Refractor suggestions please

[sshenke]

I have been fed up with dew build up on my reflector- Skywatcher 130PDS with no comprehensive solution to this problem. Every imaging session I had recently had been utterly ruined by dew build up within a maximum of an hour. hence thinking of buying a refractor scope instead, as I think dew will be much more manageable with the use of dew shields +/- dew bands. Would appreciate suggestions for a refractor scope for a modest budget (around£500) please. Thanks.

[FaDG]

Skywatcher 72ED + flattener/reducer + dew strip should be around your target point

[Skyline]

An 80Ed variant Orion, Celestron, skywatcher s/h can be had from £250

N.

[Skyline]

2 hours ago, sshenke said:

I have been fed up with dew build up on my reflector- Skywatcher 130PDS with no comprehensive solution to this problem. Every imaging session I had recently had been utterly ruined by dew build up within a maximum of an hour. hence thinking of buying a refractor scope instead, as I think dew will be much more manageable with the use of dew shields +/- dew bands. Would appreciate suggestions for a refractor scope for a modest budget (around£500) please. Thanks.

Btw how long is your dew shield on the 130pds. If you have not got one, make one out of a exercise mat. I can remember I did not get any dew problems using a home made dew shield with a 130pds.

[Louis D]

I'm guessing the 130pds has a metal tube.  One of my dobs has a sonotube tube and has never had dewing issues on either mirror.  Perhaps you could try wrapping the entire tube with closed cell foam to insulate it to see if that prevents dewing any better.  Seems like a low cost experiment to try.

[sshenke]

Thanks everyone for your helpful advice. Yes, the 130 pds has metal tube. I will cut out foam mat to cover it wherever i can. in terms of dew shield, i was informed that the ota tube itself acts as a dew shield for reflectors. I shall give it a go, but if it doesnt work, then will buy the refractor. thanks once again

[joe aguiar]

normally people dont think reflectors can have dew problems but they sometimes they do, i even had this issue before as well.

its not so much the primary mirror but if the 2nd mirror is close to the open tube it can can dewy on dewy nights.

i have sometimes just bought a dewcap and it works, thats all that you really need, since theres no front lens it should last for few hours at least if not more.

joejaguar

[Adam J]

On 13/11/2019 at 20:05, sshenke said:

I have been fed up with dew build up on my reflector- Skywatcher 130PDS with no comprehensive solution to this problem. Every imaging session I had recently had been utterly ruined by dew build up within a maximum of an hour. hence thinking of buying a refractor scope instead, as I think dew will be much more manageable with the use of dew shields +/- dew bands. Would appreciate suggestions for a refractor scope for a modest budget (around£500) please. Thanks.

There was a Altair starwave 102mm ED 2017 edition on astro buy sell recently. Probably closer to your 130pds than a 60-80mm scope. So unless you want shorter focal length specifically it's a better choice and at your budget. 

[david_taurus83]

https://stargazerslounge.com/topic/344166-skywatcher-evostar-80edreducerautofocuser-stepper-motor/

[Carbon Brush]

Dew shields, heaters and hair dryers are cheap.
Good refractors are expensive and still use metal & glass.

If you are happy with the 130 (apart from dew) why not preserve the wallet?

Just my random ramblings. David.

[sshenke]

On 15/11/2019 at 16:07, Carbon Brush said:

Dew shields, heaters and hair dryers are cheap.
Good refractors are expensive and still use metal & glass.

If you are happy with the 130 (apart from dew) why not preserve the wallet?

Just my random ramblings. David.

Thanks. I was certainly happy with the images produced using the sw130 PDS until a few months ago, but not any more as I have made hardly any image worth the effort I had put in, because of the dew problem. It is really puzzling to me as every post I have read says how rare it is to see dew in the primary mirror of a reflector. Not sure if I am doing something wrong. I have tried hair dryer and fan heater ( blowing gently from a distance of about 30 cm)  as well. Usually this leads to loss of focussing and it takes about 10 minutes to get the images back into focus, so not useful. I am going to try the insulation mat and dew shield/ cap idea as well next time there is clear sky, before deciding to buy a refractor.

 

[Carbon Brush]

For me, it is about using the minimum amount of scope heat to prevent dew - rather than remove it.
That avoids the loss of focus from a hot air blast.

Generally I reserve the hot air for ensuring kit is warm and dry at the end of a session.

This autumn is though a bit different. The garden is really soggy and there is often almost 100% humidity on the (rare) clear nights.

HTH, David.

[sshenke]

26 minutes ago, Carbon Brush said:

For me, it is about using the minimum amount of scope heat to prevent dew - rather than remove it.
That avoids the loss of focus from a hot air blast.

Generally I reserve the hot air for ensuring kit is warm and dry at the end of a session.

This autumn is though a bit different. The garden is really soggy and there is often almost 100% humidity on the (rare) clear nights.

HTH, David.

Thanks David, will keep trying!

[dweller25]

Refractors dew up too.

A cheap camping mat and some elastic webbing to make a long dew shield will work well

Edited November 17, 2019 by dweller25

[Louis D]

Just spitballing ideas here, you could also try wrapping the tube with ribbon heaters used to keep pipes from freezing underneath the foam mat insulation.  That should warm things enough to prevent dewing without causing tube currents.

[joe aguiar]

On 17/11/2019 at 07:34, sshenke said:

. It is really puzzling to me as every post I have read says how rare it is to see dew in the primary mirror of a reflector. Not sure if I am doing something wrong. I

 

are you sure its the main mirror that has the problem tho?just saying since your using blowing the  main mirror.

if you are looking down the tube your probally not seeing the issue is the 2nd mirror and most likely not the primary, reason iam saying is I had this isse b4 too and its wasn't the primary since the 2nd mirror is sometimes only 2 or 3 inches from the front its probally that.

just using a dewcap that's like 10 inch is more then fine almost all times unless your outside for the whole night.

joejaguar

[sshenke]

On 18/11/2019 at 18:51, joe aguiar said:

are you sure its the main mirror that has the problem tho?just saying since your using blowing the  main mirror.

if you are looking down the tube your probally not seeing the issue is the 2nd mirror and most likely not the primary, reason iam saying is I had this isse b4 too and its wasn't the primary since the 2nd mirror is sometimes only 2 or 3 inches from the front its probally that.

just using a dewcap that's like 10 inch is more then fine almost all times unless your outside for the whole night.

joejaguar

i understand what you are saying. i wondered about that too, as i cannot see what's going on with the secondary mirror. however, it is absolutely clear that the main mirror dews up within a maximum of 1 to 2 hours. I have been using a fan with heating option to blow air from behind the main mirror, not directly down the tube from the front end

[carastro]

I have been using refractors and dew heaters for 9 years and only time I had dew was when my dew heater failed.  I also had a SW130PDS for 2 years and never had dew on that either, but I was worried it could happen so I always had it inside an obervatory eitehr at home or a camping observatory and additionally drapped dew heaters around the tube (either side of the focusser), not sure if that helped or not.

My refactors have been out on the camping field loads of times and with dew heaters I never get a problem with dew.

Carole 

[joe aguiar]

ok I see it also depend on your climate too, there are many factors in this hobby, I guess you can try wrapping it as others suggest see if that helps

joejaguar

[sshenke]

28 minutes ago, carastro said:

I have been using refractors and dew heaters for 9 years and only time I had dew was when my dew heater failed.  I also had a SW130PDS for 2 years and never had dew on that either, but I was worried it could happen so I always had it inside an obervatory eitehr at home or a camping observatory and additionally drapped dew heaters around the tube (either side of the focusser), not sure if that helped or not.

My refactors have been out on the camping field loads of times and with dew heaters I never get a problem with dew.

Carole 

Carole, this is really puzzling for me, i have draped the sw 130 pds in exercise mat when i took it out on Monday evening. i will post the picture later, might be hilarious. Temp on Monday evening was 1 to 2 degrees at 8 pm. mirror dewed up promptly within 1 hour. never got to see a single image of interest. Can I ask, if the sw 130 pds you used, worked well in winter too?

Edited November 20, 2019 by sshenke

[dweller25]

@sshenke

Edited November 20, 2019 by dweller25
Posted in error

[dweller25]

I wonder if it might be worth sealing the rear of the OTA once the primary mirror has cooled - a plastic bag and string would work initially - just to see if it works ?

Edited November 20, 2019 by dweller25

[Louis D]

1 hour ago, sshenke said:

Carole, this is really puzzling for me, i have draped the sw 130 pds in exercise mat when i took it out on Monday evening. i will post the picture later, might be hilarious. Temp on Monday evening was 1 to 2 degrees at 8 pm. mirror dewed up promptly within 1 hour. never got to see a single image of interest. Can I ask, if the sw 130 pds you used, worked well in winter too?

Sorry to hear this.  Dewing is caused by the objective radiating energy to the clear sky and dropping in temperature below the local dewpoint.  I think your only option left is to apply dew heaters directly to the back and/or edge of the mirror.  A refractor is probably going to dew up just as quickly under these same conditions since the difference in thermal mass of the glass between a refractor objective and a mirror is going to be minor.  A dew heater is definitely in your future.

[sshenke]

On 18/11/2019 at 18:51, joe aguiar said:

are you sure its the main mirror that has the problem tho?just saying since your using blowing the  main mirror.

if you are looking down the tube your probally not seeing the issue is the 2nd mirror and most likely not the primary, reason iam saying is I had this isse b4 too and its wasn't the primary since the 2nd mirror is sometimes only 2 or 3 inches from the front its probally that.

just using a dewcap that's like 10 inch is more then fine almost all times unless your outside for the whole night.

joejaguar

i understand what you are saying. i wondered about that too, as i cannot see what's going on with the secondary mirror. however, it is absolutely clear that the main mirror dews up within a maximum of 1 to 2 hours. I have been using a fan with heating option to blow air from behind the main mirror, not directly down the tube from the front end

[carastro]

Quote

Can I ask, if the sw 130 pds you used, worked well in winter too?

Yes, but in the winter I was inside an observatory with dew heaters sometimes wrapped around the tube either side of the focusser as a precaution. 

Are you absolutely sure it is the primary that is dewing up, i.e. you have looked down the tube at the primary? 

Carole