing session did collimated mine ... I still am uncomfortable doing it though.

Still my views are much sharper now!

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8 hours ago, Raph-in-the-sky said:

Funny, I just made the same move from SW 130 to 250p. I had the same problem with collimation but someone at a club observing session did collimated mine ... I still am uncomfortable doing it though.

Still my views are much sharper now!

Interesting to hear we had the same choices! Wonder what you think of the upgrade and what have changed for you so far, that could be a different topic though.

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1 hour ago, PlanetGazer said:

Interesting to hear we had the same choices! Wonder what you think of the upgrade and what have changed for you so far, that could be a different topic though.

I think moving from the 130 to the 250 was amazing. When I looked at the moon for the first time in the 250 i felt like the difference was almost a big as the one between the 130 and naked eye (yes I m exagerating)

My first purchase after the 250 was a Telrad finder. I planning on buying a RACI finder and in time the lacerta microfocuser upgrade kit. What about you?

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3 hours ago, Raph-in-the-sky said:

I think moving from the 130 to the 250 was amazing. When I looked at the moon for the first time in the 250 i felt like the difference was almost a big as the one between the 130 and naked eye (yes I m exagerating)

My first purchase after the 250 was a Telrad finder. I planning on buying a RACI finder and in time the lacerta microfocuser upgrade kit. What about you?

Hahaha on the same boat, I was looking at dual speed focusers yesterday, and have already bought a RACI finder, I use the dot sight from the 130 on the 250 as well. I just searched the Lacerta focuser you mentioned and I'm glad that I finally found an upgrade rather than a full focuser change! Thank you

In addition, got a UHC filter for DSOs that I'll try on my next session. and thinking of getting a focus mask.

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10 minutes ago, PlanetGazer said:

Hahaha on the same boat, I was looking at dual speed focusers yesterday, and have already bought a RACI finder, I use the dot sight from the 130 on the 250 as well. I just searched the Lacerta focuser you mentioned and I'm glad that I finally found an upgrade rather than a full focuser change! Thank you

In addition, got a UHC filter for DSOs that I'll try on my next session. and thinking of getting a focus mask.

Same boat indeed... I was thinking about getting one of the finder multipliers with which you can fit 2 or 3 finders on the same shoe but I realised I could get a new Telrad (comes with its own base) for only 15 EUR more.

I have a slight advance on the filter side as I have a UHC and an OIII (and color filters but they re really low quality). The UHC is great, you should try that on the ring or the dumbell nebula, it makes a big difference.

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On 25/08/2019 at 00:02, PlanetGazer said:

Another question comes to mind, is atmospheric turbulence more noticeable in low focal ratio scopes? or is it related of how well collimated the scope is? As I noticed that my slower scope show better views of Jupiter when low in the sky, than my fast scope (250p).

 

edit: Typo

 

 

Interested in this part as well, would jupiter at same magnification, lets say 200x, with same quality Eyepieces, look better on 150mm SC f10 or on 150p f5 newtonian

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Fast scopes are more sensitive to collimating errors than slow scopes. Slow scopes also tolerate lower quality eye pieces better.

I suggest you use a permanent marker to place an ‘X’ on one of the primary mirror collimating screws and never adjust that screw again. When you collimate the mirror by adjusting all three screws the mirror will “walk” resulting in focal length error. Moving the primary in or out will change the focal point so that you will be unable to achieve sharp focus.

 

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