'Dobby'

[Bino Pete]

If I decided to buy a dobsonian mounted telescope (probably a Skywatcher 250mm one) - 

1) On one review the reviewer states - ' The low F ratio does mean that you have to choose your eyepieces carefully to avoid the seagulls effect at the edge of the field. ' can somebody explain this and check my eyepieces in my sig. below and tell me if these are ok?

2) I've also been told that these scopes take longer to cool down than my mak 127. I will be keeping the dobsonian in a garden shed, does the cool down time apply to this and also with storing my scope in a shed can you foresee any problems?

Thanks

 

[laudropb]

Hi. The X-Cel XL eyepieces work well in my F 5.9 dob! But not too sure if they are OK for you at F 4.7. I keep my Dob in a unheated garage and have not had any problems, but I do have a proper cover for it. It cools down for observing very quickly, no more than 15 minutes and often by the time I have set it up.

 

 

[MarkVIIIMarc]

For cool down I think you have the right understanding. A bigger scope will take longer to cool down than a smaller one. Keeping it in a garage or shed is closer to the outside temp than your living room so the cool down time will be less.

I will get back on eyepieces after I figure that out.

[swamp thing]

Hi Pete

Ignore "cool down' time and start observing immediately upon setting up. This waiting around for ones scope to cool down is a waste of time. If you must observe planets just avoid doing so for the first 15-30 mins after setting up. Using the scope at low to medium power on low definition objects like galaxies avoids any cool down issues. You just observe as the scope cools changing targets to higher definition ones after it has cooled.  

Have fun out there  

[Louis D]

Cool down time depends on the daytime temps as well in the shed.  If it heats up greatly during the day and then cools down rapidly at night, as in the desert, you might be better off keeping it inside (assuming A/C) until twilight when the outside temp matches the inside temp.

Maks tend to cool slowly because of their thick corrector lens at the front and closed design behind it.  The dob is completely open and you can always add a fan behind the primary mirror to speed up cooling.

Low f-ratio scopes (faster scopes) have a steeper incoming light cone to the eyepiece, stressing the design more than a slower scope would where the rays impinge on the eyepiece field lens more or less perpendicularly.  Generally speaking, simpler eyepiece designs like plossls, konigs, erfles, kellners, and orthos all will display more edge of field astigmatism (seagulls) than more modern designs like Naglers, Panoptics, and Ethos designs.  In the center 10 degrees or so, pretty much all of them will display a nice image even in faster scopes.

[brantuk]

My Mak 127 lives in the house and when I set up outside it takes about 45mins to 1hr to cool down to operating temperature. My 16" newt lives in the garage at near outside temperature but takes exactly 30 mins with the fan on to get the primary on song.

I don't know what that tells us really - but as mentioned above I just start using them soon as I'm ready and pick eyepieces and objects to suit prevailing conditions. What I can say is that the views get better after the first hour and I can choose a wider range of objects and eyepieces - pretty much with any scope.

[spike95609]

I keep my 250px indoors and usually give it about half an hour to cool down. This is the only one of the Skywatcher dobs which comes with pyrex glass, which supposedly makes it quicker to cool than a normal 10" mirror. I think too much is made of the F4.7 having an impact on eyepieces, maybe it depends on how fussy you are about edge of field defects or how wide you like your view, but I got on okay with even the much maligned 10mm Super MA that came with the scope until I got something better. I seriously considered getting some X-Cels myself, and from what I recall of my EP research back in the day, they'll work fine.

[John]

Personally I wish the 10" chinese dob's were a bit slower - F/4.7 is going to challenge most wide angle eyepieces to keep things sharp over the full field width. With such eyepieces showing astigmatism (thats the seagulls) and the fast primary mirror showing coma as the field widens it's probably best to not be too fussy as the above poster says, unless your eyepiece and coma correction budget is a few times as much as the scope costs !

My 12" F/5.3 dob takes around 30 - 40 mins to cool from dining room temp to showing decent medium to high power views and another 20 mins if I want to push things a bit more. You can use it for low power stuff more or less out of the house.

[Bino Pete]

If I was to buy a dob (250mm) and didn't have any eyepieces what would you recommend to buy?

[cotterless45]

I'd start off with BST Starguiders . At £50 a throw they give good results. I started with the 25mm, 12 and 8mm. 

For those keeping Newts in a shed , the temperature can vary . This can lead to humidity on the primary. If left for hours this can affect your mirror. I rush up and open up in warm weather, if at home. I found that a 6w reptile heating pad left on the bottom of the mirror provides enough gentle warmth to keep humidity away. Removing this before observing and putting the Dob out in shade and you're soon observing,

Nick.

[Bino Pete]

45 minutes ago, cotterless45 said:

I'd start off with BST Starguiders . At £50 a throw they give good results. I started with the 25mm, 12 and 8mm. 

For those keeping Newts in a shed , the temperature can vary . This can lead to humidity on the primary. If left for hours this can affect your mirror. I rush up and open up in warm weather, if at home. I found that a 6w reptile heating pad left on the bottom of the mirror provides enough gentle warmth to keep humidity away. Removing this before observing and putting the Dob out in shade and you're soon observing,

Nick.

What's your view of the eyepieces I've already got (see sig) in a 250mm f4.7 dob?

[Ricochet]

Starguiders and X-Cel LX's are supposed to be pretty similar in performance so there is no point essentially duplicating what you already own. Just try your existing EP's with any new scope and let your own observations guide you as to what else, if anything, you need to add.

[Louis D]

Bino Pete: I'd get either a used 5.2mm Pentax XL or a new or used 5mm Pentax XW since you don't have anything below 7mm.  I use my ancient 5.2mm Pentax XL all the time to view globular clusters, the moon, planets, and double stars.  I also have a 3.5mm Pentax XW, but it usually yields too small of an exit pupil for my floaters-filled eyes.

At the other end, pick up a used 30mm ES-82 to get a much wider field of view at a reasonable exit pupil.  If you need eye relief, try to find one of the original mushroom top ones or the Meade 5000 UWA or Celestron Axiom versions.  You can always remove the moveable eyecup with a little surgery.  There's also the 28mm UWA from Kunming United Optics in various brands like the Williams UWAN.  Meade has their new 21mm MWA 100 degree line which everyone agrees to be closer to 92 degrees for decent prices.  And there's also the Lunt and ES 20mm 100 degree offerings.  And despite mixed reviews, there's even the 31mm Celestron Luminos to consider.  You might be able to find one used for a good price.