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Posted (edited)

Last August I bought a secondhand RC F8 8", initially it seemed fine but something changed and I started to get very weird shaped stars, this coincided with my improving the dew heaters, previously I had made my own controllers up and didn't experience any problems..

The stars were shaped like the ace of clubs and I thought not much I can do about it now and stored it away until I had time to work on it.

In between time @peter shah offered his help, so last November I dropped it over to him and had a most enjoyable day with him, what a lovely patient person he is.

Anyway, Peter showed me how to strip the scope down but felt that it needed further testing, so I left it with him telling him to take his time, towards the end of January Peter called to say he had fixed it and I made plans to pop over at the beginning of February and we grabbed a bit to eat and had a thoroughly lovely meal in the local chewing about all things to put the world to right.

The clear nights were very much missing and I only managed a few nights using my Esprit's so put aside any thought of using the RC until the Galaxy season arrived.

Last week I dropped it finally on the mount and took a few images of M42, but the stars still had an odd shape, chatting to Peter he thought something might have happened in transit and suggested when I had time to drop it over to him.

Now since then I was nattered which is typical of me as I like to get everything right and when the opportunity for another RC came up of the same spec, I thought well at least I could use that whilst the other is being fixed and sell it later on, unlike mine which had a moonlite focuser, this one had a Featherlite, so a call to Ian King resulted in a Lakeside motorised focuser and control unit.

Now one of the things Peter had mentioned was the control of the dew heaters and he advocated not turning the control up full blast but having it on very gentle and to avoid using the one on the secondary as that was causing problems. So a few weeks ago I purchased a Kendrick Digifire 12 at an extortionate price (Don't ask, it was a lot more buying it direct than if I had purchased it from Europe), but it was faulty, so whilst it was away I used the Hitec Astro controller on a low to medium heat.

This evening I popped the newish RC it onto the mount and started to have a play, I was utterly gobsmacked, I was getting distorted stars and funny shapes, I had previously checked the collimation with a Howie Glatter that I had obtained and that seemed fine, it would seem nothing that I did improved the situation, so I went away and gave it some thought. Now I had left everything set up outside and had a quick glance at my laptop live as opposed to watching it from my office, and I thought, something is causing that distortion and then I noticed the dew shield, it has become a little out of shape, so I thought well it's only going to be set up another 10 mins whilst I checked out the focuser on it, so I removed it, virtually immediately there was a small improvement in the shape, not a lot but enough to be noticeable.

Now I wanted to check the focus on a real star, so I slewed to Acrturus, focused all the way in and all the way out to check the collimation, it was slightly out but nothing that was causing the odd shapes that I was getting, see below: -

5aa8847fc3b32_RCwithdewheater.png.de1262c54b02f25409886a2000182d54.png

So I am looking at the scope and thinking "Lets drop the heat even further on the primary and see if that has an affect" almost within seconds there was an improvement, so I captured an image and dropped the power completely, the image became almost perfectly round: -

RC2_Cooled_Slight_adj.PNG.0496f674bb99d8c97dbeec49a23c9159.PNG

As I played some more I had a little mess with the collimation, but thought best leave it until another night as it is so easy to collimate and I was really chuffed with myself.

So I thought OK lets see how good we can get the focusing this evening as it just didn't look good previously, Bingo SGP focused it far better than it had ever been and this was with poor seeing a lots of clouds drifting around: -

5aa88559b3805_RC_afterfocusing.thumb.PNG.bd00131b9ff74bc1cb4ef66a2d60cbd5.PNG

Now this was what I was looking for from the RC, so I thought lets just go for a better image to see what she was capable of and despite there being clouds drifting over head almost obliterating Arturus, the image came out very well, I happen to like the defraction spikes if they are done with consideration to the image, so these were really quite lovely: -

RC2_guiding_180sec.thumb.PNG.6f5c9224fe902f54ffd00d4ee9c5a818.PNG

Now the above is just a quick 180sec image to see what it would look like, I'm really pleased and if hadn't been down to Peter I would never have thought a dew heater would be the problem, the collimation still needs a small tweak but at least I know what the cause was.

I ran out of time and didn't have a chance to test the original RC but as the shapes are exactly the same, then I'm fairly convinced that the problem is now solved, the only problem I now have is that I have two RC's and only need one, so the classifieds will be taking a visit shortly once I have confirmed that the other is good as well.

So can anyone suggest some dew straps that put an even heat out rather than shoving it out all over the place?

So finally a big thanks to @peter shah and my mate @Toxophilus Mark as he has put up with my wingeing. Ta Lads.

Edited by Jkulin
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It is always a pain having to track down things like this and I'm glad you've got is sorted. It looks as if you will start getting some good results now - look forward to seeing them on the forum.

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Posted (edited)

Ive done a lot of work on introducing heat into a scope.....trying to keep the dew away without have any effects on the image is a dark art.... Great to see it working well and was a pleasure to help out.....

Edited by peter shah
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Hi Jkullin,

I see you have mentioned Dew heaters fitted to the primary and secondary mirrors. I have read about users having problems with the secondary dewing up but not many have mentioned problems with the primary. Do you know what heaters are fitted to the primary and secondary?

I have a secondhand RC10C which should be ready to go in a few weeks. I am going to give it a try as it is collimation wise. Then in the summer I plan to carry out a few modes. I have a mirror isolation/focuser collimation plate to fit. This will mean taking the scope apart. So I had thought I might fit a dew heater to the primary while it was in pieces. After your experience I may have a rethink about its worth. Heater is fairly easy to fit to secondary when the instrument is assembled.

Graham

 

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Hi Graham,

I had fitted a Kendrick dew heater directly onto the back of the secondary and sealed it was sealant and then coincidentally I started to get weird shaped stars, so Peter offered to check it out as initially he thought the primary maybe pinched, so he showed me how to strip it down.

Peter ran it in his Obsy and he was reasonably satisfied that there were no distortion, but he wasn't using the secondary heater and I don't think I left the primary with him.

When I got it back I had a play with it, but things really weren't right, so Peter offered to have another look when we planned to meet up again. In between time I had acquired this other RC and rather than be without a longer Focal Length for some Galaxies that I was after I set that up and experienced a similar distortion, so then it was a matter of stripping everything off so that you were dealing with a bare scope and that proved that the primary dew strap was the cause..

Now I do plan to use some form of dew prevention, so I ordered today a temperature sensor meter and will fit that under the dew strap until I can work out precisely what temperature the distortion occurs, I shall refit the secondary dew heater and similarly check that out as well. I had purchased a Kendrick DigiFire 12 to control on variances the power to the dew straps, but that was sent away with a warranty fault and I have just discovered that they have sent it back with the fault unresolved, so I will now have to wait until that has been fixed so that I can establish a controlled routine for Dew Control.

My dew heater strap can only fit right at the base of the RC as moving it up would not allow 100% contact due to the Vixen and Losmandy rails.

I can't advise what the effect would be of fitting a heater direct to the primary, but I imagine you could suffer similar problems that I have.

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Many thanks for you comprehensive reply.

So you primary heater was in direct contact with the back of the mirror. On the 10inch I have seen heaters inside the back plate and around the outside of the mirror there is supposed to be sufficient room to place it without touching the mirror. I am not sure there is the same clearance on the 8 inch which you have. I will see what actual room there is when I get round to taking it to apart. I may fit a some sort of primary heater anyway as taking the scope to pieces is not something I plan to do very often.

Sorry to here about your Kendrick experience. Did you buy it direct from them?

Graham

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18 minutes ago, Fellside said:

Many thanks for you comprehensive reply.

So you primary heater was in direct contact with the back of the mirror. On the 10inch I have seen heaters inside the back plate and around the outside of the mirror there is supposed to be sufficient room to place it without touching the mirror. I am not sure there is the same clearance on the 8 inch which you have. I will see what actual room there is when I get round to taking it to apart. I may fit a some sort of primary heater anyway as taking the scope to pieces is not something I plan to do very often.

Sorry to here about your Kendrick experience. Did you buy it direct from them?

Graham

Hi Graham,

I wrapped my dew heater band around the rear of the primary on the outside, not fastened to the back or in direct contact with the mirror, I would have thought you would have considerable problems putting it too close or too high a temp.

Yep not a very nice experience with Kendrick, much has been edited, PM me if you want further details.

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