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gravityguy

Skywatcher 250PX Dobsonian (was certain, now unsure) :)

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Hi all, I've been hanging around here for a while now reading various posts and trying to decide what telescope I want to buy as my first. After lots of deliberation I had chosen the Skywatcher 250px dobsonian due to its large aperture and relative low cost.

I called sherwood photo for some advice and to be honest it has put a bit of a spanner in the works. He said that if I wanted to see decent detail of planets then a 10" is not the best choice and maybe an 8" would be better. I do want to see good detail on the planets but I would also like to see DSO's as well as possible. He did say that I could make a mask for the front to reduce the aperture which would effectively lower the focal length making the planets viewable with higher detail.

As a newb I am unsure which I am going to prefer (either planetary of DSO's) and so I don't want to limit myself either way. I have to say, it was a bit of a surprise for a shop to steer me towards a smaller and therefore cheaper scope, but I was grateful for their honesty rather than trying to tell me tat actually what I need is a 12" instead! lol

I know this is yet another question about a scope that has been talked about a lot on here but I would really appreciate other peoples view on this.

Mike

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The 10" is faster at F4.7 than an 8" which I believe is F6. Not sure exactly how this effects planetary viewing if im honest. My 250px provides very pleasant views of eveything, including planets.

For instance on Jupiter I can see seaguls in the banding, I can see very minor bands in the upper and lower hemispheres, not just the main bandings. I can see very well resolved shadows on it's face when moons transit in front. I can see the GRS. All from an urban environment at 200-250x magnficiation. A doddle for this scope which can handle 500x (this is its maximum and it's not very pleasant at this magnification in my personal opinion).

Saturn will show the cassini division in good seeing. Mars shows polar caps and detail on the surface. Venus shows a cresecent.

The Moon is just lovely in this scope too. You can get so up close and personal, with a widevifeld eyepiece you have to wonder if the view isnt far off what an Appolo astronaut would've seen with their own eyes out of the porthole windows!

The 10" will be more demanding on eyepieces than the 8", that much is mathematically unavoidable, but in my opnion its worth it.

Pesonally I plan to upgrade to a 12" at some point in the next year and that has a slighltly slower F ratio at 4.9

Obviously these are all fantastic DSO scopes and the bigger you get the better however 14" and 16" are very demanding on eyepieces. you'd need Tele Vues or other high end optics to prevent quite drastic coma i'd imagine. My scope at F4.7 does test my eyepieces which range from £150 up each.

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Thanks for that. Sounds like you get excellent detail using a 10" - even better knowing that it is the same scope as the one I am looking at getting.

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In theory he is right because of the increased size of the secondary in relation to the mirror it will reduce the contrast slightly. it's one of the reasons that planetary newts tend to be longer focal lengths. A planetary purist will see the difference us mere mortals probably woudn't notice that much. It's the usual thing about compromise and different scopes no scope does it all perfectly every one has its own advantages but apparture will get you over most of the differences

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The 10" is faster at F4.7 than an 8" which I believe is F6. Not sure exactly how this effects planetary viewing if im honest. My 250px provides very pleasant views of eveything, including planets.

For instance on Jupiter I can see seaguls in the banding, I can see very minor bands in the upper and lower hemispheres, not just the main bandings. I can see very well resolved shadows on it's face when moons transit in front. I can see the GRS. All from an urban environment at 200-250x magnficiation. A doddle for this scope which can handle 500x (this is its maximum and it's not very pleasant at this magnification in my personal opinion).

Saturn will show the cassini division in good seeing. Mars shows polar caps and detail on the surface. Venus shows a cresecent.

The Moon is just lovely in this scope too. You can get so up close and personal, with a widevifeld eyepiece you have to wonder if the view isnt far off what an Appolo astronaut would've seen with their own eyes out of the porthole windows!

The 10" will be more demanding on eyepieces than the 8", that much is mathematically unavoidable, but in my opnion its worth it.

Pesonally I plan to upgrade to a 12" at some point in the next year and that has a slighltly slower F ratio at 4.9

Obviously these are all fantastic DSO scopes and the bigger you get the better however 14" and 16" are very demanding on eyepieces. you'd need Tele Vues or other high end optics to prevent quite drastic coma i'd imagine. My scope at F4.7 does test my eyepieces which range from £150 up each.

I think I've just gone and bought one.... :rolleyes2: (gulp). I obviously didn't need much convincing! lol

I think the fact that you own one and get good images has really helped. Thank you (can I then blame you if I dont like it? lmao)

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Blame the clouds, we all do. Oh an congratulations, I dont think you'll be disappointed :)

If you havent already, get a copy of Turn Left at Orion. It's the perfect book to start out with.

Edited by Stargazing00

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Blame the clouds, we all do. Oh an congratulations, I dont think you'll be disappointed :)

If you havent already, get a copy of Turn Left at Orion. It's the perfect book to start out with.

I've seen a lot of people recommend that so I have now purchased it for my kindle! hehe

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It's a very diagramatical book, in that it guides you visually. Not sure how it would render on a kindle to be honest. It's printed on large pages and all diagrams are red light friendly (white with black detail).

Let me know how you get on with it on a Kindle.

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In theory he is right because of the increased size of the secondary in relation to the mirror it will reduce the contrast slightly. it's one of the reasons that planetary newts tend to be longer focal lengths. A planetary purist will see the difference us mere mortals probably woudn't notice that much. It's the usual thing about compromise and different scopes no scope does it all perfectly every one has its own advantages but apparture will get you over most of the differences

Actually on this; the larger secondary on the 250px is actually quite useful in my opinion. I have a growing range of 82' AFOV eyepieces and one reason I am able to use these well on this scope is because of that larger secondary mirror. Larger AFOV gives more pleasing views (in my opinion) and you end up nudging the scope along a little less as well. :)

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Size of optics doesn't effect coma, it's speed. A 16" f/8 will be no more demanding, eyepiece wise than an 8" f/8. The issue is often as we increase aperture to the larger sizes we shorten the f/ratio to keep the scopes length down. The large SCT (14"-20") have long f/ratios and don't need expensive eyepieces to perform very well indeed. It's only that most large Dob owners don't wanna be scrambling up and down ladders all night that makes them build faster, more demanding scopes. :)

?????

Not quite sure what you are saying here, but my Naglers work very nicely in my observing buddies 8"f/6 skyliners. There's no loss of AFOV, or FOV.

On a side note. Not sure I agree with the shops advice. A 10" will be better on planets. The fact it's f/ratio is faster than an 8" skyliner is neither here nor there. Resolution depends solely on aperture, period. You will always see more with a bigger scope, although there is very little difference between the two that I can see when pointed at planets. Seeing effects planetary views more than anything else (except clouds :D ) and can often prevent you getting the most out of your scope, but TBH if seeing is bad for a 10" the 8" will fair no better.

On masking down a 10". On the nights where you need to mask it down, TBH I would just forget the planets and observe something else. IMO there is too much resolution lost when using a masked down 10" scope, because to avoid the secondary percentage becoming too large and destroying contrast, the masked down scopes aperture is too small to be that useful. I've masked down my 10" with very mixed results, yes, it improved on the terrible seeing conditions but, the image produced was so lacking in resolution it was a waste of time. Only scopes 12" and larger are worth masking down, as when they are masked down the smaller aperture is still large enough to be useful.

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I meant the speed of the 14" and 16" scopes is faster than the 10" and the 10" is F4.7 although I can see now I didnt actually say that. I believe the others ae F4.5 and F4.4 respectively so would test eyepieces even further than a 10". The 12" is a nice one because they extended the FL from 1200mm in the 10" to 1500mm in the 12" it got slightly slower despite the aperture increase.

On the secondary mirror, I've been told that the 10" has an oversized secondary mirror and that this would be beneficial to achieve maximum AFOV from eyepieces. It may be mis information or I just misunderstood.

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I think for a first scope the 250PX will blow your socks off whatever you look at. Sure, other scopes might be more suitable if you really want to specialise in lunar study or planetary imaging or something like that, but if you want an all-rounder a big dob will be very hard to beat.

James

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It is true about eyepieces though. My experience is that it's not kind at all to cheap eyepieces. It's worth seeking advice about what works and what doesn't if you decide you want to add to your collection. You don't have to go mental on price, but you do need to choose wisely.

James

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Thanks for the of the adcive, it is very much appreciated :)

My scope actually arrived yesterday (I only ordered it at about 2pm on Tuesday and so was expecting delivery today) so a massive thank you to Martin at first light optics who was very helpful! I have to say, even though I knew the dimensions of the scope, I was still amazed at just how big it is! I thought FLO had made a mistake and sent me hubble (it is advertised on their site afterall)! hehe. I am also surprised at just how portable it is. It took just a couple of minutes to take off the tube and then the same again the reattach making it very quick indeed. On that note, is there a specific way to carry the tube as I was finding that the finderscope kept pressing down on my arm. Is that normal or will that eventually cause damage?

The only thing I actually managed to do last night was to align the finderscope. I did this on my driveway as it gives a better all round view to find. A couple of people walked past and looked at me like I was some kind of nutcase! lmao

I also made my first mistake. I put the 25mm eyepiece on tope of the dust cover after I had put it back on the scope, but then forgot to take it off again before I took the tube off the mount. It rolled off and hit the ground pretty hard. It was in its case but I hope that I havene't gone and damaged it before I've even had chance to use it!!!

Looking forward to some clear skies now :)

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Hi I am beginner also and have this scope, I wouldn't worry about the eyepieces that came with it they aren't very good anyway they are just to get you started... Its a great scope but too big to hump around I find... I don't know if its a good idea or not because I thought of it myself but I have fixed a big bag of those crystals that soak up moisture that you get in camera bags onto the inside of my main 10" dust cover... I also bought an Ostara 2x Barlow, red LED torch and got SkyWalk on my Iphone, a set of decent lenses and that's it, that's all you need really... I did buy loads of books and a planisphere but to be honest you can get everything you need to know from the net and the SkyWalk shows you where everything is... its a great app you can put it on night vision (red) mode also so it doesn't ruin your night vision... I haven't touched my books or planisphere since I the got SkyWalk app, you can even track satellites with it like the ISS although you will have a job getting it on your viewfinder you can follow it with binocs ok I find.

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I hold the OTA with on hand gripped around the mirror and the other arm cradling it at the elbow like you would a babies head. Lower to the ground gently so as not to disrupt the primary mirror. Move the base to where you want to set up first and then carry the OTA to it. The finderscope should never get near your body if you are carrying it like this.

Mistakes are easy to make. The other night I switched from 2" to 1.25" and forgot to put the reducer in. When I went to offer up a 1.25" eyepiece I damn near posted it through the 2" hole, first class delivery to my primary mirror!

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Hi I am beginner also and have this scope, I wouldn't worry about the eyepieces that came with it they aren't very good anyway they are just to get you started... Its a great scope but too big to hump around I find... I don't know if its a good idea or not because I thought of it myself but I have fixed a big bag of those crystals that soak up moisture that you get in camera bags onto the inside of my main 10" dust cover... I also bought an Ostara 2x Barlow, red LED torch and got SkyWalk on my Iphone, a set of decent lenses and that's it, that's all you need really... I did buy loads of books and a planisphere but to be honest you can get everything you need to know from the net and the SkyWalk shows you where everything is... its a great app you can put it on night vision (red) mode also so it doesn't ruin your night vision... I haven't touched my books or planisphere since I the got SkyWalk app, you can even track satellites with it like the ISS although you will have a job getting it on your viewfinder you can follow it with binocs ok I find.

Thanks for that, much appreciated. I used to use skywalk but have since found skysafari which was highly recommended on here. I definitely prefer it to skywalk as it seems more stable when moving it around the sky. Bit more pricey than skywalk but I think the added features and stability are worth it. How do you find the ostara barlow? This is probably going to be the first piece of kit I buy so am interested as you have the same scope. Do you use it with the original EP's or have you bought others?

I hold the OTA with on hand gripped around the mirror and the other arm cradling it at the elbow like you would a babies head. Lower to the ground gently so as not to disrupt the primary mirror. Move the base to where you want to set up first and then carry the OTA to it. The finderscope should never get near your body if you are carrying it like this.

Mistakes are easy to make. The other night I switched from 2" to 1.25" and forgot to put the reducer in. When I went to offer up a 1.25" eyepiece I damn near posted it through the 2" hole, first class delivery to my primary mirror!

Ouch, that could have been costly...!!! I will try your method of carrying (when the skies clear).

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Thanks for the of the adcive, it is very much appreciated :)

My scope actually arrived yesterday (I only ordered it at about 2pm on Tuesday and so was expecting delivery today) so a massive thank you to Martin at first light optics who was very helpful! I have to say, even though I knew the dimensions of the scope, I was still amazed at just how big it is! I thought FLO had made a mistake and sent me hubble (it is advertised on their site afterall)! hehe. I am also surprised at just how portable it is. It took just a couple of minutes to take off the tube and then the same again the reattach making it very quick indeed. On that note, is there a specific way to carry the tube as I was finding that the finderscope kept pressing down on my arm. Is that normal or will that eventually cause damage?

The only thing I actually managed to do last night was to align the finderscope. I did this on my driveway as it gives a better all round view to find. A couple of people walked past and looked at me like I was some kind of nutcase! lmao

I also made my first mistake. I put the 25mm eyepiece on tope of the dust cover after I had put it back on the scope, but then forgot to take it off again before I took the tube off the mount. It rolled off and hit the ground pretty hard. It was in its case but I hope that I havene't gone and damaged it before I've even had chance to use it!!!

Looking forward to some clear skies now :)

I will have a look at skysafari I haven't heard of that, I bought the ostara barlow from Amazon but you can get from flea bay also or probably anywhere that sells a range of eyepieces, the moon with a 2x barlow and the original short eyepiece and a moon filter was my first big major wow with this scope... its amazing but the moon travels so fast through the field of view.... fantastic though !! The missus is always telling people about it even though she isn't interested in the rest she loves looking at the moon through this scope...

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Hi all, I've been hanging around here for a while now reading various posts and trying to decide what telescope I want to buy as my first. After lots of deliberation I had chosen the Skywatcher 250px dobsonian due to its large aperture and relative low cost.

I called sherwood photo for some advice and to be honest it has put a bit of a spanner in the works. He said that if I wanted to see decent detail of planets then a 10" is not the best choice and maybe an 8" would be better. I do want to see good detail on the planets but I would also like to see DSO's as well as possible. He did say that I could make a mask for the front to reduce the aperture which would effectively lower the focal length making the planets viewable with higher detail.

As a newb I am unsure which I am going to prefer (either planetary of DSO's) and so I don't want to limit myself either way. I have to say, it was a bit of a surprise for a shop to steer me towards a smaller and therefore cheaper scope, but I was grateful for their honesty rather than trying to tell me tat actually what I need is a 12" instead! lol

I know this is yet another question about a scope that has been talked about a lot on here but I would really appreciate other peoples view on this.

Mike

I recently bought a 10" skywatcher dobson. Didn't view the planets, but did observing of some DSOs. Even from the light polluted sky of New Delhi, I had good views of the ring nebula. Was able to split the 4 component stars of epsilon lyrae at 120x. If you are opting for a dobson, then couple of DIY stuff would help. i fixed a piece of foam in the inside of the dobson mount to prevent the tube banging in. Check the collimation every time you use the OTA. I have taken a laser collimator. It really helps to collimate in minutes. Its a great scope...perfect value for money.  

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Congratulations on your new purchase.


I could have suggested that 8" 12" and 10" in that order is the way to go? Affordability puts the 8" first. First Light Optics ( our site sponsor) quotes it as the uk's best selling Dobsonian telescope for good reason. Its an f/6 focal ratio.


Next is the 12". Much dearer, but it comes with more performance and a focal ratio of f/5. Newtonian telescopes have comatic aberration, COMA. Put simply the faster the telescope the more prominent the coma is visually.


The 10" f/4.8 telescope is the fastest out of the three, therefore,eyepiece selection is now more critical in order to prevent the visual effects that coma present to you in your final image. The lower focal ratio affects the cone of light  though the telescope, the side effect is that you dont have a flat field of view, so images start looking like little 'comets' as you view them towards the edge of the field of view. This can only be improved by the use of selective eyepieces. Not everyone is bothered by coma, if the image is affected off-centre, they just move the telescope accordingly. But for astro photography the results would not look too good.

Edited by Charic

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 Newtonian telescopes have comatic aberration, COMA. Put simply the faster the telescope the more prominent the coma is visually.

The do suffer from coma as a result of the mirror shape but they most certainly do not suffer from chromatic aberration as they use a mirror, not a lens. Chromatic aberration is a refractive problem only, not a reflective one. An eyepiece can suffer with it though.

To the OP... the 10 inch is the best choice of the 3 in my opinion.  I have the 10 and a friend has the 12 and and 8 inch newt. The 8 inch doesn't show as much as the larger ones (10 and above seems to be the magic number for the feint stuff out there) and the 12 inch is just a monster. The 10" is the best compromise for size, portability, price and views. Good choice. You are going to love it.

Edited by Tiny Small

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The do suffer from coma as a result of the mirror shape but they most certainly do not suffer from chromatic aberration as they use a mirror, not a lens. Chromatic aberration is a refractive problem only, not a reflective one. An eyepiece can suffer with it though. To the OP... the 10 inch is the best choice of the 3 in my opinion.  I have the 10 and a friend has the 12 and and 8 inch newt. The 8 inch doesn't show as much as the larger ones (10 and above seems to be the magic number for the feint stuff out there) and the 12 inch is just a monster. The 10" is the best compromise for size, portability, price and views. Good choice. You are going to love it.

To be fair Charic did type comatic aberration not chromatic.

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Tiny Small.........did I put  chromatic? Oops .  My mind, brain fingers  all wanted to write comatic, and it looks like comatic aberration is in there.  :shocked:

You're right though about the failure of a lens to focus all colours to the same convergence. That is chromatic aberration.

ps. How's Kingston these days. I used to school there, many many Years ago, but everything I remember has gone? School gone, first factory I wanted to work in, Hawker Siddeley gone , many things all gone!

Edited by Charic

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To be fair Charic did type comatic aberration not chromatic.

LMAO! Ooopps.

Sorry... long day at work and I read that about 40 times. Appologies :)

Kingston is terrible. I can't wait to move back up north.

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